Morphological studies on some species of blechnum, doodia, woodwardia and stenochlaena. I. The gametophytes and the juvenile sporophytes

Abstract

Morphology of the spores, prothalli and juvenile sporophytes of 6 spp. of Blechnum, 4 spp. oi Doodia, 3 spp. of Woodwardia and 2 spp. of Stenochlaena is described. The spores are monolete, bilateral, with tenuimarginate laesura, and ranging in size from 30-35 times 45-55 μ in Doodia to 40-50 times 70-75 μ in Woodwardia. The exine is 2-3 ´ thick and smooth, except in Stenochlaena in which it is prominently verrucate to rugulose. All genera, except Stenochlaena, are perinate, the perine being granulose and either loose and wrinkled as in Woodwardia or closely adherent to the exine and devoid of wrinkles as in Doodia. The spores germinate within 2-3 days of sowing and develop within 3 or 4 days of germination into densely chlorophyllous germ filaments, 3-9 cells long. The rhizoids are light brownish in colour and may include a few scattered plastids. There is some variation in the sequence of cell divisions during the development of a prothallial plate and establishment of a meristem, the same species often exhibiting a fair amount of plasticity in developmental history. The simplest condition is one in which the anterior cells of the germ filament, including the terminal cell, divide longitudinally and an obconical meri-stematic cell is differentiated by an oblique division in one of the daughter cells of the terminal cell (as in B. gibbum, B. orientate, D. dives and often in D. maxima). The prothallial plate then expands and develops a cordate apex; the meristematic cell is later replaced by a multicellular meristem in the usual way. In most spp. of Blechnum, however, the terminal cell of the germ filament produces an apical hair before dividing longitudinally. Also, a meristematic cell may not be differentiated soon after the first longitudinal division, one of the daughter cells continuing growth of the germ filament. In Woodwardia spp., D. aspera and D. media the terminal cell (sometimes 2 cells at the anterior end) becomes quiescent when the germ filaments are 5-8 cells long. The cell behind i t divides longitudinally and one of the daughter cells develops an obconical meristematic cell adjacent to the quiescent apex, as in the Doodia spp., or by further divisions form an asymmetric, broad, prothallial plate in which a meristematic cell is differentiated in a marginal cell on the broader side of the plate. A marginal hair is produced prior to the establishment of a meristematic cell. The meristematic region later becomes 'apical' by unilateral growth of the young prothallus. In Stenochlaena there is a perceptible retardation in growth at the anterior end of the germ filaments when they are 6 7 cells long; the terminal cell often ends in a hair and a prothallial plate is developed from cells behind the quiescent apical region of the germ filament. The meristematic cell is developed laterally. The mature prothallus is of the cordate type, reaching maturity in about 8 weeks from spore germination, but often continuing growth for long and sometimes elongating markedly with age. In Stenochluena the prothallus is often profusely branched, with a dichotomized midrib. Unicellular, capped, papillate hairs are borne by the prothalli from early stages of development, and are usually profuse all over the margin and lower surface of the mature prothallus. Sex organs are of the usual type reported in advanced ferns. The antheridium is rather elongated (often oblong) and c. 40-100 p across. It has the usual structure, but the basal cell is often barrel-shaped and the opercular cell is very wide. At maturity the sperms are released through a pore-like opening (which soon widens) in Stemchlaena and Blechnum; in Doodia and Woodwardin the opercular cell collapses to release the sperms. Juvenile sporophytes are characteristic in appearance. The early juvenile leaves have a broadly cuneate lamina supplied by a single vascular strand forked 2 or 3 times, and with the margin shallowly lobed, each lobe corresponding to an ultimate veinlet. In many cases the early leaves possess many closely placed forked veinlets. A midrib is found in the 2nd or 3rd leaf onwards. The juvenile lamina becomes broadly 3-lobed soon after midrib formation and later becomes pinnatifid in Doodia and Woodwardia. In Blechnum the lamina remains entire for a long time, becoming elongated and broadly lanceolate. In Stemchluena, soon after the formation of the midrib the lamina becomes ovate with broadly rounded apex; the margin progressively becomes less lobed and may become nearly entire. However, in later formed leaves the apex becomes pronounced and the margin distinctly toothed, each tooth corresponding to an ultimate veinlet. The primary veins are rather distant from each other and each forked unequally twice. The venation of the juvenile leaves becomes reticulate in Stenochlaena, by the formation of a row of large costal areoles when the plants are about 3 months old. The areoles are formed in goniopteroid manner, with an excurrent tertiary veinlet a t the point of fusion of the secondary veins; later on other tertiary veinlets are formed parallel to it from the outer margin of the areole. The juvenile lamina in Stemchlaena becomes lobed (and later pinnate) at quite a late stage in development; by the time lobing of the lamina starts the venation pattern of the leaf is nearly similar to that of the adult leaves. The rhizome of the juvenile plants is short, except in Stemcklaena in which it is elongated and slender from the very early stages. All juvenile leaves bear unicellular papillate hairs on the stipe and lamina. I n addition, uniseriate multicellular gland-tipped hairs occur on the stipe base and on the rhizome. As the plants grow, these hairs develop into small lanceolate paleae by longitudinal division of the cells in the basal half, accompanied by broadening of the daughter cells.