EMBRYOGENESIS IN GYMNOSPERMS 



209 



inverted pyramid, and on the completion of wall formation there are 

 tiers of 5, 4, 3, 3, 1 or 6, 4, 3, 2, 1 cells, the uppermost 5 or 6 being 

 cells which are open above to the cytoplasm of the proembryo. All 

 the nuclei again divide: the upper tier produces five or six prosuspensor 

 (suspensor) cells, and an equal number of nuclei which degenerate; 

 the nine to eleven cells below, which on the elongation of the pro- 

 suspensor are seen to constitute the embryo proper, become binucleate, 

 a feature already noted in Saxegothaea. This binucleate condition of 

 the embryonic cells is also common to Podocarpiis and Dacrydium. 

 Comparative studies show that the principal differences in the embryo- 

 geny of the several genera and species can be referred to the relative 

 developments of the suspensor and the binucleate embryonic cells. 

 This is illustrated in Fig. 49 and in the accompanying Table (after 

 Buchholz, 1941): the larger the suspensor, the smaller the number of 

 distal embryonic cells. 



Rosette cells are rarely observed in this group, but they do occur 

 and may form small embryos in Podocarpus urbanii. Simple and cleav- 

 age polyembryony occur though in some, e.g. P. macrophyllus maki, 

 no cleavage takes place in the terminal embryo. In this species some 

 of the prosuspensor cells may become detached and form small 

 embryonic cells distally {see also Brownlie, 1953). 



CAPHALOTAXACEAE AND TAXACEAE 



The Taxaceae are held by some botanists to be the most advanced 

 order in the Coniferales, but others, e.g. Saxton (1934) and Florin (1939, 

 1951), consider that both the fossil evidence and the morphology and 

 life history of the living genera indicate the Taxaceae as an ancient 

 phyletic line, distinct from the Coniferales, though probably sharing 



