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What is a cephalium? Root Gorelick Department of Biology and School of Mathematics & Statistics and Institute of Interdisciplinary Studies, Carleton University, 1125 Raven Road, Ottawa, Ontario K1S 5B6 Canada (e-mail: [email protected]) Photographs by the author unless otherwise stated.

Summary : There are problems with previous at - gibt meist einen abgrenzbaren Übergang vom tempts to define ‘cephalium’, such as via produc - photosynthetisch aktiven Gewebe zum nicht pho - tion of more hairs and spines, confluence of tosynthetisch aktiven und blütentragenden areoles, or periderm development at or under - Cephalium, die beide vom gleichen Triebspitzen - neath each areole after flowering. I propose using meristem abstammen. Cephalien haben eine an - the term ‘cephalium’ only for a combination of dere Phyllotaxis als die vegetativen these criteria, i.e. flowering parts of cacti that Sprossabschnitte und sitzen der vorhandenen have confluent hairy or spiny areoles exterior to a vegetativen Phyllotaxis auf. Wenn blühende Ab - thick periderm, where these hairs, spines, and schnitte nur einen Teil der oben genannten Merk - periderms arise almost immediately below the male aufweisen, schlage ich vor, diese Strukturen shoot apical meristem, and with more hairs and als „Pseudocephalien“ zu bezeichnen. spines on reproductive parts than on photosyn - thetic parts of the shoot. Periderm development Introduction and confluent areoles preclude of Most cacti (Cactaceae) are peculiar , cephalia, which therefore lack or mostly lack even for angiosperms, with highly succulent stomata. There is almost always a discrete tran - stems, numerous highly lignified leaves aka sition from photosynthetic vegetative tissues to a spines, lack of functional photosynthetic leaves, non-photosynthetic -bearing cephalium, CAM photosynthesis, huge sunken shoot apical both of which arise from the same shoot apical meristems, and fantastic stem architectures meristem. Cephalia have different phyllotaxy (Buxbaum, 1950; Gibson & Nobel, 1986; Mauseth, than vegetative parts of the shoot and appear to 2006). The few that lack most of be on top of existing vegetative phyllotaxy. If flow - these traits – such as many species of ering parts only have a subset of the above char - Mill., with broad photosynthetic leaves, non-suc - acteristics of cephalia, then I propose calling these culent woody stems, and C3 photosynthesis – are structures ‘pseudocephalia’. thus antithetically considered anomalous, even though they superficially resemble typical an - Zusammenfassung : Es gibt Probleme mit bisheri - giosperms (Leuenberger, 2008; Griffith, 2008; gen Versuchen, den Begriff „Cephalium“ zu Butterworth & Edwards, 2008). Here, I focus on definieren, etwa über die Bildung von mehr more stereotypical succulent cacti in the subfam - Haaren und Dornen, die Verschmelzung von Are - ily , which includes all cacti other than olen oder die Periderm-Entwicklung auf oder un - Pereskia (sensu lato, including Leuenbergeria terhalb jeder Areole nach der Blüte. Ich schlage Lodé), Maihuenia Phil., and the subfamily Opun - vor, den Begriff „Cephalium“ nur für eine Kombi - tioideae (prickly pears and chollas). In particular, nation dieser Kriterien zu verwenden, also für I focus on cacti that only flower from specialized blühende Abschnitte von Kakteen, die zusam - reproductive structures, called cephalia (singular: menfließende behaarte oder bedornte Areolen cephalium) or pseudocephalia (singular: pseudo - außen an einem dicken Periderm besitzen, deren cephalium), both of which only occur in some Haare, Dornen und Peridermen fast unmittelbar species of the Cactoideae. unter dem Triebspitzenmeristem entspringen und Cephalium-bearing cacti are the platypus of die mehr Haare und Dornen auf den reproduk - the kingdom insofar as they look like a hoax: tiven Abschnitten als auf den photosynthetisch two very different looking organisms seemingly aktiven Sprossteilen haben. Periderm-Entwick - grafted onto one another to resemble a chimera lung und zusammenfließende Areolen schließen between a photosynthetic non-flowering part and eine Photosynthese der Cephalien aus, die daher a non-photosynthetic flowering part. Or, as keine oder fast keine Spaltöffnungen besitzen. Es Charles Darwin more eloquently said in his letter

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Figures 1a, & b. densiareolatus This is a pseudocephalium because the flowering b regions are discontinuous, photosynthetic, and possibly not produced directly at the shoot apical meristem. to Joseph Dalton Hooker about Welwitschia mirabilis Hook.f., dated 18 December 1861, “a vegetable Ornithorhyncus ”. Like the platypus, cephalium-bearing cacti are real. However, most of us do not consider cephalium-bearing cacti to be quite as striking as the platypus because west - ern science has been familiar with the Me - locactus Link & Otto since Christopher Columbus returned from his first trip to Hispaniola in 1492, while the platypus was introduced to western sci - ence three centuries later, in 1799. Mauseth (2006) defined a cephalium as the re - productive parts of those cactus plants in which there is a distinct juvenile-to-adult transition in morphology. This is a great starting point for defining a cephalium, but is too circular because the only way to genuinely distinguish juvenile from adult morphology is by whether or not axil - lary buds (aka areoles in cacti) are capable of flow - ering. This is why paedomorphism is so hard to define in plants (Olson, 2007). Clearly Mauseth (2006), Buxbaum (1964), and other botanists re - alized the circularity of this definition of a cephal -

