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223 Tropical Bryology 2: 223-237, 1990
A functional evolution of the Leucobryaceae
Harold Robinson Department of Botany, National Museum of Natural History, Smithsonian Institution Washington, D.C., 20560, U.S.A.
Abstract. Recent conclusions on the limits of the Leucobryaceae and on the function of the Leucobryaceous leaf are used as basis for further observations on evolution of the group. Eight genera are recognized in the family; 1. Leucobryum, 2. Steyermarkiella, 3. Ochrobryum, 4. Arthrocormus, 5. Schistomitrium, 6. Holomitriopsis, 7. Cladopodanthus, and 8. Octoblepharum. The leaf form, capsule shape, and peristome substructure of Leucobryum seems to derive from a Campylopus-type member of the Dicranaceae. Four basic stages are noted in the functional evolution of the family. 1. The stratification of the leaf into leucocysts and chlorocyst layers with leucocysts holding water and internally generated gas; 2. The shift from soil substrates seen mostly in Temperate Zone Leucobryum to rotten wood substrates or epiphytism; 3. Increasing reliance on vegetative reproduction with reduction of reliance on sporophytes; 4. Morphogenetic increase of the number of chlorocysts in the leaf. Geographical concentrations of the genera are noted, and distributions between hemispheres are apparently mostly by way of the South Atlantic. The use of functional considerations in evolutionary studies is emphasized. The retention of paraphyletic groups in taxonomy is defended. Members of the Leucobryaceae are known family begun by Robinson (1985), and to all bryologists on the basis of their thick many of the changes observed in the whitish leaves, and most students have evolution of the family are seen to correlate sectioned the leaves to see the unique with various functional considerations. mass of large leucocyst cells surrounding a network of slender green chlorocyst For purposes of the present paper a series cells (Figs. 1-5). However, most of separate steps are taken. First, the basic bryologists, after superficially conclusions of Robinson (1985) regarding distinguishing the commonly encountered the limits of the family are briefly revisited. members of the family give no further Next, the apparent origin of the family is thought to its evolution. Genera have considered, establishing the evident been distinguished on the basis of capsule outgroup. The third step involves a review form, peristome, calyptra, and details of of the characters by which the genera of cell structure in the leaves. The differences the family are distinguished, with a involved have caused some bryologists summary of apparent subgroups within such as Cardot (1900) and Andrews (1947) the family. Finally, certain anomalous to assume that different elements of the aspects of the results are discussed with a Leucobryaceae are directly related to two defense of function as a character. The other totally different families of mosses, apparent paraphyletic nature of the Dicranaceae and Calymperaceae. The Dicranaceae in relation to the present study extends the study of the Leucobryaceae is noted, and the value of
225 paraphyletic groups is defended. perspective, noting that the leaf structure imposed certain functional limitations. The Limits of the Leucobryaceae theoretical approach led to a successful search for evidence of gas in the leucocyst Older treatments of the family such as that cells of living leucobryaceous leaves. The of Brotherus (1924) have included the gas was considered necessary for proper standard genera Leucobryum Hampe, gaseous exchange at the surfaces of the Ochrobryum Mitt., Schistomitrium Dozy included chlorocysts, even as the leucocyst & Molk., Cladopodanthus Dozy & Molk., layers continued to function for water Octoblepharum Hedw. and Arthrocormus storage. It was further noted that the Dozy & Molk., as well as Leucophanes genera that least needed such gas in the Brid. and Exodictyon Card. Two other leucocysts were the same genera that genera, Cardotia Besch. and Carinafolium departed most from the leaf structure Williams have been synonymized interpreted as a broadened midrib (Lorch respectively with Leucobryum (Andrews 1894), and the same genera were the only 1947) and Octoblepharum (Bartram ones showing clear indications of 1960). The only other genera that have relationship to the Calymperaceae. On been included in the family are that basis, Leucophanes and Exodictyon Holomitriopsis H. Robins. and Steyer- were excluded from the Leucobryaceae markiella H. Robins. The diversity within by Robinson (1985) and the two genera this series of genera caused some authors were placed within the broad concept of to extract at least Leucophanes as a separate the Calymperaceae although they were family Leucophanaceae. Herzog (1926) not closely related to each other within treated only Leucophanes in the latter that family. Leucophanes is the one genus family while Fleischer (1904) included outside of the presently defined Cardotia, Octoblepharum, Arthrocormus, Leucobryaceae that has a similar pattern and Exodictyon. Most, but not all, of the of leucocyst layers enclosing a chlorocyst genera included by Fleischer have some