Why Vegetative Propagation of Leaf Cuttings Is Possible in Succulent And

Haseltonia 20: 51–57. 2015 51 WHY VEGETATIVE PROPAGATION OF LEAF CUTTINGS IS POSSIBLE IN SUCCULENT AND SEMI-SUCCULENT PLANTS ROOT GORELICK Department of Biology, School of Mathematics & Statistics, and Institute of Interdisciplinary Studies, Carleton University, 1125 Raven Road, Ottawa, Ontario, K1S 5B6 Canada email: [email protected] Abstract: Simple leaves are usually conceptualized as terminal organs that cannot develop new roots or shoots. However plants with succulent or semi-succulent leaves in many unrelated families defy this rule. They do so by differentiating leaf parenchyma – which are probably often phloem parenchyma or companion cells, the two of which are often indistinguishable – into new root and shoot apical meristems. Succulent and semi-succulent leaves can survive longer than non-succulent leaves once detached from shoots of their parent plant, possibly enabling them to differentiate and establish a root system before desiccation and/or starvation. Curiously, almost no succulent plants have compound leaves, even though compound leaves are often considered to have properties of both stems and leaves and hence theoretically should have the capability of developing new structures, such as new root and shoot meristems. I discuss some testable predictions arising from these notions and ask whether differences between ability to regrow roots and shoots from detached leaves of monocots versus eudicots is better explained by reticulate versus parallel venation or phylogenetic constraints. Keywords: phloem parenchyma; companion cell; intercalary meristem; apical meristem; cicatrix, cicatrices; Wiesner test; lignin; phylogenetic constraint; Crassulaceae LEAVES USUALLY ARE TERMINAL ORGANS Leaves are usually conceptualized as terminal or- It is impossible to root leaf cuttings from most gans. While a shoot can grow more shoots, as well tree or grass species or even most herbaceous tem- as grow leaves, flowers, and roots, a leaf is terminal perate weeds, such as dandelions and hawkweeds. In hence usually cannot grow any additional leaves, fact, there do not seem to be any plants with non- shoots, roots, or flowers. Identical arguments apply succulent leaves of terrestrial plants that are capable to leaves being terminal organs in all other vascular of being vegetatively propagated via leaf cuttings, plants (gymnosperms, monilophytes, lycophytes), except for maybe via highly technical micro-propa- except that flowers are replaced with other forms of gation. By contrast, almost anybody that has grown reproductive structures. Thus, Agnes Arber (1950) plants with succulent and semi-succulent leaves, ex- called leaves “partial shoots”. The terminal nature of cept aloes and some opuntioid cacti, knows that it is leaves is especially true with simple leaves. Simple often easy – sometimes too easy with some species of leaves, as well as leaflets on compound leaves, are Kalanchoe Adanson – to vegetatively propagate these generally considered the end of the ontogenetic plants by leaf cuttings. How is this possible? line. Leaves begin their growth from leaf ground tis- The terminal developmental nature of leaves is sue and later grow broader leaf blades via marginal a possible reason that most plants cannot be easily blastozones, aka marginal meristems, which are small propagated from leaf cuttings, in stark contrast with and ephemeral in simple leaves (Dengler & Tsukaya shoot and root cuttings. Roots and shoots contain 2001; Mauseth 2009) (the term ‘blastozone’ unin- dormant meristems, such as numerous shoot axillary tentionally conveys the very short-lived nature of buds at the base of each leaf. For example, areoles these zones of mitotic activity). In leaves, “subse- in cacti are merely short shoots that can grow new quent meristematic activity is intercalary and diffuse” branches or flowers, as well as grow leaves vis-à-vis (Beck 2005: 327; citing Donnelly et al. 1999). By spines (Buxbaum 1950; Gibson & Nobel 1986). A contrast, mitotic cell divisions via primary growth in vascular cambium can be grafted onto another vas- shoots and roots occurs at their distal end, via api- cular cambium, including natural grafting that oc- cal meristems. Growth from leaf intercalary meri- curs frequently with roots (Basnet et al. 1993; Je- stems, marginal blastozones, and plate meristems is linkova et al. 2009; also see discussion and references usually conceptualized as an increase of length and in Gorelick 2012). Unlike roots and shoots, leaves sometimes width of an already existing leaf, especially lack dormant apical meristems. Furthermore, simple those leaves with indeterminate growth, as we see leaves lack any secondary growth vis-à-vis lateral with growing blades of grasses and even the millenni- meristems, aka