Bradleya 34/2016 101 According to Buxbaum (1952), Schumann (1897–1899) used the term cephalium for Pfeiff. and and no other genera. Schumann (1897–1899) used the term cephalium for reproductive structures completely enveloped in hairs, in which the flowering tissues widened in both vertical and circumferential di - rections – thereby excluding most lateral cephalia – and the hair-bearing areoles are helically or spi - rally arranged. He then, quizzically, applied the term ‘cephalium’ to both Melocactus and Cephalo - , even though flower-bearing areoles in Cephalocereus are not helically arranged, but maintain the vertical phyllotaxy of juvenile ribs. Berger (1907: 61) provided the first explicit definition of a cephalium as “the floriferous region of the plant is differentiated from the rest”. How - ever, this was for Cephalocereus , which many re - searchers today consider to have a pseudocephalium (e.g. Valverde et al ., 2007). Gürke (1908) also used the term ‘cephalium’ for Cephalocereus , although his notion of the genus included what we now call Backeb. and Britton & Rose. In their four-volume monograph, Britton & Rose (1919–1923) confined use of the term ‘cephal - ium’ to Melocactus Link & Otto and Pfeiff., with all other cacti having a pseudo - cephalium or having neither a cephalium nor Figure 2 . Pilosocereus gounellei subsp. zehntneri . pseudocephalium. This is a pseudocephalium because the flowering Berger (1926, 1929) concurred with Britton regions are discontinuous and photosynthetic. and Rose and only used the term cephalium to ium and therefore proposed a suite of other crite - refer to the terminal (aka apical) cephalium, such ria for what constitutes a cephalium or pseudo - as in Melocactus and Discocactus . Berger (1926, cephalium. In this paper, I synthesize their 1929) used the term pseudocephalium for Espos - attempts and, in so doing, highlight the contin - toa , Cephalocereus , and Mattf.. Berger uum in degrees of cephalium development, espe - (1926, 1929) declined to apply the term ‘pseudo - cially where gradual transitions shed light on the cephalium’ even to the hairier species of Pilo - otherwise abrupt transition to cephalium devel - cereus Lem. (now known as Pilosocereus Byles & opment, as well as the exceptions that seemingly G.D. Rowley), but referred to the long-lived are - prove the rule. oles of Britton & Rose and Neoab - bottia Britton & Rose (subsequently subsumed in History of the term cephalium Britton & Rose) as being real For a history of the term ‘cephalium’, see the cephalia, albeit he distinguished long-shoot from first few pages of Buxbaum (1952) and Gorelick short-shoot cephalia. (2013), which I summarize here. Werdermann (1933) and Buxbaum (1952, In 1831, William Jackson Hooker first used 1964) defined hairy floriferous regions of the shoot the term ‘cephalia’ in Latin diagnoses in Curtis’s to be cephalia if they originated from the shoot Botanical Magazine, albeit without definition or apical meristem and to be pseudocephalia if their elaboration, in reference to Melocactus intortus hairy areoles that bore grew hairs later in (Mill.) Urb. However, between 1831 and 1845, he development, i.e. hairs developed on areoles from also used the term to refer to hairy flower buds in far below the shoot apical meristem. Buxbaum other genera that we do not currently consider to (1975) realized that there is a continuum between be cephalium-bearing, such as cephalia and pseudocephalia – How soon after mi - rigidissimus Rose, eyriesii Pfeiff. & totic divisions of the shoot apical meristem do the Otto, glaucescens Britton & Rose, and hairs need to grow? – but never proposed an al - ottonis (Lehm.) N.P. Taylor. ternative.

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c d Figure 3. ( ) schottii . This is a pseudocephalium because the flowering regions are discontinuous and photosynthetic ( 3a,b ). Furthermore, flowers are sometimes produced from mor - phologically juvenile areoles with short stout spines ( 3c ) or from areoles that are intermediate between juvenile and adult morphologies ( 3d ).

Bradleya 34/2016 103 Figure 4. Arrojadoa eriocaulis . Each cephalium Figure 5. Pachycereus marginatus . Flowers arise here is smaller than a single areole in Neoramon - from confluent areoles. But does this make it a dia arequipensis . pseudocephalium? In a paper titled “What is a cephalium?” lateral cephalia are, according to Rauh (1957), Croizat (1942: 169) noted that, “neither Britton & only found in Neoraimondia and a few species of Rose nor Werdermann seem to define the cephal - Neoabbottia , now known as Leptocereus , although ium”. But then Croizat declined to define a cephal - Rauh also toyed with – but ultimately dismissed – ium, instead asserting that there is no way to do the notion that short-shoot cephalia may also so. exist in (Englem.) Britton & Buxbaum (1952) noted that Britton Rose. Rauh’s (1957) other contribution was his & Rose lateral cephalia had lower ribs, lower than Figure 10, appended at the end of his paper with - vegetative ribs, and the vascular cylinder was de - out any discussion. This is a schematic of radial pressed underneath the cephalium, making the sections of shoots showing reduced thickness of cephalium appear sunken. But Buxbaum did not cortex/periderms underlying both apical and lat - consider the sunken nature of flowering areoles to eral cephalia, but no change in cortex/periderm be a defining characteristic of lateral cephalia, thickness underlying both apical and lateral thereby subsuming Pseudoespostoa Backebg. pseudocephalia. within Espostoa . Finally, Mauseth (1989: 1–2; citations omitted) Rauh (1957) largely reiterated the work of oth - provided the following extensive characterization ers, extolling Buxbaum (1952). Rauh’s main con - of cephalia: tributions were introducing the nomenclature “Cephalium-bearing cacti … are species which ‘terminal cephalium’ and ‘lateral cephalium’, the are strongly dimorphic: during their juvenile latter of which he subcategorized as either long- phase, they resemble most other cacti in hav - shoot lateral cephalia (“LangtriebecephaIien”) or ing monopodial globular or columnar bodies short-shoot lateral cephalia (“Kurztrieb - covered by spine clusters (short shoots called cephalien”). Long-shoot lateral cephalia are found areoles). In this stage they are incapable of in Espostoa , , etc. Short-shoot flowering. When plants reach reproductive

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Figure 6. Many relatively long stout spines on ju - venile growth and shorter, sparser spines more flex - ible on later growth from which flowers arise. a. Carnegiea gigantea juvenile growth b. Carnegiea gigantea adult growth c. & d. candelaris Photographs: e James D. Mauseth e. .

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f h Figure 7. Gradual transition from juvenile to adult morphology — gradual transitions are only found in aberrant individuals. a. Melocactus oreas normal discrete transition from vegetative to reproductive tis - sue of cephalium. b. Melocactus oreas highly unusual gradual transition from vegetative to reproductive tissue of cephalium. c. Espostoa (Vatricania ) guentheri normal discrete transition from vegetative to re - productive tissue of cephalium. d. Espostoa (Vatricania ) guentheri highly unusual gradual transition from vegetative to reproductive tissue of cephalium. Photograph: Jürgen Menzel. e. (opposite page) Micranthocereus (Coleocephalocereus ) albicephalus normal discrete transition from vegetative to repro - ductive tissue of pseudocephalium. f. Micranthocereus (Coleocephalocereus ) albicephalus normal discrete transition from vegetative to reproductive tissue of pseudocephalium. g. & h. Micranthocereus (Coleo - cephalocereus ) albicephalus highly unusual gradual transition from vegetative to reproductive tissue of pseudocephalium.