network, but the structure in Leucophanes irregularity in leucocyst positions that is derived much more from laminal than results in some or all the chlorocysts being from costal material. A differentiated costa triangular. The capsules of the latter group, is present in the middle of the leaf of where known, were also erect with Leucophanes. All the remaining genera variously reduced 'Pottioid' peristomes that share the Leucobryaceous leaf with (Andrews 1947), and the group was internally generated gas were kept together considered to be related to the Calympera- in the Leucobryaceae. The ceae. The evident relationship of the latter Leucobryaceae are like many other Land group to the Calymperaceae, the relation Plants in having trapped gas in close of Leucobryum to the Dicranaceae, and proximity to their photosynthetic tissue, the apparent basic unity of most of the but they are distinctive in having the gas Leucobryaceae led Cardot (1900) to trapped inside cells instead of between suggest the Leucobryaceae was primitive cells. and the Dicranaceae and Calymperaceae derived. The latter view was rejected by Problems in relating such genera as Leu- Andrews (1947) but the unsatisfactory cobryum and Octoblepharum to each other division of the family proposed by Fleisch- remained after the Robinson (1985) study, er (1904) was retained with the assumption but the problem of a Calymperaceous the Leucobryaceae had two origins. relationship for genera such as Octoblepharum was removed. This latter Robinson (1985) approached the study of view is retained in this study. the Leucobryaceae from a functional
228 Relationships of the Leucobryaceae characters is one of the most disparate elements in the family. For capsules like The present perception of the relationships that of Ochrobryum, a simple formula of the family are based almost entirely on must be applied. Anything that is totally the nature of the one genus, Leucobryum. freakish is essentially useless for That genus shows what are considered determination of what is or what is not unmistakeable characters of the closely related. Nothing has been detected Dicranaceae. The usually curved capsules in the redelimited Leucobryaceae of this bear a clearly Dicranaceous type peristome study that bars possible relationship to the (Figs. 10-13) with closely set vertical bars Leucobryaceae or shows reason to relate on each external sector of the tooth. The any of the genera to any other known same type of ornamentation is seen in family. Dicranum (Robinson, 1971) and Campy- lopus Brid. (Fig. 7, 8). If the family Leucobryaceae consisted only of Characters of the genera of the Leucobryum, there would be ample reason Leucobryaceae to place the family totally within the Dicranaceae. In the Dicranaceae the Capsules, calyptrae, propagula, planation structure of the Leucobryaceous leaf is of the leaf, number or relative thickeness also anticipated in the broadened costa of the leucocyst layers in the leaf, and the and large leucocyst-like cells of many shapes of the leucocyst walls can all be species of Campylopus (Fig. 6). In such useful indistinguishing and grouping the species as Campylopus cavifolius Mitt. genera of the Leucobryaceae. the structure suggests that gas is produced within the leucocysts as in the Leucobrya- Capsule.—The capsule with its peristome ceae. Frahm in this symposium reports is one of the most obvious features for seeing such internally generated gas in the distinction within the family. Curved latter genus. capsules with well-developed vertical striations on the outer surface like those of The problem with the Dicranaceous origin the Dicranaceae are known in the family of the Leucobryaceae is the need to explain only in Leucobryum and the erect capsule the great diversity within the latter family in L. incurvifolium is treated here as a from a seemingly less variable ancestral minor exception where the peristome group. The topic is further explored below, remains unchanged. Two elements placed but it is notable that some of the characters in or close to Leucobryum in this study, in which the Leucobryaceae vary are found Cardotia of the Indian Ocean and in the Dicranaceae, even within the Steyermarkiella of eastern Venezuela are Campylopus relationship. Erect capsules placed in the absence of any knowledge of with slender peristomes are found in their capsules, and it is suspected that their Campylopus and in its close relative capsules if found would also be curved Pilopogon Brid. (Fig. 9). Erect capsules with vertically striated peristomes. are even found within Leucobryum in L. incurvifolium C. Müll. (Robinson 1965a). The erect symmetrical capsules of all other The peristomes of various Leucobryaceae genera of the family are treated here as that do not seem like Dicranaceae such as derived. It seems notable that they are Schistomitrium (Figs. 14, 15) and associated with genera that occur on Octoblepharum (Figs. 16, 17) are types organic substrates or have become that could be derived by reduction from a epiphytic. As such, the genera with erect Dicranaceous peristome, especially that capsules do not necessarily bear those of Octoblepharum which in other capsules pointed upward from the ground.
