The Gametophyte of <Emphasis Type="Italic">Cystopteris Fragilis
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THE GAMETOPHYTE OF CYSTOPTERIS FRAGILIS BY SURJIT KAUR (Pteridology Laboratory, National Botanic Gardens, Lucknow) Received May 6, 1963 (Communicated by Dr. V. Purl, F.A.~C.) 1NTRODUCTION Cystopteris is a small cosmopolitan terrestrial genus of Athyrioid ferns, typified by C. fragilis (L.) Bernh. Eighteen species are included.under the genus by Christensen (1906-34), but according to most pteridologists (Christensen, 1938; Copeland, 1947; Holttum, 1954), many of these species may well be transferred to Athyrium. Recently Blasdell (1963) has published a detailed taxonomic account of the genus based on material from North America and East Asia including herbarium material from various herbaria. He has recognized ten species with a few varieties. The present study deals with the morphology of the spore and prothallus of C. fragilis. Spores for the present study were obtained through the courtesy of Dr. A. Lawalree from Belgium. Spore morphology is based on acetolysed preparations mounted in glycerine jelly (Erdtman, 1952). Prothallial morphology is based entirely on cultures raised on nutrient agar medium maintained at a temperature of 24 4-2 ° C. and a light intensity of 600 ft.-c. (Nayar, 1962). SPORES The spores of C. fragilis (Figs. 1, 2) are monolete (bilateral) of plano- convex, with dark brown, spinate exine (the spines are deep reddish-brown, ca. 6/~ long and with broad base and tapering apex). Surface of the exine between the spines is granulate. The laesura is tenuimarginate. The spores measure on an average 29 × 43 × 28 tz (P x E1 × E2, exclusive of spines). Blasdell (1963) describes two types of spores in this species : one with spines and the other with smooth exine on which is superimposed a layer which has given it a rugose appearance. The latter type, however, was not observed in the present study. PROTHALLI The spores germinate within a fortnight after sowing. On germination the exine ruptures at the laesura and a green germ filament emerges, generally 148 The Gametophyte of Cystopteris fragitis 149 preceded by the first rhizoid, which often contains a few sparse chloroplasts. The germ filament usually becomes 4-6 cells long and the basal cells bear rhizoids (Fig. 3). Generally the rhizoids are nearly hyaline, non-chlorophyllous l 200 .~ FIGS. 1-17. Fig. 1. Spore in equatorial view. Fig, 2. Exine stratification of the same. t~igs. 3-13. Stages in the development of the prothallus. Fig. 14. Mature prothallus. Fig. 1.~ Sl~X of the same. Fig. 16. Mature antheridium. Fig. 17. Marginal hair. (s, sexine; n, pexine.) and with a slightly swollen base: Some of the rhizoids are pale-brown in olour, and often sparsely chlorophyllous. The basal cell of the germ fila- ment is similar to the other cells. When the germ filament is 4-6 cells long the terminal cell divides longitudinally by an oblique wall into two daughter cells (Fig. 4), the bigger cell of which again divides into two daughter cells B3 150 SURJIT KAUR by a wall oblique to the first, thus cutting off an obconical apical meristematic cell (Fig. 5). In some cases the terminal cell of the germ filament divides twice before the establishment of the apical meristematic ceil (Fig. 6). Often the first division of the terminal cell is nearly vertical: an apical meristematic cell is then formed as a daughter cell in one of the halves. In some cases an apical meristematic cell is not formed and the terminal cell divides anticlinally and periclinatly, resulting in an ameristic obcuneate thallus (Figs. 7-10). Even in those prothalli which develop an apical meristematic cell, the first daughter cell of the terminal cell may undergo divisions and it often results in the formation of a small lobe on one side. Longitudinal divisions occur in the penultimate cells and later in the other cells of the germ filament leaving usually two of the basal cells uniseriate. The prothallus soon becomes spatulate with a fiat apex which soon becomes notched. At this stage margi- nal hairs start developing (Fig. 11). In some thalli, the appearance of margi- nal hairs may, however, be postponed, hairs appearing only when the thallus becomes conspicuously notched. The marginal hairs are unicellular, papil- late, chlorophyllous and devoid of any waxy cap-like covering (Fig. 17). The apical cell maintains its identity till the prothallus is cordate (Fig. 12). The apical meristematic cell divides by a transverse wall, the outer cell so formed being again divided by vertical divisions, resulting in the formation of a multicellular meristematic plate (Fig. 13). In some of the thalli the meristematic plate is established rather late, the apical cell persisting even when the thallus becomes distinctly cordate. Usually after the establish- ment of the meristematic plate the prothallus