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maturity, new shoot growth is strikingly dif - cephalia. I briefly also describe two other traits ferent: it is narrower than the juvenile portion, that cephalia may have, namely that cephalia are its phyllotaxy is altered, and areoles are so not an inflorescence and that cephalia have spines closely packed that the cephalium epidermis that are shorter and more flexible than spines on is completely hidden by spine and trichomes. photosynthetic parts of the shoot. Due to the dense, opaque covering, the cephal - ium is incapable of photosynthesis, and is Definition 1: Area of dense spines and hairs nourished by translocation up from the per - from which flowers arise sistently photosynthetic juvenile portion of the A cephalium could be defined as a region of ex - shoot. Structural dimorphism is correlated traordinarily dense development of modified with functional dimorphism: the juvenile por - leaves (spines) and unbranched uniseriate tri - tion is autotrophic and nonreproductive but chomes (hairs) from which flowers originate. A the adult portion is autotrophic and reproduc - cephalium is produced by the same shoot apical tive. It is important to emphasize that these meristem as vegetative (photosynthetic) tissue, ei - plants are monpodial and the cephalium is the ther simultaneously with a lateral cephalium or direct continuation of the juvenile shoot; once sequentially with an apical (aka terminal) cephal - the transition occurs, there is no reversion.” ium. Note that in cacti, spines are modified leaves Four possible definitions of cephalium or possibly highly modified bud scales, which are I present four criteria for defining cactus themselves highly modified leaves. A few species, cephalia, highlighting the advantages and disad - such as Pfeiff. and Espostoa vantages of each: (1) an area of denser spination guentheri (Kupper) Buxb. (synonym Vatricania and hairs from which flowers arise, (2) an area of guentheri (Kupper) Backeb.), begin their repro - confluent areoles, (3) an epi-phyllotactic repro - ductive shoots with a lateral cephalium that even - ductive part of the shoot, and (4) a flowering re - tually encircles the entire shoot, thereby gion with extensive periderm development that eventually becoming an apical cephalium. lacks stomata and chlorenchyma. While individu - The above definition captures our usual naïve ally each of these four criteria have problems, to - gestalt of cephalia as woolly and spiny specialised gether they provide a decent definition of flowering structures. This definition seems to

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d Figure 8. Various levels of pseudocephalium for - mation in Micranthocereus . a. Micranthocereus purpureus b. Micranthocereus purpureus c. Micranthocereus polyanthus d. Micranthocereus violaciflorus e. Micranthocereus auri-azureus e f.(opposite page) Micranthocereus auri-azureus

108 Bradleya 34/2016 f nicely encompass cap-like apical cephalia, ring- well below the stem apex (Figure 1), with these like apical cephalia in which reproductive and long dense tufts of hairs being discontinuous, i.e. vegetative growth alternate from the shoot apical in not contiguous thatches vertically arranged on meristem (and in which cephalia are definitely not a shoot (Taylor & Zappi, 2004). By contrast, Pi - terminal), and lateral cephalia. Furthermore, at losocereus gounellei (F.A.C. Weber ex K. Schum.) first blush, this definition appears to apply across Byles & G.D. Rowley subsp. zehntneri (Britton & all clades in which cephalia supposedly occur, Rose) Byles & G.D. Rowley (synonym: P. braunii namely the -- Esteves) grows long hairs from the shoot apical (BCT) clade in the Core Cactoideae II and the meristem (Figure 2), implying that, with this def - Pachycereeae plus Leptocereus and Neoraimondia inition, Pilosocereus gounellei subsp. zehntneri in the Core Cactoideae I (sensu Hernández- has no less of a cephalium than found in Cephalo - Hernández et al ., 2011). cereus senilis and C. columna-trajani (Karw. Ex This definition implies that cephalia are part Pfeiff.) P.V. Heath (see below regarding the next of primary growth, arising directly from the shoot criterion/definition of a cephalium for discussion apical meristem (Buxbaum, 1952, 1964). This of these two Cephalocereus species, which, in fact, would also apply to some species of Pilosocereus , have pseudocephalia). It seems plausible that the which are usually acknowledged to have either a genus Pilosocereus has some taxa with true pseudocephalium or neither a cephalium nor cephalia or pseudocephalia because the closest pseudocephalium. A pseudocephalium is gener - relatives of Pilosocereus are probably Arrojadoa , ally considered to be composed of dense tufts of Melocactus , and Discocactus (Hernández-Hernán - hairs (uniseriate trichomes) that grow from flow - dez et al ., 2011). ering areoles, but often long after areoles were Members of both the Core Cactoideae I and II formed from the shoot apical meristem. Areoles in present potential problems for this definition of a cacti are short shoots, like spurs on an apple or cephalium. In the Core Cactoideae I, Pachycereus gingko , hence can have secondary growth, schottii (Englem.) D.R. Hunt (synonym Lopho - often called ‘indeterminate growth’ of areoles cereus schottii ) would seem to form a cephalium (Gibson & Nobel, 1986; Taylor, 1991; Gorelick & by this and several other definitions. Its flowers Machado, 2012). For example, Pilosocereus den - are usually only produced from larger areoles with siareolatus F. Ritter forms dense tufts of hairs longer denser spination per areole than on the