cambia. um-old leaves of Welwitschia mirabilis Hook.f. (Pear- 52 GORELICK—LEAF CUTTINGS OF SUCCULENTS son 1929; Evert 2006; van Jaarsveld & Pond 2013) New root and shoot meristems probably develop A few plants even have the antithesis of apical meri- de novo from phloem tissues in the detached leaf. stems in their leaves, namely basal meristems that This would provide new apical meristems a way to undergo virtually all mitotic divisions at the proxi- tap into the vasculature of the detached leaf. Angio- mal end of the leaf, as occurs with the highly ligni- sperm primary phloem in leaves is comprised of con- fied leaves of cactus spines (Mauseth 2009). ducting sieve tube members, companion cells, phloem parenchyma, and phloem fibers. Gymnosperms, SUCCULENT LEAVES ARE NOT TERMINAL monilophytes, and lycophytes have sieve cells in lieu ORGANS of sieve tube members (Evert 2006). Gymnosperms Many succulent plants, especially in the Crassu- have albuminous cells (aka Strasburger cells) in lieu laceae, but also in other families, can be grown from of companion cells, whereas monilophytes and ly- leaf cuttings (Kerner & Oliver 1902 [1895]; Hage- cophytes lack anything like companion cells (Evert mann 1932; Yarbrough 1936, citing Schubert 1913). 2006). Phloem parenchyma are cells surrounding Furthermore, several Kalanchoe species in the sub- and supporting conducting phloem, but the term genus Bryophyllum Salisb. grow new plantlets along ‘phloem parenchyma’ is a bit of a misnomer insofar as their leaf margins. Cuttings of simple leaves can be sieve tube members, sieve cells, companion cells, and successfully propagated from a phylogenetically di- albuminous cells are also forms of parenchyma (al- verse group of succulent and semi-succulent plants, though some developing primary phloem sieve tube including eudicots, monocots, basal angiosperms, members in some monocots have thicker cell walls, gymnosperms, and ferns, many of which are listed in which eventually thin out, so that their mature con- Table 1. Kerner & Oliver (1902 [1895]) and Hage- ducting sieve tube members are once again paren- mann (1932) provided particularly extensive lists of chymous; Evert 2006). Phloem parenchyma may be species that can be propagated from leaf cuttings. indistinguishable from or intergrade with companion My table 1 is largely an update of August Hage- cells in angiosperms (Esau 1969; Evert 2006). Devel- mann’s (1932) table 1. opment of incipient apical meristems from leaves of Why do only some leaves grow new roots and angiosperm phloem tissue are probably from phloem shoots? There is some literature on this, mostly from parenchyma/companion cells because mature sieve the 1930s, mostly on Kalanchoe subgenus Bryophyl- tube members lack nuclei and phloem fibers have lum (Howe 1931; Naylor 1932; Clamp 1934; John- sufficiently thick cell walls to preclude cell division son 1934; Stoudt 1934, 1938) and the gesneriad (Evert 2006). Saintpaulia (Naylor & Johnson 1937). Here I argue It is not too surprising that plants can be grown that leaf primary vascular tissues (there is no second- from cuttings of succulent or semi-succulent leaf ary growth in simple leaves) contain some undiffer- cuttings because leaves have intercalary (i.e. diffuse) entiated parenchyma that can later differentiate to meristematic activity (Donnelly et al. 1999; Beck form new meristematic regions in fallen or severed 2005), from which new root and shoot apical meri- leaves or even in unfallen leaves of Kalanchoe subge- stems can develop. Leaf intercalary meristems are nus Bryophyllum. Parenchyma are merely plant cells even known to contain functional conducting xylem with thin cell walls (cells with thick cell walls are and phloem (Evert 2006). While attached to a stem, known as sclerenchyma and those with intermediate leaf intercalary meristematic activity produces new width cell walls are known as collenchyma). Undif- leaf tissue. But once no longer attached to stems, ferentiated parenchyma in leaf vascular tissue is com- there seems to be no reason why intercalary leaf mer- mon in stems of succulent plants. Undifferentiated istems could not produce any other cell types. This parenchyma is also especially evident in secondary may have never been explicitly noted in the literature growth of the many xerophytic plant stems with in- because many leaves are not succulent, so could not cluded phloem, i.e. phloem that differentiates from survive long enough after being detached or severed xylem parenchyma rays, as in Ipomoea arborescens to develop incipient apical meristems. Sweet (Convolvulaceae), Simmondsia chilensis (Link) I also want to briefly discuss two other ideas that C.K. Schneid (Simmondsiaceae), and Bougainvillea do not provide the reason for being able to

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