230 In such a situation regular curvature of the capsule would serve no purpose. Arthrocormus is still another example of reduced teeth in the family. As drawn by The capsule of Ochrobryum thoroughly Fleischer (1904) they are like reduced distinquishes the genus from all others. Schistomitrium teeth. The capsule is reduced to the form of an inverted operculum, evidently by some The reduction of the peristome in the 'morphogenetic trick'. The latter term Leucobryaceae reaches its extreme in refers to an abrupt and often unique Ochrobryum which has both a reduced rearrangement of the developmental capsule and a complete loss of peristome. process. Such “tricks” do not require The trend in the family is for capsule and intermediates and offer little aid in peristome reduction correlated with establishing relationships. The leaves of increased epiphytism. The new habitat Ochrobryum with their propagula better seems to reduce the need for fully formed demonstrate a phyletic distance between and fully functional capsules. The general the genus and others in the family. trend for reduction is so strong that the trend toward loss of the vertical division Peristome.—The basic peristome of the on the outer surface of the teeth in family is the Dicranaceous / Octoblepharum, Schistomitrium, and Fissidentaceous type with vertical striations Arthrocormus is not regarded as evidence on the outer surface, the type that is found of a particularly close relationship between fully developed in the Leucobryaceae only those genera. in Leucobryum. Other forms in the family are all considered derived from the type in Calyptra.—Most members of the Leuco- Leucobryum. bryaceae have cucullate calyptrae, the condition that is found in most members of The eight peristome teeth, or perhaps more proposed outgroup in the Dicranceae. properly eight pairs of teeth, of Genera with cucullate calyptrae include Octoblepharum in some views seem to the one considered most like the have essentially identical markings on both Dicranaceae, Leucobryum, and the ones the inner and outer surfaces (Figs. 16, 17). considered most modified in chlorocyst They were drawn by Fleischer (1904) disposition, Octoblepharum and Arthro- with four vertical cell rows on the outer cormus. surface on the basis of Indonesian material. Examination of American material that Ochrobryum has narrowly mitrate seems to show less rows (Figs. 16, 17) calyptrae that cover the long- rostrate actually shows that two of the rows are opercula. Fleischer (1904), Brotherus reduced to narrow marginal vestiges. (1924), and Bartram (1949) all characterize the calyptra of Ochrobryum as hairy. The The SEM views of the individual teeth of American specimens surveyed seem nearly Schizomitrium also seem to lack a vertical if not completely glabrous. The material median line on either surface (Figs. 14, from Asia usually has dense spreading 15). In this case the Fleischer illustration hairs at the base similar to those of (1904) agrees. Nevertheless, the closely Schistomitrium. Other features of related Cladopodanthus as drawn by Ochrobryum and Schistomitrium indicate Fleischer (1904) and Holomitriopsis that they are not close, and provide no (Robinson 1965b) have rather ordinary support for the idea of common origin of bifid teeth. The Schistomitrium teeth are the character. evidently a rather limited reduction within the Leucobryaceae. The three genera, Schistomitrium, Holo- 231 mitriopsis, and Cladopodanthus, that are the Calymperaceae, but the margins related by the cross-sections of the leaves, involved are costal material while the also all have mitrate calyptrae. The similarly positioned margins of the calyptrae of Holomitriopsis and Calymperaceae are laminal tissue. The Cladopodanthus are glabrous while those propagula in the Calymperaceae are on of Schistomitrium have coarse spreading specialized leaf apices or along the sides hairs at the base. The hairs of of the costa. The presence of monoseriate Schistomitrium are generally like those of propagula in Arthrocormus and Ochrobryum and some Dicranaceae in Octoblepharum is not accepted by the the relationship of Campylopus such as present author as evidence that those Mitrobryum H. Robins. (Robinson 