Bradleya 34/2016 109 vegetative parts of the shoot with their short stout ingly, Rauh (1957) speculated that Pachycereus spines (Figures 3a,b). However, in this species, (Lophocereus ) schottii flowering areoles were each flowers are occasionally produced on areoles with individual cephalia. But to be somewhat of a juvenile morphology, i.e. on those areoles with just devil’s advocate, this would make every areole on a few spines that are short and stout (Figures every cactus a cephalium, which therefore be - 3c,d). Furthermore, juvenile portions of shoots of comes a vacuous definition. In some ways, this Pachycereus (Lophocereus ) schottii grow very was Croizat’s (1942) point. So, instead, the area slowly, whereas a huge increase in shoot growth over which dense development of spines and tri - rate occurs once cephalium/pseudocephalium de - chomes develop, with lots of underlying cork and velopment and flowering begin (Parker, 1988; no stomata, would have to be larger than a single Martorell et al ., 2006; Gorelick, 2016). A different areole to constitute a cephalium, which bring us to problem for this definition may occur in Core Cac - a second definition of a cephalium. toideae II with some species of Discocactus . While the vegetative portions of all Discocactus species Definition 2: Contiguous/confluent axillary are produced by the same shoot apical meristem buds/areoles as the cephalium, the possibility exists that a new A cephalium could be defined as a large flow - meristem seems to grow at the base of the cephal - ering area comprised of contiguous or confluent ium that produces new photosynthetic non-repro - axillary buds/areoles. This area could be large in ductive tissue and typical juvenile spines and a vertical direction (lateral cephalia), horizontal areoles, even though these tissue may end up direction (ring-like apical cephalia), or both direc - being produced after the cephalium (Gorelick, tions (cap-like apical cephalia). 2014c). Thus, in Discocactus heptacanthus Britton This second definition of cephalia means that & Rose subsp. catingicola (Buining & Brederoo) individual flowering areoles would not be consid - N.P. Taylor & Zappi, D. placentiformis K. Schum., ered small cephalia. The problem is that several and D. bahiensis Britton & Rose subsp. gracilis plants that we typically think of as possessing P.J. Braun & Esteves, the cephalium and parts of cephalia, would no longer be considered cephal - the vegetative body on which it sits may be pro - ium-bearing. In the Pachycereeae, the so-called duced by different meristems. However, we may cephalia of Cephalocereus senilis , C. columna-tra - be able to dismiss these concerns with Discocac - jani , and Pachycereus militaris (Audot) D.R. Hunt tus if vegetative shoot development is extremely (synonym Backebergia militaris (Audot) Sánchez- slow, at least for photosynthetic parts that de - Mej.) are formed of discrete (non-contiguous) are - velop immediately before cephalium development oles (Vázquez-Sánchez et al ., 2005, 2007, 2016). (see Gorelick, 2014c for what Jim Mauseth termed Their flowering areoles are close together, but still the ‘Mount St Helens effect’). In many ways this is with some space between them. These flowering very similar to Pachycereus (Lophocereus ) schottii areoles each reside on their own tubercle, with with very slow shoot apical meristem growth be - stomata between areoles. I do not know whether fore cephalium/pseudocephalium development the flowering areoles of the ring-like cephalium of and with a discrete transition to rapid growth Cephalocereus apicicephalium E.Y. Dawson are once the cephalium/pseudocephalium has formed. contiguous/confluent, but suspect that they are Pachycereus (Lophocereus ) schottii also high - not, but instead are on discrete areoles on discrete lights the delicate matter of how dense and con - tubercles with stomata between them like their tinuous the development of spines and hairs must congeners. be. Each areole is a dense mass of trichomes and How large of a spatial area does something modified leaves. Like cephalia, at least per the need to be to be considered a cephalium? The ring- next definition, areoles are shoots in which every like cephalia of some species of Arrojadoa , such as epidermal cell produces a leaf or trichome, with - A. eriocaulis Buining & Brederoo (Figure 4), are out stomata and without chlorenchyma. Building of much smaller area than the area of a single Ne - on earlier observations by Rauh (1957), Mauseth oraimondia arequipensis flowering areole/cephal - & Kiesling (1997) hinted that individual areoles ium. But for this definition of cephalium to be can be cephalia in Neoraimondia roseiflora effective, we need some measure of spatial extent. Backebg. (synonym: Neoraimondia arequipensis And we cannot simply count number of flowering (Meyen) Backeb.) and Neocardenesia herzogiana areoles if areoles are confluent. Backebg. (synonym: Neoraimondia herzogiana Another problem with this second definition is (Backebg.) Buxb. & Krainz) because their flower - that many members of the Pachycereeae that we ing areoles have huge amounts of cork, no stom - usually do not think of as cephalium-bearing ata, indeterminate growth for several decades, would qualify as cephalium-bearing. Pachycereus and many flowers. And maybe not too surpris - marginatus (DC.) Britton & Rose has confluent

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c Figure 9. Epi-phyllotaxy — Cephalium appears to drift across the underlying parallel structure of photosynthetic ribs in 9a–9c. Rib development is invisible in cross-section 9d. a. Espostoa melanostele b. dybowskii — see the rightmost shoot c. Coleocephalocereus purpureus d. Coleocephalocereus purpureus cross-section e. Micranthocereus streckeri — unlike the other species in this figure, this species has a pseudo - e cephalium

Bradleya 34/2016 111 flowering areoles, implying that we should con - what appear to be pseudocephalia with virtually sider each rib of mature shoots to be a lateral contiguous areoles (Gorelick, 2013), albeit on sep - cephalium (Figure 5). Pachycereus (Lophocereus ) arate tubercles, much as in Cephalocereus schottii is really little different from Cephalo - columna-trajani and C. senilis , while other species cereus senilis , with longer denser modified leaves never form pseudocephalia (e.g. M. auri-azureus on larger areoles in flowering portions of a shoot. Buining & Brederoo), while with other species it is Actually all species of Pachycereus Britton & Rose uncertain whether the flowering-bearing areoles sensu lato, including Carnegiea gigantea Britton are contiguous (e.g. M. violaciflorus Buining, M. & Rose, show such vestiges of cephalium produc - polyanthus (Werderm.) Backeb.) (Figures 8a–d). tion. Flowering portions of C. gigantea shoots markedly differ from non-flowering portions in Definition 3: Epi-phyllotactic reproductive having fewer spines, almost confluent areoles, and structure larger areoles (Figures 6a,b). Lateral cephalia seem to be developmentally The differentiation between relatively spine - ‘painted’ over existing phyllotaxy. In some speci - less flowering portions of the shoot and relatively mens of Espostoa melanostele (Vaupel) Borg veg - spiny portions of the non-flowering portions of the etative ribs remain vertical while the lateral shoot in the Pachycereeae (Core Cactoideae I) is cephalium seems to helically drift across those also seen in (Meyen) Brit - ribs (Figure 9a). We see the same pattern in some ton & Rose and Stetsonia coryne (Salm-Dyck) Brit - specimens of Espostoopsis dybowskii (Rol.-Goss.) ton & Rose (Core Cactoideae II) (Figures 6c-e). Buxb., in which the cephalium seems to wander Are we ready to say that Pachycereus spp., across and grow on top of ribs, something that Carnegiea gigantea (which is very closely related could be termed ‘epi-phyllotactic’ (Figure 9b). The to Pachycereus ), Browningia candelaris , and Stet - epi-phyllotactic nature of lateral cephalia is par - sonia coryne all have cephalia? ticularly evident in the globose species of Coleo - A few species of Core Cactoideae II shed some cephalocereus Backeb. (i.e. subgenus Buiningia light on whether confluent/contiguous flowering (Buxb.) P.J. Braun), such as C. aureus F. Ritter areoles make for a good criterion of whether a true and C. purpureus (Buining & Brederoo) F. Ritter, cephalium exists. However, while almost all where the vegetative rib architecture disappears cephalium formation seems virtually instanta - underneath the contiguous white cephalium hair neous, a few specimens of a few species show a and spines (Figures 9c). The ring-like terminal more gradual transition from vegetative to repro - cephalia of Arrojadoa spp. and ductive structures. This is more than a few extra leucostele are certainly not associated with the ribs being added to the vegetative growth imme - vertical vegetative ribs. By contrast, Cephalo - diately before cephalium production, but up to cereus senilis and C. columna-trajani reproductive 15cm of transitional growth, where areoles are structures are on specific ribs and flowering are - much bigger but not quite confluent nor produc - oles are not contiguous. Having flowering are - ing spines typical of cephalia. Figures 7a–h show oles/tubercles ascribed to specific ribs makes these this transitional behavior in cephalium develop - two species of Cephalocereus not epi-phyllotactic, ment in Melocactus oreas Miq. and Espostoa (Va - hence they possess pseudocephalia. tricania ) guentheri , and, to a lesser extent, in Epi-phyllotactic cephalia, i.e. cephalia existing pseudocephalia of Micranthocereus albicephalus as being on top of vegetative phyllotaxy, is also ev - (Buining & Brederoo) F. Ritter (synonym: Coleo - ident in other taxa. In those Discocactus species cephalocereus albicephalus (Buining & Brederoo) that grow new photosynthetic tissues after cephal - F.H. Brandt). In the Core Cactoideae I, Pachyc - ium formation – which may be all species in the ereus (Backebergia ) militaris also shows this tran - genus given the wide flattened eponymous disc- sition between vegetative and reproductive like nature of their vegetative shoot – no new ribs growth, although the transitional area is usually grow from this novel meristem, but instead the no longer than 8cm in vertical extent (Mauseth et original vegetative ribs continue to lengthen, i.e. al ., 2005; Vázquez-Sánchez et al ., 2016). phyllotaxy is maintained after the cephalium Not only is cephalium/pseudocephalium de - grows (admitting that photosynthetic portions of velopment in Micranthocereus (Coleocephalo - shoots in some Discocactus species have helical cereus ) albicephalus sometimes seemingly phyllotaxy in lieu of ribs). In Micranthocereus gradual, but there is a large variation in whether streckeri , the width of the cephalium/pseudo - cephalia or pseudocephalia are formed in the rest cephalium both increases and decreases as a shoot of the genus Micranthocereus Backeb. Some grows, encompassing more or fewer ribs (Figure species, such as M. purpureus (Gürke) F. Ritter 9e). Note that only with this and the first defini - and M. streckeri Van Heek & Van Criek. form tion of cephalia (epi-phyllotactic and dense spina -

