THE GYMNOSPERMAE : CYCADALES, ETC. 759 



ventral canal nucleus seem to break up and form the nuclei for a number of 

 small ephemeral cells at the top of the oosphere, but the details are uncertain. 

 This tissue is regarded as a physiological " endosperm," for it is soon absorbed 

 by the growing embryo. Meanwhile the cells of the jacket layer fuse with 

 the oosphere, and their large nuclei and their cytoplasm mingle with the 

 contents of the oosphere. The fertilized oospore nucleus divides into eight, 

 of which from three to five may give rise to embryos. Those which begin 

 development commence by surrounding themselves with cell walls. 



Embryogeny. 



Each primary embryonic cell begins development by the division of its 

 nucleus into two of unequal size. A tubular outgrowth appears into which 

 the larger nucleus passes and moves down to the tip of the tube, and behind 

 it a cross wall is formed. This terminal cell divides and becomes a pro-embr}^o. 

 The remainder of the tube forms the suspensor, which elongates and 

 pushes the young embryo down into the nutritive tissue at the base of the 

 prothallus. The basal cells of the pro-embr\o likewise elongate, though not 

 greatly, and form a secondar\' suspensor, which is not clearly distinguish- 

 able from the root cap of the embryo proper. Only one embryo usually 

 matures. 



While the embryo is developing the apex of the prothallus becomes meri- 

 stematic and builds a layer of protective tissue which closes the base of the 

 pollen chamber and prevents the suspensor from being pushed upwards out 

 of the prothallus. 



Germination. 



The seed has no obligatory resting period and may germinate immedi- 

 ately. The two cotyledons are linear and grow steadily until they are several 

 centimetres long. As the plumular leaves are reduced to scales from the 

 beginning, the cotyledons are responsible for all photosynthesis in the early 

 stages of growth. Each cotyledon contains two parallel bundles which unite 

 basipetaly and pass into a diarch root system. 



Gnetum 



The genus Gjietum is superficially very distinct from Ephedra but has many 

 points of coincidence in structure. It is distributed all over the Tropics, 

 and the thirty-four species are all lanky trees or climbers. 



Vegetativelv the most striking feature is the dicotyledonous type of 

 foliage, with net- venation, which is quite indistinguishable from that of an 

 Angiosperm (Fig. 760). Anatomically the stem is characterized by the 

 presence of wood vessels (Fig. 761) and by the peculiarity, in the climbing 

 species at least, of forming successive concentric zones of vascular bundles, 

 as in the stems of some Cycads. 



