180 EMBRYOGENESIS IN PLANTS 



recorded; but in Bowenia serrulata there are only 64 (Lawson, 1926). 

 These data are of interest for comparison with those of gymnosperms 

 with small zygotes. For example, in Sequoia, only four nuclei are 

 present, a wall being formed at the first division. Cycas and Stangeria 

 are characterised by a second phase of simultaneous free nuclear 

 division at the base of the zygote, these nuclei being subsequently 

 organised into the embryo apex and suspensor. 



The numerous free nuclei are at first fairly uniformly distributed in 

 the zygote, each nucleus being surrounded by a comparatively small 

 amount of cytoplasm. Before wall-formation begins there is usually 

 some movement of the nuclei towards the basal end. This is particularly 

 noticeable in Stangeria : the cytoplasm at the basal end of the zygote 

 becomes dense and contains many nuclei, whereas the cytoplasm in the 

 upper region becomes vacuolated and thinly populated with nuclei. 

 Important but as yet little understood biochemical and biophysical 

 processes underlie these developments. Cell formation now begins to 

 take place, the manner of this process being somewhat specific. After 

 the free nuclear phase in Encephalartos, Macrozamia and some species 

 of Cycas, the embryo becomes cellular throughout, this development 

 being considered to be phylogenetically primitive. In Dioon and 

 Stangeria, the deposition of the permanent walls is preceded by an 

 evanescent segmentation throughout the whole zygote, both processes 

 having their inception at the lower end and subsequently progressing 

 towards the upper, i.e. neck end. In some others, cell formation takes 

 place in the lower region while the upper region still remains in the 

 free nuclear state, e.g. in Bowenia. After some hundreds of cells, with 

 dense protoplasmic contents, have been formed, the visible organisation 

 of the embryo begins. The oval cell mass, or partial cell mass, is known 

 as the proembryo. This term is used to include the early embryonic 

 development up to the point where the distal embryonic region begins 

 to form the distinctive organs of the embryo proper. 



Fig. 39. Embryogeny in Cycads 



A-D, Zamia umbrosa. A, Formation of cellular proembryo at end of second phase of 

 simultaneous nuclear division. B, Cellular proembryo; wall formation is com- 

 pleted but there is as yet no regional difterentiation. C, Beginning of differentiation; 

 young cap cells can be distinguished, also beginning of elongation of suspensor cells. 



D, Differentiation of proembryo completed; cap meristem, suspensor cells and 

 buffer cells are now clearly differentiated (A, C, x 40; B, D, x 35; after Bryan). 



E, F, Macrozamia spiralis. E, Development of proembryo; the elongation of the 

 suspensor cells has forced the embryo down into the prothallus; an enzymic diges- 

 tion of that region is taking place. F, Five embryos derived from five archegonia; 

 the suspensors are coiled and closely entangled; all but one of the embryos have 

 become abortive; note the persistent egg membranes (E, x 33; F, x 2-4; after 

 Brough and Taylor). G, Bowenia serrulata. Developing embryo with suspensor 



(x 75, after Lawson). 