1968), genera belong to the Calymperaceae. but not sufficiently alike to indicate direct relationship. Instead, hairs on the calyptrae The specialized propagula would seem to in the Dicranaceae such as Campylopus serve little purpose in a family where seem to demonstrate the lack of consistency unspecialized leaf fragments are so easily of the character in at least that group. In distributed. Such easily distributed leaves this study the hairs on the calyptrae are are common in the Leucobryaceae, and considered a localized apomorphy in apparently even in Ochrobryum. A Schistomitrium since they are lacking in specimen, Kellerman 7397b (US) both related genera, and since they are determined as Ochrobryum from associated with a genus that has a more Guatemala by Peterson and O. derived form of peristome tooth. The propaguliferum Dix. from Ceylon have genera with glabrous calyptrae, Cladopo- deciduous leaves on some stem apices, danthus and Holomitriopsis, are also the but the identification of these specimens genera with a less modified form of with Ochrobryum needs confirmation. peristome. Such development of vegetative Propagula.—The vegetative reproduction by whole leaves and leaf reproduction of the Leucobryaceae fragments might be a factor in reduction of involves various specializations. It consists dependence on the sporophyte for of specialized deciduous leaves in many reproduction. With most distribution being species of Leucobryum. Many other by vegetative means, the sporophyte would Leucobryaceae have leaf fragments that be reduced to its most essential evolutio- seem capable of growing into new plants. nary role of providing for occasional Specialized budlike propagula are borne genetic segregation and recombination. distally on the leaves of some species of The leucocyst covered chlorocysts of the Ochrobryum such as O. gardneri (C. leaf fragments would seem capable of Müll.) Lindb. (Correns 1899), O. crumii survival for reasonably long periods of H. Robins. and O. obtusifolium (C. Müll.) time. Such fragments would reduce the Mitt. The character is unlike anything in dependence on climatic conditions that other Leucobryaceae. Propagulae that are neither too wet or too dry for adequate have been reported in two species of spore distribution. Octoblepharum (Harrington & Egunyomi 1976) and in Arthrocormus (Enroth 1988) The deciduous leaves and leaf fragments are monoseriate more like those of the of the Leucobryaceae would easily be Calymperaceae, and Enroth emphasizes distributed by animals such as birds. The the fact that both genera had been placed distribution by birds would be highly in the latter family by Ellis (1985). The beneficial for the epiphytic taxa, and in the propagula in the Leucobryaceae are more present view has been a major factor in commonly found on leaf margins than in evolution of the advanced members of the 232 Leucobryaceae. the chlorocysts has been much noted in the previous taxonomic studies of the Leaf cell shapes.—The leaf cells vary in Leucobryaceae. The alternate (Fig. 2) two ways that are useful to taxonomists. rather than opposite (Figs. 1, 4) positions One is by differentiation of the cell layers, of the cells in the two different layers has the other is by the shape of the leucocysts been used to distinguish genera such as at the insections with the chlorocysts. Cardotia as well as whole series of genera that Fleischer (1904) and Andrews (1947) Three genera, Schistomitrium, Cladopo- seemed willing to relate to totally different danthus, and Holomitriopsis share a sources outside of the family. Andrews marked difference in the depth of the cells (1947) discussed the variation of the in the two layers of leucocysts (Fig. 4). character in relation to Cardotia as he The character was noted by Florschütz reduced that genus to the synonymy of (1964) in South American material later Leucobryum. Andrews said the Cardot named Holomitriopsis (Robinson 1965b) (1900) drawings and material named by when he suggested its relationship to the Thériot as Cardotia from Madagascar had eastern Hemisphere Schistomitrium. The all the appearance of a Leucobryum except character is regarded here as a marker for that the chlorocysts in the upper