108 EMBRYOGENESIS IN PLANTS 



apical end and progresses towards the basal end. There is differential 

 development as between the cells at the apex and base of the megaspore, 

 the former being small and dense, whereas the latter are large and 

 irregular. These large cells, which are described as being filled with oil 

 and albuminous granules, provide the nutrients on which the young 

 embryo will depend until it emerges from the prothallus and becomes 

 a rooted, green, autotrophic plant. 



The first archegonium, as in S. kraussiaua, is formed from one of 

 the small distal cells about the time the megaspore is shed ; and others 

 are formed later. The archegonial neck, of four cells in cross section, is 

 characteristically short, with but two cell tiers and a single neck-canal 

 cell. Numerous rhizoids, which develop at the apical region, may have 

 a nutritive function (Campbell, 1940). The retention of the megaspore 

 within the sporangium, the full development of the gametophyte 

 therein, and even the fertilisation of the ovum prior to shedding, have 

 been indicated by Lyon (1904) for S. apus and S. rupesths. These 

 developments are of interest because of their analogy with seed plants. 



THE OVUM AND YOUNG EMBRYO 



The available illustrations do not disclose any asymmetry or 

 protoplasmic differentiation in the ovum or zygote. Nevertheless, the 

 embryogeny is invariably endoscopic and polarity is determined from 

 the outset. As in Lycopodium, a suspensor is typically present. The 

 very considerable differences in the size and protoplasmic contents of 

 the embryonic cell and the suspensor indicate that the metabolism at 

 the two poles is very different indeed. Figs. 25, 26, 28. The outermost 

 segment of the two-celled embryo, i.e. the suspensor, may enlarge but 

 usually undergoes little further development; the innermost or 

 embryonic cell gives rise to the body of the embryo. The growing 

 embryo becomes more or less deeply embedded in the prothallial 

 tissue, but eventually it curves upwards and emerges as a leaf-bearing 

 shoot. As the megaspore lies on its side, the young embryo actually 

 grows in a horizontal position until it begins to respond to the stimulus 

 of gravity. In some species it will be convenient to refer to the two-celled 

 embryo as consisting of the embryonic cell and the basal cell. 



SELAGINELLA SPINULOSA 



In this temperate species, which is characterised by radial symmetry, 

 a simple stele, and a ' Selago-condliion' in the distribution of its 

 sporangia, botanists have recognised a phylogenetically primitive 

 member of the genus. The suspensor and embryonic cell are determined 

 by the first transverse division of the zygote which soon elongates in 



