182 EMBRYOGENESIS IN PLANTS 



The coherent mass of densely protoplasmic cells which occupies the 

 basal end of the archegoniiim will in due course develop into the body 

 of the embryo proper. This region of the proembryo will here be 

 referred to as the embryonic region. In the subjacent cells, which 

 will develop as the suspensor, the first evidence of differentiation can be 

 seen, Fig. 39, in that they begin to elongate conspicuously in the axis 

 of the proembryo. The proembryo thus affords evidence of differential 

 metabolism, the end adjacent to the base of the archegonium becoming 

 the locus of apical growth and morphogenetic activity. Further 

 evidence of differential metabolism is seen in the formation of mucilage 

 cavities which begin to appear about this time. 



Bryan (1952) has illustrated in detail the early embryonic develop- 

 ment in Zamia umbrosa. Fig. 39. Following the phase of free nuclear 

 division, a cellular proembryo is formed at the base of the archegonium. 

 This proembryo differentiates into four regions, namely: (i) a distal 

 meristem whose outer cell layer eventually forms (ii) a conspicuous 

 cap; (iii) a suspensor, subjacent to the distal meristem; and (iv) buffer 

 cells subjacent to the suspensor and extending for some distance up the 

 sides of the archegonium. The cap cells become conspicuously en- 

 larged in the axis of the embryo and have a haustorium-like appearance. 

 They have densely granular contents, large nuclei and thick cell walls. 

 The posterior region of the meristem continually adds new elongating 

 cells to the suspensor which eventually becomes coiled and twisted. 

 When eventually the cap cells disintegrate, the meristematic region 

 enters on an active phase of growth. While cap cells have not previously 

 been reported in the Cycadales, Bryan considers that on critical exami- 

 nation they will probably be found in other genera. Similar cap-like 

 developments are known in various conifers. As the suspensor cells 

 continue to divide and elongate to a remarkable extent, the embryonic 

 region is thrust down into the tissue of the prothallus. Even more 

 remarkable suspensor developments have to be recorded. As we have 

 seen, several archegonia are present at the micropylar end of the 

 prothallus ; the ovum in each of these may be fertilised and the pro- 

 embryonic and embryonic developments indicated above may ensue. 

 One embryo, however, becomes predominant, and it alone becomes 

 fully developed, all the others sooner or later being inhibited, Fig. 39 

 (Brough, 1940; de Silva and Tambiah, 1952). But the several sus- 

 pensors, some of v/hich are short and some considerably elongated, 

 grow side-by-side and become twisted and coiled together. This 

 compound structure is topped by the upper region of the suspensor of 

 the single embryo that eventually attains to full development. In these 

 developments, which are reminiscent of the interweaving of fungal 

 filaments in the formation of complex fructifications, evidence of the 