part of the the related group of three genera. Robinson leaf were triangular in section rather than (1985) noted in Holomitriopsis the quadrangular. Andrews noted that in the tendency for the shallower dorsal series of lower part of the leaf the cells were cells to more easily lose their outer walls quadrangular, “Agreeing in both these thus potentially exposing the chlorocysts respects with Octoblepharum, though to external air (Fig. 4). If this trait is true of otherwise there is no great resemblance the group it would seem a partial reversal between the two genera.” According to of the basic Leucobryaceous leaf strategy Andrews, “This character may be of in which the advantage of one layer of importance, but that Cardotia belongs in ventral water- storage cells is retained. close relationship with Octoblepharum rather than Leucobryum is very There are examples in diverse elements of questionable indeed.” Andrews the family such as Leucobryum and confirmed Cardot`s claim of similar Octoblepharum of extra layers of considerable development of triangular leucocysts in the leaves. The extra layers chlorocysts in the apical part of the leaf in are evidently minor elaborations of the L. albidum (Brid.) Lindb. of North basic two leucocyst layers during their America. Andrews reduced Cardotia to developmental phase (Ruhland 1924). One the synonymy of Leucobryum and also genus, Arthrocormus, has a more complex discounted the value of the alternate-celled development of many leucocyst layers in condition as a division between groups of which there are two or three layers of genera in the family. Nevertheless, the chlorocysts (Fig. 5). The character must alternate condition of the leucocysts has involve a basic morphogenetic been used to distinguish another probable rearrangement. Ruhland (1924) illustrates close relative of Leucobryum, namely a leaf apex of Leucobryum glaucum Steyermarkiella (Figs. 2, 3), but here the (Hedw.) Ångstr that has some extra alternate condition is throughout the leaf chlorocysts outside of the central layer. and is reinforced by a highly anomalous The latter example is a rarity in modification in the cell shape (Fig. 3). Leucobryum, but shows how the condition in Arthrocormus could have arisen. The alternate leucocyst arrangement is strongly developed in Octoblepharum, The pattern of leucocyst intersections with and alone could normally distinguish that 233 genus from Leucobryum. However, one and subtropical zones in the World, but in functional aspect of the alternate versus reality the family seems to occur mostly in the opposite pattern of leucocysts has been areas that are ecologically temperate. overlooked in the considerations of the character. The alternate pattern has the net Leucobryum.—The literature indicates 2 result of approximately doubling the species in eastern North America with a number of chlorocysts in the central plane third in Florida, 6 in Central America, 4 in of the leaf in relation to the number of northern South America, 1 species south leucocysts with which they intersect. In into Argentina, and 4 in southeastern this way the alternate leucocyst Brazil. Europe has 2 species with one the arrangement is like the Arthrocormus same as in North America and northern modification where chlorocysts occur in Asia. Nine species are cited for Gabon more than one layer. Both mechanisms and 16 for central Africa, 3 reach South increase the ratio of chlorocysts to Africa, and 19 occur in Madagascar. leucocysts. These changes increase the Eastern India has 9 species, China 14, ratio of live functional cells in relation to Japan 9, southeast Asia 16, the Philippines dead surrounding cells by simple 5. Four species reach Australia with 2 morphogenetic tricks. It would seem to be reaching the south, one species is in New one trend in the family that represents Zealand. Hawaii has 3 species. The many continuing elaboration of a character species once listed for Indonesia have instead ofdegeneration. It would seem to been reduced to 8 by the work of Enroth indicate what is comparatively important (1989) indicating that similar reductions in the survival strategy of the may occur in other areas when studies are Leucobryaceae. complete.