becomes cordate, often being broader than long. Superficial hairs appear at this stage. The superficial hairs are unicellular, chlorophyllous and similar to those present on the margin, but usually larger. Usually, the prothalli attain maturity within three months of germination. The mature prothallus (Fig. 14) is cordate with a shallow ~pical notch, and heavy midrib. The wing cells are uniformly thin-walled. Antheridia and archegonia are restricted to the midrib region on the lower surface. Antheridia begin to appear when the thalli are distinctly cordate, and all antheridia are superficial. Development and structure of the antheridium are of the usual type. The antheridium is subglobose and the opercular cell is small and undivided. The wall of the central cell does not touch the basal wall (Fig. 16). Archegonial neck is curved away from the apex of the thallus and is swollen at the apex. The neck is 4 cells long and is composed of usually 4 tiers (sometimes 5 or 6) of cells. The Gametophyte of Cystopteris fragilis 151 DISCUSSION Cystopteris fragilis agrees with some species of Athyrium (Diplazium) in the morphology and development of the prothallus (Nayar, 1960): in both cases plate formation is initiated by the formation of an obconical apical meristematic cell in the terminal cell of the germ filament and prothallial hairs are produced rather late in development. Also, the mature prothallus is cordate, bearing simple papillate hairs both marginally and superficially. Christensen (1938) remarks that the spores of Cystopteris differ from those of Athyrium and Dryopteris in lacking a perine. Recently the spore morphology of several species of Athyrium (including Diplazium) has been described (Nayar and Devi, 1963) in which there is a progressive reduction of the perine in the different species Of Athyrium. The absence of a perine in Cystopteris, thus does not serve to differentiate the genus from A thyrium. Many pteridologists include Cystopteris in the Asplenioid phylum (Ching, 1940; Christensen, 1938). The spores of the Asplenioid ferns are perinate and there is no report of any species where a perine is absent. Also, during prothallial development in Asplenioid ferns the germ filament terminates in a papillate hair; a meristematic cell is formed lateral to the hair later on. Also, the Asplenioid prothalli are hairy from very early stages (Momose, 1959 a, 1960 a, b, 1961 a, b, 1962). Thus morphology of the gametophyte of Cystopteris suggests the affinity of the genus to Athyrium rather than to the Asplenioid ferns. SUMMARY The spores of Cystopteris fragilis are monolete with spinate exine and devoid of perine. On germination a germ filament is produced which when 4-6 cells long has the terminal cell dividing obliquely twice to form an obconical apical me- ristematic cell. Marginal hairs are produced by young thalli. The apical cell is replaced by a multicellular meristem when the thalli become cordate. An apical cell stage is frequently omitted and spatulate, ameristic thalli develop a multicellular meristem directly. The mature prothallus is cordate with a broad midrib and bears margiv,~l and superficial unicellular hairs. ACKNOWLEDGEMENTS The author wishes to express her deep gratitude to Prof. K. N. Kaul, Director, National Botanic Gardens, for his keen interest in the work. I 152 SURJIT KAUR am indebted to Dr. B. K. Nayar for his guidance and to Dr. A. Lawlree for kindly providing me with spores for the present study. Thanks are also due to Shri P. C. Roy for helping in making the diagrams. REFERENCES Blasdell, R. F. .. "A monographic study of the fern Cystopteris," Memoirs Torrey Bot. Club, 1963, 21 (4), 1-102. Ching, R. C. .. "On natural classification of the Polypodiaceae," Sunyatsenia, 1940, 5, 201-68. Christensen, C. .. Index Filieum : with Supplements I, II, Ili, Copenhagen, 1906-34. .. "Fflicineae". In Verdoorn's Manual of Pteridology, The Hague 1938. Copeland, E. B. .. Genera Filieum, Waltham, Mass., 1947. Erdtman, G. .. Pollen Morphologyand Plant Taxonomy, Uppsala, Sweden, 1952. Holttum, R. E. .. Flora of Malaya, Vol. II, Ferns, Singapore, 1954. Momose, S. "Protha!li of Aspleniaceae (1)," J..lap. Bot., 1959 a, 34, 231-38~ "Prothalli of Aspleniaceae (2)," Ibid., 1959b, 34, 293-302. "Prothalli of Aspleniaceae (4)," Ibid., 1960a, 35, 229-35. "Prothalli of Aspleniaceae (6)," Ibid., 1960b, 35, 321-26. "Prothalli of Aspleniaceae (7)," Ibid., 1961 a, 36, 33-40. "Prothalli of Aspleniaceae (8)," Ibid., 1961 b, 36, 153-63. "Prothalli of Aspleniaceae (9)," Ibid., 1962, 37, 169-78. Nayar, B. K. .. "The gametophyte and young sporophyte of Athyrium escu- lentum," Amer. Fern. J., 1960, 50, 194-203. .. "Morphology of the spores and prothalli of some species of the Polypodiaceae," Bot. Gaz., 1962, 123, 223-32. ~--~ and Devi, S .. "Spore morphology of Indian Ferns I. The Aspidiaceae," Grana Palynologica, 1963 (in Press). .