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c Figure 10. Ring-like cephalia (or possibly pseudo - cephalia). a. Arrojadoa rhodantha Britton & Rose b. Arrojadoa rhodantha subsp. aureispina (Buin - ing & Brederoo) P.J. Braun & Esteves c. Arrojadoa marylaniae Soares Filho & M. Machado d. Stephanocereus leucostele d

Bradleya 34/2016 113 tion and uniseriate hairs in flowering regions) is confluent flowering areoles than on juvenile por - M. streckeri considered to have a cephalium, tions of its shoot with discrete areoles or between whereas with the other two definitions (confluent ribs on reproductive parts of the shoot and areoles and increased cork/decreased thereby qualify as a cephalium? By this definition, chlorenchyma) this species would have a pseudo - areoles of both Neoraimondia species and some cephalium. Leptocereus species each constitute a cephalium Figure 9a is a cultivated rooted top cutting of (Rauh, 1957; Mauseth & Ross, 1988; Mauseth & Espostoa melanostele , in which the long lower por - Kiesling, 1997). It remains to be seen whether tion of the cephalium grew before the cutting was deeming Pachycereus marginatus , some Lepto - taken. The cutting was planted with a different cereus species, and Neoraimondia cephalium- compass orientation and subsequently underwent bearing is a problem. about a year of growth without a cephalium, al - Mauseth (1999) looked at Espostoa lanata , E. beit with a depression in the shoot. Eventually a mirabilis , E. ritteri , Pseudoespostoa (Espostoa ) new lateral cephalium formed. Despite this being melanostele , Thrixanthocereus (Espostoa ) senilis , an artificial situation, notice how growth of the and Vatricania (Espostoa ) guentheri . “Only sam - lateral cephalium stopped and then restarted, ples of E. mirabilis , T. senilis, and V. guentheri which is really no different from what happens in had patches of bark in the cephalium” (Mauseth, Arrojadoa and Stephanocereus leucostele (Figures 1999: 36). This contradicts what I found in Espos - 10a,b) with their ring-like apical cephalia. toa (Pseudoespostoa ) melanostele — where not only did I see bark underlying the cephalium, but Definition 4: Flowering region with in - felt the resistance of the scalpel as I was cutting creased cork/periderm and decreased stom - through that cork – and seems to contradict what ata and chlorenchyma Paul Hoxey depicted for Espostoa (Thrixantho - A cephalium could be defined as a flowering cereus ) senilis that does not show any obvious region with reduced number of stomata, reduced bark formation (Charles, 2015). It might be worth chlorenchyma, and increased periderm (cork) for - taking another look at Espostoa sensu lato for mation that is produced from proximal epidermal bark underneath cephalia. layers. “The development of periderm in each are - Defining cephalia by periderm development ole after flowering and fruiting is a distinctive and after flowering and fruiting seems too restrictive. defining feature of lateral and apical cephalia” While this developmental chronology applies to (Vázquez-Sánchez et al ., 2016: 245). Periderm pro - Pachycereus (Backebergia ) militaris , in most duction from epidermis, hypodermis, or cortex in - cephalium-bearing members of the Core Cac - variably results in the cephalium having a much toideae II, such as Melocactus , Discocactus , Es - narrower or non-existent cortex than does the veg - postoa , and Coleocephalocereus , periderms seem etative portion of the shoot (Rauh, 1957). For lat - to form before flowering. Plus most arborescent eral cephalia, the narrower cortex results in cacti eventually form periderms on older portions titling of the shoot towards the cephalium and the of shoots, sometimes long after flower and fruit shoot supposedly can no longer safely have axil - production, but clearly without a cephalium, e.g. lary branching above the point on the shoot where Brasiliopuntia brasiliensis (Willd) A. Berger. the lateral cephalium started growing (Valverde Because cephalia are generally not photosyn - et al ., 2007), although there are exceptions with thetic, it is often believed that cephalia could do relatively common axillary branching of cephalia without stomata, replacing cephalium epidermal in Coleocephalocereus Backeb. subgenus Coleo - tissue with periderm. This pattern is seen in cephalocereus and Coleocephalocereus subgenus cephalia of the Core Cactoideae II and some Simplex N.P. Taylor (Gorelick & Machado, 2012), species of Leptocereus and both species of Neorai - as well axillary branching of a cephalium in at mondia in the Core Cactoideae I. But, given that least one cultivated specimen of Espostoa lanata areoles in cephalia/pseudocephalia of the Pachyc - Britton & Rose (Gorelick, 2014a). ereeae in the Core Cactoideae I are not usually This definition of a cephalium suffers in a way contiguous or confluent, it seems that stomata we have seen above: How reduced do the number and chlorenchyma are found between tubercles in of stomata and amount of chlorenchyma need to their cephalia/pseudocephalia. But even in the be and how increased does the underlying layer of Pachycereeae, photosynthesis is highly reduced in periderm need to be? When does this definition be - cephalia/pseudocephalia, sometimes with addi - come sufficiently attenuated that most or all tional cork formed under the cephalia/pseudo - Pachycereus species are considered cephalium- cephalia, at least relative to nearby bearing? For instance, does Pachycereus mar - non-reproductive epidermal tissue. ginatus have fewer stomata underlying its

