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Trends of Specialization in the Sporocarp and Spores in the Living and Fossil Marsileaceae

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Trends of Specialization in the Sporocarp and Spores in the Living and Fossil Marsileaceae TRENDS OF SPECIALIZATION IN THE SPOROCARP AND SPORES IN THE LIVING AND FOSSIL MARSILEACEAE T. s. MAHABALE Botany Department, University of Poona, Poona, India ABSTRACT the petiole a little above the ground is The paper gives an account of the trends of slightly conical at the top. It is profusely specialization in the structure of the sporocarp and covered with ramental scales or hairs. It spores in three living members of the Marsileaceae, gives an impression to be radially sym­ J',la/'silea, Pi/ularia and Regnellidium, and in the only definitely known fossil member of this family, metrical, but a careful examination of its belonging to the genus Regnellidiutll named as serial sections shows bilateral symmetry Rodeites dakshini by Sahni ( 1943 ), from one of the (TEXT-FIG. 2 and PLs. 1, 2, FIGS. 7, 20). Tertiary horizons of India, the Deccan Intertrap­ There are four or in some species three or two pean Series of Mohgaon Kalan ( Dist. Chhindwara). chambers in it, in which the sori are lodged. Apart from morphological differences in shape, size, position of spines and their position on a sporocarp, In M arsilea the kidney-shaped sporocarp is it is the sporocarp wall which shows a good deal of definitely bilateral (TEXT-FIG. 3) and the variation and specialization in the three genera. coenosorus is partitioned into a number of The tabular layers of elongated cells forming a chambers 8-12 on each side (TEXT-FIG. 3 column below epidermis, sporoderm, and the mode of germination of mega- and microspores differ in and PL. 2, FIG. 19). This number, however, different species. The spore wall is 4-layered in all varies in different species. In Regnellidium genera, but the spore size and episporial ornamenta­ the round and large sporocarp is without a tion are different in them. In the spores of all hump, like that in the sporocarp of Pilularia these plants prismatic layer is conspicuous in both kinds of spores, but more so in megaspores. The (TEXT-FIGS. 11, 14) or spines as those in sori are bilaterally arranged in the living members Marsilea (TEXT-FIG. 10). It possesses only and also in the fossil member, Rodeites. The sporo­ a red-coloured raphe. It is also few-cham­ carp wall, however, seems to have been less differ­ bered, 6-7, and bilaterally symmetrical entiated in the fossil member, Rodeites dakshini Sahni than in the living species Regnellidium diphyllum (TEXT-FIG. 1), though externally round Lind., as the tabular cells a.re not apparent in the for­ ( PL. 2, FIGS. 18, 23, 24). mer. The raphe, on the other hand, in this member The structure of the sporocarp wall in is more prominent than that in the living species. these three genera is quite different (TEXT­ FIGS. 4, 6, 7-9 and PL. 1, FIGS. 1-3). In YDROPTERIDINEAE comprise two M arsilea the wall having peltate scales or families, the Salviniaceae and Marsi­ hairs is heavily thickened and mucilaginous H leaceae, both of which possess a (TEXT-FIG. 7 and PLs. 1, 2, FIGS. 2, 19). structure, sporocarp, in which sori bearing Two distinct layers of elongated, tabular mega and microsporangia are lodged. On cells lie below epidermis, in close association the basis of the morphology of sporocarp, the with the parenchymatous layers secreting two families are considered to be related to mucilage and filling the cavities of the Hymenophyllaceae and to two other families, chambers (TEXT-FIG. 7 and PL. 1, FIG. 2). the Schizaeaceae or Cyatheaceae. The Marsi­ In Pilularia the outer epidermis is not thick, leaceae is considered to be related to the but there are two contiguous layers of cells Schizaeaceae on the basis of its soral struc­ below it having heavy sclerenchymatous ture, particularly the indusium. This family thickening on opposite sides (TEXT-FIG. 6 consists of three genera, M arsilea with about and PL. 1, FIGS. 3, 7). These two layers 6S species, Pilularia having 6 species, and the constitute a strong, subdermal, thick wall in monotypic Brazilian genus Regnellidium, the sporocarp of Pilularia (TEXT-FIG. 6 and with its single species, R. diphyllum Lind. PL. 1, FIG. 3-scl.). The cells below these The sporocarps in these genera show a good thick subdermal layers are mucilaginous and deal of variation in their morphology, they fill the cavities of sporocarp chambers structure of the wall, scales or hairs on them, (PLS. 1, 2, FIGS. 7, 20). In Regnellid'ium sori, and the method of spore dispersal. In the wall of the sporocarp is quite massive Pilularia the round sporocarp arising from (TEXT-FIG. 1 and PL. 2, FIG. 18-scl.). It 66 tb t> p 2 p ml '~ ~ ~+------- "."',':"• j 6 .... ::. '. .' p TEXT-FIGS. 1-9 - Sporocarp and its wall in the Marsileaceae. (Abbrev.: ep, epidermis; sci, scleren­ chymatous cells; 51, stomium; Ib, tabular, columnar palisade-like cells; p, parenchyma secreting mucilage; mi, microspores; mg, megaspores; p, prismatic layer; ep, warty episporium; end, double-layered endospor­ lUm; ap, appendages.) 