Geography of the Leucobryaceae As represented, Leucobryum is the most widely distributed genus in the The distributions of the genera of the Leucobryaceae. It is the only one with a family can be summarized with estimates few species ranging northward into of numbers of species for various regions. Temperate North America and Europe, The numbers are derived from both and one occurring as far south as southern herbarium records and some literature such New Zealand. There are other examples as Brotherus (1924), Bartram (1933, 1939) of the most primitive members of groups for Hawaii and the Philippines, Gangulee having the widest distributions of any part (1971) for eastern India, Schultze-Motel of a group, a phenonmenon that gave rise (1973, 1974, 1975) for Melanesia, Sa- to the now largely discredited Age and moa, and West Africa, Crosby et al. (1983) Area Hypothesis of Willis (1915). In the for Madagascar, Enroth (1989) for Borneo, case of Leucobryum this concept might Magill (1981) for South Africa, Scott et al. explain the diversity found in both (1976) for Australia, Sainsbury (1955) for Hemispheres. However, the occurrence New Zealand, and Florschütz (1955) for of the genus in the Temperate Zones seems American Octoblepharum. The high correlated with the ability of the species numbers cited for some areas may indicate involved to live on a soil substrate, a trait the need for synonymizations like those of not seen in other genera of the family. In Enroth (1989) dealing with Borneo. addition, the genus contains a number of Precise limits on distribution are inevita- specialized species in the tropics which bly uncertain, but some limits are assumed either grow on organic substrates or are on the basis of the general absence in sometimes epiphytic. regions of extreme cold or extreme aridity. The family is mostly limited to the tropical The species of the genus seem most 234 numerous in the Paleotropics, but a few graphical distribution that is completely distinctive elements such as L. distinct from that of Schistomitrium which incurvifolium, L. martianum (Hornsch.) is the other member of the family with Hampe, and the L. crispum C. Müll. group fringed mitrate calyptrae. occur in the Neotropics. The synonymized Cardotia is in the Madagascar region, a Arthrocormus.—The genus is known only region credited with the largest from the Paleotropical region where the concentration of species in the genus. The one species ranges from Ceylon eastward only evidence of direct interchange through Melanesia. The range scarcely between hemispheres involves L. glaucum overlaps with that of Ochrobryum but in the north between eastern North overlaps rather closely with that of the America, Europe and Asia. The Schistomitrium / Cladopodanthus group distribution of the genus does not seem to and with a large number of Leucobryum be of recent origin, and the dispersal species. between the hemispheres may have originally been strictly between tropical Schistomitrium, Cladopodanthus, and regions. Holomitriopsis.—The genera are credited respectively with 3, 4, and 1 species. The Steyermarkiella.—The genus is known first two genera are restricted to the area of only from the eastern Guayana Highlands Malaysia, Indonesia, The Philippines and of northern South America. It is regarded Melanesia. The presence of the related here as a distinct genus but a close relative Holomitriopsis in the eastern part of the of Leucobryum. The alternate leucocysts Guayana Highlands of South America is suggest that closest relationship is to the in an area notable for some Paleotropical Cardotia element of the Madagascar elements (Robinson 1986). Distribution region, and the genus shows no of the elements associated floristically with resemblance to any Neotropical elements Holomitriopsis was evidently across the of Leucobryum. Steyermarkiella occurs South Atlantic, but at this time the in an area where other bryophytes are Paleotropical genera of the Schistomitrium found with close Paleotropical group are not known from Africa or India. relationships (Robinson 1986). Species may have been in those areas previously or they might prove to be there Ochrobryum.—Three species are cited at present identified as Leucobryum. Ho- from Central America and northwestern lomitriopsis was originally described as a South America, 1 species is from western Leucobryum and its true relationship was Mexico, and 5 are cited from southern not suspected until the study by Florschütz Brazil. West Africa has 3 cited species (1964). Holomitriopsis is considered here and 1 species is cited from each of Central as a comparatively recent extension of the Africa and Madagascar. The Himalayas range of this basically Paleotropical group and Ceylon are credited with 1 species into the Western Hemisphere. each and 4 are cited from southeast Asia. Octoblepharum.