114 Bradleya 34/2016 Figure 11.(below & top right) Narrower cortex with more periderm in apical and lateral cephalia. Cross-sections. a. Discocactus zehntneri var. araneispinus (Buin - ing & Brederoo ex J. Theun.) P.J. Braun b. Espostoa melanostele c. Coleocephalocereus purpureus

b

a

a

c Figure 12.(right) Two diametrically opposite cephalia/pseudocephalia per shoot. a. Espostoa melanostele b. Micranthocereus streckeri b

Bradleya 34/2016 115 are also slightly narrower than the juvenile vege - tative parts of their shoots, with slightly narrower cortex, although this is not obvious without sec - tioning because the long spines of the cephal - ium/pseudocephalium hide the slightly narrower underlying reproductive portion of the shoot (Mauseth et al ., 2005). I have not sectioned the ring-like apical cephalia of Arrojadoa spp. or Stephanocereus leu - costele (Gürke) A. Berger to look for increased periderm production and thinner cortex underly - ing their flowering aroeoles. However, Rauh (1957) shows no such changes in shoot architec - ture in his Figure 10.II. Nonetheless, it would be worth sectioning Arrojadoa spp. and Stephano- cereus leucostele because I do not agree with Rauh’s labelling of Figure 10.IV, in which he shows narrower cortex/periderm underlying cephalia of Espostoa , Facheiroa Britton & Rose, Cephalocereus , and Thrixanthocereus Backeb., which is definitely not the case for Cephalocereus senilis and Thrixanthocereus senilis F. Ritter, also known as Espostoa senilis (F. Ritter) N.P. Taylor (see discussion below). This is an odd error for Rauh (1957) to have made given that he recog - nized that Thrixanthocereus did not have sunken cephalia/pseudocephalia. Cork layers underlie lateral cephalia. Lateral cephalia, at least in the Core Cactoideae II, ap - pear sunken because the cortex underlying the cephalium is narrower than under photosynthetic non-reproductive portions of the shoot. This causes the shoot apical meristem to tilt in the di - rection of the lateral cephalium. Thus, juvenile shoots of many species of lateral cephalium-bear - Figure 13. Coleocephalocereus goebelianus (Vau - ing cacti are erect and extremely vertical, but once pel) Buining. Phyllotaxy changes once the cephal - they start growing a cephalium, the apex of the ium is formed. Below the cephalium, vegetative shoot tilts towards the cephalium growing side of ribs are parallel and virtually unbranched. Once the shoot. Photos of cultivated specimens of the cephalium is formed, vegetative ribs branch Cephalocleistocactus chrysocephalus F. Ritter and are no longer parallel. (synonym chrysocephalus (F. Ritter) Mottram) show this exact behaviour: vertical ju - Narrower cortex is evident in both apical and venile shoots that start growing with a significant lateral cephalia (Figures 12a–c). With cap-like tilt as soon as the lateral cephalium or pseudo - apical cephalia in Melocactus and Discocactus , cephalium develops, with its long flexible spines, cephalia are noticeably narrower than the photo - and the plant starts to flower (Hunt et al ., 2006; synthetic parts of the shoot below. Melocactus veg - Lodé, 2015). etative parts do not grow again once a cephalium A few species of lateral cephalium-bearing and starts nor do their cephalia get any wider due to pseudocephalium-bearing plants do not have secondary growth. Discocactus shoot radial sec - tilted shoot apexes. Two ways have evolved to tions also show this narrower cortex in cephalia avoid apical tilting of lateral cephalia: (1) having (Buining, 1980; Gorelick, 2014c). However, the the cephalium widen around the circumference of vegetative parts of some (all?) Discocactus grow the shoot so that it eventually becomes a cap-like wider after cephalium production, so the cephal - apical cephalium (e.g. Cephalocereus senilis , C. ium looks disproportionally narrow compared columna-trajani , and some shoots of Espostoa with Melocactus (Gorelick, 2014). Pachycereus (Vatricania ) guentheri ) and (2) growing a second (Backebergia ) militaris cephalia/pseudocephalia lateral cephalium on the diametrically opposite

116 Bradleya 34/2016 a

c

b

e Figure 14. Cephalia with longer spines than on juvenile photosynthetic portions of shoot. a. Arrojadoa penicillata Britton & Rose b. & c. Siccobaccatus dolichospermaticus (Buin - ing & Brederoo) P.J. Braun & Esteves [synonym Micranthocereus dolichospermaticus (Buining & Brederoo) F. Ritter] d. Discocactus horstii Buining & Brederoo d e. Discocactus zehntneri subsp. boomianus

Bradleya 34/2016 117 Figure 15. Stephanocereus luetzelbergii reproductive portions of shoots are narrower and have more hair per areole. side of the shoot, e.g. some shoots in Espostoa spp. grow (Gorelick, 2014a). The one glaring difference and Micranthocereus streckeri (Figures 12a,b) in architecture once a lateral cephalium starts (Gorelick, 2013). growing is in the arrangement of ribs. Prior to for - While the apex of lateral cephalium-bearing mation of a lateral cephalium, ribs are vertical shoots often tilt, ultimately all shoots manage to and evenly spaced and of constant number. As grow vertically, except possibly in Espostoa soon as the lateral cephalium starts growing, how - cremnophila Hoxey which we still know too little ever, the shoot adds photosynthetic ribs and these about (Hoxey, 2014). The world is not filled with morphologically juvenile ribs – although they are lateral cephalium-bearing shoots that form spirals chronologically and developmentally of the same nor is the ground littered with cephalia that have age as the cephalium – are almost never vertical broken off because of the stress on tilted cephal - (Figure 13). This cobbled together architecture ium-bearing shoot. Instead, shoots bearing lateral probably keeps the shoot growing vertically, prob - cephalia manage to straighten themselves as they ably also with some help from extra tension wood.