1-3 - T.S. of sporocarps showing bilateral symmetry in Regnellid-ium, Pilu­ lari and Marsilea. 1, Regnellidiun! diphyllum Lind. T.S. of sporocarp. X 6. 2, Pilularia antericana A. Br. T.S. of sporocarp. X 13. 3, Marsilea poonensis Kolhat. T.S. of sporocarp. >< 13. 4, Regllellidium diphyllum Lind. T.S. of sporocarp wall. X 44. 5, Ibid. A megaspore of the same lying below micro­ sporangia. X 44. 6, Pilularia americana A. Br. T.S. of sporocarp showing structure of the sporocarp wall and two microspores below it engulfed in mucilage. X 166. 7,ll1arsilea poonensi5 Kolhat. T.S. of sporocarp showing epidermis with a stomium (51), small tabular cells, elongated palisade-like cells below them, parenchyma secreting mucilage, a vascular bundle ( vb) and a sporangium with two developing megaspores. X 166. 8, 9, wall of the sporocarp in young sporocarps of Regnellidium diphyltum Lind. Showing differentiation of the tabular cells in young condition. X 166. 68 THE PALAEOBOTANIST contains elongated tabular cells below epi­ degenerate, resulting in a varying number of dermis and several layers of parenchymatous megaspores in each sporangium (ct. TEXT­ cells, about 15, traversed by mucilage ducts FIGS. 13, 17,29 and PL. 1, FIGS. 4-7). Conse­ and vascular bundles. The elongated cells quently in different species of Marsilea become differentiated at a very early stage in and Pilularia, and in Regnellidiurn diphyllurn the development of the young sporocarp the number of megaspores in a ripe sporocarp (TEXT-FIGS. 8, 9). The mucilage-secreting varies (PLS. I, 2, FIGS. 4-7 and 20, 23). cells are very conspicuous in the subdermal The largest number of megaspores is in tabular cells of this genus. The sori and Regnellidiurn (PL. 2, FIGS. 17, 23), next to spores lie fully enveloped in the mucilage that in Pilularia ( PLS. 1, 2, FIGS. 7, 20) and secreted by these cells in all species of the the smallest in Marsilea ( PL. 1, FIG. 5). In three genera (TEXT-FIGS. 1-7 and PL. 2, FIGS. different species of Marsilea this number 17-20), but the development and extent of varies from 1 to 9 per sporocarp. In Regnel­ the tabular, parenchymatous and mucilage­ lidiurn diphyllurn there are 40-80 megaspores secreting ceIls in the sporocarp of different engulfed by numerous microspores lying in a species of M arsilea and Pilularia greatly mass of mucilage. In Pilularia the number vary (TEXT-FIGS. 2-7 and PL. I, FIGs. 1-3). of megaspores is also generally large, but in Dehiscence of the sporocarp in M arsilea, some species it is comparatively small, vary­ as is well known, is by means of a long ing from 8 to 25 ( ct. PLS. 1, 2, FIGS. 7, 20). mucilage cord, the sorophore. to which the Both mega- and microspores lie together in sori are attached ventrally (TEXT-FIG. 10). the mucilage collected in sori lying in cham­ It emerges generally from the distal end of bers. The mucilaginous mass is mostly the sporocarp and is quite long, several times dorsal ( PL. 2, FIG. 19) and in the form of a longer than the sporocarp. In Pilularia the cord in Marsilea (TEXT-FIG. 10), globular sporocarp lies just in flush with the soil and and lying in the distal terminal half in dehisces by means of more than one valve or Pilularia, and spread all around the sporocarp pores at the top, generally 3 or 4, producing in Regnellt'diurn (TEXT-FIG. 17 and PL. 2, a drop of mucilage which, absorbing water, FIG. 17). The peculiarities of the wall struc­ enlarges considerably (TEXT-FIGS. 11, 14). ture and the mucilage-secreting cells, thus, The spores are spread in it. They germinate largely account for the difference in the mode on moist soil of the pond in which the plants of dehiscence of the sporocarp in the three grow and produce sporophytes which root in genera and in their different species. It the soil. In Regnellidiurn the sporocarp looks as if the whole sporocarp of Marsilea­ dehisces irregularly and gives out a large ceae, after early stages, is a large mucilage irregular mass of mucilage, in which nume­ or gelatine-secreting body, in the midst of rous sori containing both mega- and micro­ which the sori and spores get dispersed. spores lie together in the mass of mucilage Spores in the Marsileaceae, like the sporo­ secreted (TEXT-FIG. 12 and PL. 2, FIG. 17). carp wall, greatly differ in the three genera, The development of microspores and but all of them have some common features, megaspores in the three genera proceeds namely the presence of mucilage and other more or less along a similar pattern of the substances in the spore wall which enable Polypodiaceae up to 16 spore mother cells them to absorb lot of water and to swell.
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