EMBRYOGENESIS IN GYMNOSPERMS 205 



small embryo initial and the enlarging subjacent suspensor respectively, 

 both elongate, Fig. 46h-l. Johansen (1950) regards each of these cells 

 as a potential embryo initial. Sometimes the cells of the subterminal 

 tier grow more rapidly and push aside those of the terminal tier, but 

 usually the latter hold their own. They elongate and become lobed 

 distally. Nuclear division, providing a nucleus for each lobe, may 

 follow, or the nucleus may enter one lobe which then grows ahead of 

 the other lobes. After a cell has elongated, the nucleus divides; one 

 daughter nucleus occupies the small distal cell, the other the elongated 

 suspensorial segment. In this way the distal cell may add new elongated 

 segments to the intertwined suspensor. Fig. 46k. These developments 

 impose a delay in the organisation of the embryo proper. However, 

 the distal cells of those embryos which have penetrated deeply into the 

 prothallus eventually enter on a new phase. This is characterised by 

 the formation of a mass of meristematic cells and is the beginning of 

 definitive embryo formation. The actively growing apical cell produces 

 a broadening mass of meristematic cells, below which the single 

 suspensor cell becomes greatly distended radially. Fig. 46l. As the 

 embryo enlarges, cells in the subapical region elongate and form the 

 massive secondary suspensor, Fig. 46m. The latter more or less effectively 

 obliterates the embryos which lie behind it. Mature embryos have 

 usually two cotyledons (see Cook, 1939). 



PODOCARPACEAE 



In Saxegothaea conspicua the archegonium is an elongated structure 

 with a narrow pointed base. The four free nuclei from the first two 

 mitoses of the zygote nucleus move into this basal region and again 

 divide twice, the sixteen nuclei (approximately) being disposed in some- 

 what irregular tiers (Looby and Doyle, 1939; Doyle and Looby, 1939). 

 These tiers, which occupy only a small part of the archegonium, 

 include an upper one of three to five nuclei, open above to the cyto- 

 plasm of the proembryo, an adjacent tier of three to five cells which will 

 elongate and become the primary suspensor, and a conical mass of 

 some six to ten embryonic cells at the base of the archegonium, the 

 most distal cell being an apical initial. Fig. 46n. Some of these cells 

 become binucleate and persist for some time in this condition, but 

 partition walls are eventually laid down. The subsequent embryogeny 

 is usually characterised by the absence of cleavage polyembryony — 

 though that may sometimes occur — by the elongation of the suspensor 

 cells and the early collapse of some of them, and by the early formation 

 of abundant embryonal tubes. 



In several species of Podocarpus there is the unusual feature that 

 fertilisation only takes place after the 'seeds' are shed from the plant, 