118 Bradleya 34/2016 [I have, thus far, alluded to all genera of cephalia, includes Pachycereus (Backebergia ) mil - cephalium- and pseudocephalium-bearing cacti, itaris , all Arrojadoa species, most Micrantho- except for the enigmatic Venezuelan endemic cereus species (at least those with pseudocephalia, Cereus mortensenii , (Croizat) D.R. Hunt & N.P. including those segregated into Siccobaccatus ), Taylor, which has sometimes been considered Pi - many Espostoa species, Espostoopsis dybowskii , losocereus mortensenii (Croizat) Backeb. Mauseth Facheiroa ulei (Gürke) Werderm., Discocactus (1999) illustrates a cross section through a horstii Buining & Brederoo, and D. zehntneri Brit - pseudocephalium-bearing shoot of C. mortensenii , ton & Rose subsp. boomianus (Buining & showing discrete hair-bearing ribs that are lower Brederoo) P.J. Braun, all of which have longer and than the non-hairy ribs and showing more robust spines in their cephalia or pseudo - chlorenchyma underlying the pseudocephalium. cephalia (Figures 14a–d). But I know too little about this taxon to say any more.] Integrated approach to defining cephalium and pseudocephalium Traits often associated with cephalia I propose defining a cephalium as part of a cac - Cephalia are usually thought of as possessing tus shoot arising directly from the shoot apical two other characteristics – not being inflores - meristem, with the cephalium composed of con - cences and having shorter spines than on juvenile fluent areoles from which flowers originate, bear - photosynthetic parts – neither of which should be ing copious spines and trichomes, and underlain thought of as definitions of cephalia. by a thick periderm in lieu of an even thicker cor - Cephalia are specialized perennial reproduc - tex. Hairs on flowering parts are longer than tive structures that live for as long as the shoot on those on non-flowering parts. Cephalia lack which they grow. Cephalia are thus not inflores - chlorenchyma and stomata. Cephalia are epi- cences because inflorescences are ephemeral. This phyllotactic with very different phyllotaxy from characterization of cephalia seems to apply to all photosynthetic portions of the shoot. All of these taxa, except Pachycereus (Backebergia ) militaris , criteria must be met for something to be a cephal - in which cap-like apical cephalia/pseudocephalia ium. and a small portion of the vegetative tissue just I propose defining a pseudocephalium as a dis - below cephalia/pseudocephalia abscise after sev - crete portion of a cactus shoot from which flowers eral years of reproductive growth (Mauseth et al ., originate, but a portion of the shoot that is miss - 2005). The area around a mature plant is often ing one or more – but not all – of the above crite - filled with decomposing abscised cephalia/pseudo - ria for being a cephalium. cephalia, which die and never seem to form ad - As an aside, I do not like the prefix ‘pseudo-’ ventitious roots. These cephalia/pseudocephalia in pseudocephalium because of its pejorative con - can grow to about a meter tall, but all eventually notations, but am reluctant to create new jargon. are abscised. After abscission, one or more new There is absolutely no evidence that plants with shoots grow from the remaining vegetative part of pseudocephalia are any less fit than closely re - the plant, often just below the abscission layer. lated taxa with cephalia. I wonder whether this is Thus, while Pachycereus (Backebergia ) militaris why Berger (1926) replaced the term ‘pseudo - cephalia/pseudocephalia are perennial structures, cephalium’ with the more descriptive ‘wollzonen’, they are also ephemeral compared with all other i.e. ‘wool-zone’. cephalia considered herein. Therefore Pachyc - A few species and a few individuals of other ereus ( Backebergia ) militaris reproductive parts species show a gradual transition from juvenile could be considered long-lived inflorescences, not photosynthetic parenchymous portions of shoots true cephalia. Neoraimondia cephalia can also be with stomata to flowering non-photosynthetic considered inflorescences if they later de-differen - periderm-laden portions of shoots without func - tiate back in to vegetative long-shoots, which they tional stomata, such as Pachycereus (Backebergia ) sometimes do in N. arequipensis . militaris and some specimens of Espostoa (Vatri - The second trait often associated with cephalia cania ) guentheri . As with many aspects of biology, is that their spines are more slender and flexible reproductive parts of these plants do not fall than spines in vegetative parts of the shoot. This neatly into the cephalium category, but only be - seems to be true in all cephalium-bearing taxa. cause their developmental transition from juve - However, Mauseth (2006) says that spines on nile to adult stages is not instantaneous. cephalia are shorter than spines on vegetative However, analogously, day and night are still portions of the shoot, but this seems to not be uni - well-defined concepts despite the existence of versal. A diverse assemblage of counter-examples, dusk and dawn. some of which contain cephalia and some pseudo -

Bradleya 34/2016 119 Figure 16. Facheiroa cephaliomelana var. estevesii

Definitions are peculiar things in science. Hy - opment of cephalia or pseudocephalia between potheses can be false or not. Theories can be right species. Stephanocereus A. Berger contains one or wrong. Theories can even be falsifiable or not, species, S. leucostele , with ring-like cephalia or although we usually strive for the former in order pseudocephalium from which flowers originate, to consider something scientific. Data may be typ - similar to that found in Arrojadoa , and another ical or anomalous. Facts may or not be context species, S. luetzelbergii (Vaupel) N.P. Taylor & sensitive. But definitions are rather arbitrary, Eggli, that has only a few extra spines or tri - with no decent gauge except for their utility and chomes in flowering areoles, but has modified consistency (Wagner, 2010; Gorelick, 2011a,b). shoot shape in flowering regions, much like Melo - There is even more than utility to make some - cactus (Figure 15). Given that the flowering por - thing a decent definition. Definitions should re - tions of S. luetzelbergii shoots are photosynthetic flect commonsense, which will reflect current all the way around the circumference, it is doubt - paradigms. And, to quote the famous title of Theo - ful that there are periderms formed underneath dosius Dobzhansky’s (1973) popular article, their flowering areoles. Micranthocereus contains “Nothing in biology makes sense except in the species with and without modified flowering are - light of evolution.” To this end, it would also be oles, i.e. with and without pseudocephalia. nice if a definition of cephalium reflected phy - Facheiroa contains species without any specially logeny, although this could be a pipe dream given differentiated flowering areoles or periderms un - the flux in accepted (i.e. inferred) evolutionary re - derlying those areoles, such as F. squamosa lationships (Gorelick, 2014b). It would be nice to (Gürke) P.J. Braun & Esteves, to species that know whether a trait such as presence of a cephal - have copious long spines and hairs on flowering ium is a homology versus an analogy, but this may parts, such as F. ulei (Gürke) Werderm. and F. be too much to ask at this time. cephaliomelana Buining & Brederoo var. estevesii Several cactus genera have a range of devel - (P.J. Braun) N.P. Taylor & Zappi (Figure 16). I