—Eleven species occur The species are apparently concentrated in the American tropics, 2 species are in in South America, West Africa, and West Africa, 4 in central Africa, and only southeast Asia. The concentrations seem the 1 supposedly pantropical species is rather evenly distributed, and the cited from India eastward into the Pacific. Hemisphere of origin is not evident. The The latter species is also credited to Hawaii, only reasonable avenue of dispersal but it may have been introduced from between the Hemispheres is across the America rather than Melanesia. South Atlantic. Ochrobryum has a geo- 235 The genus is primarily tropical American The leucocysts hold both water and in distribution with most of the internally generated gas. paleotropical representation apparently consisting of a single nearly pantropical 2. The shift from soil to rotten wood or species. As such the genus is regarded as adoption of epiphytism with restriction to neotropical in origin. The most divergent tropical or subtropical regions. Change of element in the genus, with a carina near substrates resulting in less consistently the base of the leaf, is the northern South vertical sporophytes reducing the reason American O. tatei (Williams) Bartr. that for curved capsules. The shift of habitats was first described as the distinct genus to less persistent substrates increasing Carinafolium (Williams 1931). The genus evolutionary rates. Octoblepharum has been in the neotropics for at least 20-25 million years on the basis 3. Increasing reliance on vegetative repro- of a Hispaniola specimen in amber loaned duction and potential bird distribution. by Dr. F. Hueber (Robinson 1985). The Accompanying loss of reliance on age of the genus in the area is probably sporophytes with reduction of sporophyte much greater. structure.
General Considerations and 4. Morphogenetic increase of number of Conclusions chlorocysts in the leaf.
The earlier discovery of a basic functional The stages listed above can be elaborated peculiarity in the Leucobryaceous leaf as follows: (Robinson 1985) has been enlarged upon to offer a projected functional evolution of 1. The functional strategy described for the family at both the ecological and the Leucobryaceae by Robinson (1985) morphogenetic levels. Direct experiment must be of some limited benefit to the has not been attempted, but a number of mosses involved if their distribution is any inevitable conclusions can be derived from guide. The specialization has not conferred the basic continuing experiment found in the ability to survive in places where other nature. The functional progression could mosses do not occur, but it seems to have be presented in a cladistic form, but a left the genera fully competitive within the review of the characters of the family is more moist and less frigid areas where too incomplete. Only two synapomorphies other mosses are found. The Leucobrya- are presently known that would unite any ceae are successful, but they are not signi- of the derived genera into a distinctive ficantly out-competing other mosses in subgroup, the closeness of chlorocysts to any specific habitat. The specialization the ventral surface and the mitrate calyptrae involves a reduction in the photosynthetic generally unite Schistomitrium, Cladopo- tissue of the leaf, but it insures a more danthus and Holomitriopsis. It is also continuous optimal vapor pressure of both notable that the important functional gas and water for gaseous exchange at the changes noted in this study often occur surfaces of the chlorocysts in those tissues. within rather than between genera. The two effects might mostly offset each other. Still, any one species with such A number of stages are recognized in the leaves could compete in a wider range of functional evolution of the Leucobryaceae, habitats than other single species. some having secondary effects. The functional change inherent in the 1. The stratification of the leaf into inner structure of the Leucobryaceous leaf seems chlorocyst networks and outer leucocysts. to have offered only limited advantages 236 over the function of leaves in the supposed cladistic study should not be limited to a Dicranaceous ancestors, but the change paraphyletic group, that is, failing to unquestionably set the evolution of the include any derived groups in the study. I group in a different direction from that of would totally disagree with cladists that the Dicranaceae. The physiology changed the taxonomic groups should be first and structural changes followed later. redelimited to eliminate paraphyly when Thus, Leucobryum has the Leucobryace- the recognizeability of the group by ous strategy but retains most of the form of ordinary taxonomists would be impaired. the ancestral Dicranaceae. Thus, there is no mandate to redefine the Dicranaceae to include the functionally 2. The shift of the Leucobryaceae from and structurally distinct Leucobryaceae, soil