120 Bradleya 34/2016 suspect that these latter two taxa have pseudo - cephalia because their areoles do not seem to be confluent and seem to be phyllotactic, not epi- phyllotactic. The most poignant and, in some ways, the most contentious genus regarding cephalium and pseudocephalium development is Espostoa sensu lato, including Vatricania Backeb. and Thrixan - thocereus Backeb. Espostoa sensu stricto contain what seem to have bona fide lateral cephalia with contiguous areoles and massive amounts of un - derlying periderm formation. The monotypic genus or subgenus (or nothogenus?) Vatricania a also has contiguous flowering areoles, massive amounts of underlying periderm, but what starts as a lateral cephalium eventually encircles the en - tire shoot. Furthermore the transition from juve - nile to reproductive morphology can be more gradual in Vatricania than in Espostoa sensu stricto. While there may be problems with cactus phylogenies based solely on chloroplast DNA – be - cause chloroplast genomes in cacti are not strictly maternally inherited (Corriveau & Coleman, 1988; Gorelick, 2014b) – one such phylogeny (Schlumpberger & Renner, 2012) places Vatrica - nia in the Cleistocactus sensu stricto clade and Es - b postoa sensu stricto in the not very closely related Figure 17. Pseudocephalia have non-confluent clade. Should chloroplast DNA be suf - areoles and are photosynthetic. ficient to segregate Vatricania from the rest of Es - a. Espostoa (Thrixanthocereus ) senilis postoa , especially in light of (1) Vatricania and Photograph: Paul Hoxey Espostoa sensu stricto having morphologically b. Micranthocereus streckeri similar cephalia and (2) it being unknown whether chloroplasts are maternally inherited in stomata, a thin cortex, many highly lignified (and these taxa? Thrixanthocereus , which contains be - suberized?) cells underlying the epidermis that tween two and four species, seems to not have were probably mostly from epidermally-derived true cephalia, but instead pseudocephalia. Espos - periderms. While standard reference works (e.g. toa senilis (F. Ritter) N.P. Taylor (synonym: Thrix - Anderson, 2001; Hunt et al ., 2006) mention these anthocereus senilis F. Ritter) has long hairs in its cephalia, they do not illustrate them. Clearly Lep - flowering areoles, but flowering areoles are not tocereus cephalia are apical, on the ends of long confluent and do not have any substantial under - shoots, growing from the same shoot apical meris - lying cork, and have a photosynthetic epidermis tem as the photosynthetic juvenile portion of the (Figure 17a) (Charles, 2015). Reproductive struc - shoot. Yet for some inexplicable reason, Rauh tures of T. senilis look almost identical to pseudo - (1957) unambiguously referred to these as short- cephalia in Micranthocereus streckeri (compare shoot cephalia (“Kurztriebcephalien”). Leptocereus Figures 17a & b) (Gorelick, 2013). Should lack of and Neoraimodia , the two taxa Rauh (1957) de - a true cephalium be sufficient to segregate T. se - scribed as having short-shoot lateral cephalia, are nilis into a separate genus from the rest of Espos - relatively ancestral members of the Core Cac - toa (cf. Buxbaum, 1959)? toideae I (Hernández-Hernández et al ., 2011) and Leptocereus provides a challenging case. Only possibly the only two genera of the Core Cac - three of the roughly fifteen species contain toideae I with true cephalia; Cephalocereus and cephalia ( L. grantianus Britton, L. paniculatus Backebergia have pseudocephalia. Leptocereus (Lam.) D.R. Hunt, L. quadricostatus Britton & pretty much eliminates the possibility that the Rose). Mauseth & Ross (1988) showed that L. term ‘cephalium’ will have any phylogenetic basis. quadricostatus only flowers from true apical My integrated definition of ‘cephalium’ is in cephalia that are small, old and slow growing, some ways in accord with and in some ways di - with no chlorenchyma, probably with every epi - verges with the definition given by Mauseth dermal cell producing trichomes or spines and no (1989). The similarities are that we both agree

Bradleya 34/2016 121 in Arrojadoa , Stephanocereus leucostele , and Cephalocereus apicicephalium (Figure 10). Rever - sions from reproductive to non-reproductive growth occur frequently in Neoraimondia rosei - flora (synonym: N. arequipensis ), where long-lived flowering short-shoot cephalia often eventually elongate to form photosynthetic long shoots (Mauseth & Kiesling, 1997). While much less fre - quent, in cultivation, specimens of Espostoa and Melocactus are known to revert from cephalium to photosynthetic non-reproductive tissues and sometimes back again to cephalium (Figures 9a & 18). How do these new integrated definitions of cephalium and pseudocephalium bode for some of the other more enigmatic taxa? While Stephanocereus luetzelbergii has a silhouette re - sembling that of a large Melocactus , such as M. levitestaus Buining & Brederoo, Stephanocereus luetzelbergii has a pseudocephalium by virtue of the more copious hairs on flowering areoles (Fig - ure 15). These flowering areoles arise just below the shoot apical meristem, are not confluent, and are on photosynthetic ribs, further indicating that these are pseudocephalia. Those taxa that have fewer spines and trichomes on flowering areoles than on juvenile areoles, such as with Browningia Figure 18. Buining & Bred- candelaris , Stetsonia coryne , and Carnegiea gi - eroo showing reversion from a cephalium to vege - gantea , have neither pseudocephalia nor cephalia tative growth. Note the gradual transition during by the definitions herein. The famed San Pedro this reversion, as the plant still has floral remains cactus, Trichocereus pachanoi Britton & Rose while the cephalium-bearing part of the shoot (synonym: (Britton & Rose) grew wider. However, flowering stopped once H. Friedrich & G.D. Rowley), is very similar, hav - phyllotaxy straightened and chlorenchyma re - ing fairly long spines as a seedling, but becoming turned. This plant is in cultivation at the Hunt - increasingly spineless as it matures. More prob - ington Botanical Garden. lematic are the reproductive portions of shoots of Photograph: Matt Ritter. some Pachycereeae, such as Pachycereus mar - that cephalia arise from strongly dimorphic ginatus , Pachycereus (Lophocereus ) schottii , and monopodial growth in which photosynthetic por - Pachycereus (Backebergia ) militaris . Pachycereus tions of the shoot are not reproductive, while re - marginatus has confluent flowering areoles, de - productive parts of the shoot are not spite no hairs and no thick periderms, hence by photosynthetic. Mauseth (1989) claimed that the definition herein, this species has a pseudo - cephalia show a change in phyllotaxy, whereas I cephalium. Pachycereus (Lophocereus ) schottii claim something slightly different, namely that seems to have a pseudocephalium, unless looking cephalium phyllotaxy is on top of original phyl - at specimens that flower from juvenile areoles lotaxy of the juvenile shoot. I place much greater with only a few short stout spines per areole (Fig - emphasis on periderms underlying cephalia being ure 3c), so I am not sure how to classify their re - a defining trait of cephalia than Mauseth (1989), productive parts. Pachycereus (Backebergia ) with such periderm formation resulting in the militaris has periderm formation that occurs sunken look of cephalia and their lack of probably a year after new growth from the shoot chlorenchyma. I have shown that transition from apical meristem, flowering areoles that are never vegetative and reproductive growth is not always quite confluent, and reproductive structures that abrupt, although usually is. But the most impor - are an inflorescence. This species therefore has tant difference between our definitions is that I pseudocephalia. allow for reversions between reproductive and Ultimately, I hope the modified definitions of non-reproductive growth. Such reversions are ob - cephalium and pseudocephalium herein will pro - vious with ring-like cephalia and pseudocephalia vide a convenient way to answer questions in evo -

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