to rotten wood or living plant substrates nor redefine Leucobryum to include any has a number of important results. It derived groups. In the case of Leucobryum, seems at least partly responsible for the the lack of known apomorphies between limitation of all advanced Leucobryaceae it and the common ancestor of most other to the tropics and subtropics. The change Leucobryaceae does not mean that none from soil substrates also reduces the exists. In fact, only Steyermarkiella of the tendency for sporophytes to be borne erect distinguished genera is suspected here of which reduces the value of a curved being derived directly from Leucobryum. capsule. Finally, experience seems to indicate that attempts to reduce the paraphyletic A more profound effect of the change of condition are not accepted by taxonomists substrates involves the comparative when they conflict with marked structural stabilities of those substrates. Many characters (Robinson 1987). Leucobryum species and many Dicranaceae occur on more stable subtrates, a feature correlated with the 3. Vegetative reproduction is found in more conservative evolution of those many mosses including many Dicranaceae groups. The more advanced so that the phenomenon is not limited to Leucobryaceae occur on less stable the Leucobryaceae. Nevertheless, most if substrates which correlates with their not all the Leucobryaceae shed whole apparent accelerated evolution. The less leaves or leaf fragments capable of stable environment would impose producing new plants. The prevalence of generally shorter life cycles and could also the vegetative reproduction would be more subject to catastrophic change. inevitably reduce reliance on sporophytic The greater vulnerability to catastrophic reproduction, and is certainly correlated change correlates with greater with forms of Leucobryaceae in which the discontinuities in structural features sporophyte is variously reduced. The between various derived genera. capsules and peristomes are reduced, sometimes to extremes as in Ochrobryum, One by-product of unequal rates of and sporophytes in some of the genera are evolution is the presence of paraphyletic rare. Sporophyte reduction is more groups. Certainly, the Dicranaceae is a prevalent and more extreme than in the paraphyletic group without the Dicranaceae. Sexual reproduction would Leucobryaceae. Leucobryum seems continue to serve the more limited func- technically near enough to the ancestral tion of genetic segregation and stock of the Leucobryaceae to be recombination but would be of reduced considered paraphyletic in relation to other significance in maintaining or increasing genera of the family. species distribution. I would agree with cladists that a proper 237 Another factor reducing the importance of the sporophyte in the Leucobryaceae is the wet habitats in which many of the species occur. As noted by Robinson (1986), too much moisture can adversely effect the dispersal function that is highly developed in many sporophytes. The unique capsule design in Ochrobryum might circumvent the problem if any of the spores are retained in the cavity of the long-rostrate, deciduous operculum.
Octoblepharum albidum Hedw. commonly has many sporophytes and also has one of the widest distributions of any member of the family, apparently being nearly pantropical. The species has some fragmentation of leaves, and both methods of distribution must contribute to its broad geographical range. It seems notable that the increased presence of sporophytes is found in a species that seems to occur in somewhat drier habitats than most other members of the family.
The deciduous leaves and leaf fragments of the epiphytic Leucobryaceae would easily come in contact with birds and other arboreal animals. Contact with such animals would result in direct transmittal of the reproductive structures to other appropriate habitats.
4. The basic leaf form of the Leucobryaceae has two subsequent modifications of interest, the shift to alternate arrangement of leucocysts in genera such as Octoblepharum and Ste- yermarkiella, and the development of more than one layer of chlorocysts in Arthrocormus. Both specializations have in common the increase in relative number of chlorocysts in the leaf. The functional significance of both specializations seems obvious, and the fact that two such modifications occur in the family seems to emphasize the importance of the chlorocysts to the survival strategy of the family.
This review of the Leucobryaceae has demonstrated the value of a functional approach to taxonomy. The importance of function is hard to over-estimate. Organisms that could not function can be found only in theory such as in some cladograms. Taxonomists should not forget physiological or morphogenetic realities as they study their groups.
The final pattern that is revealed in the Leucobryaceae supports many theoretical 238