568 Morphologie, Betruchtung, Teratologie, Cytologie. 



A Single axial sporogenous cell is organised in the nucellus. 



The mother-cell divides to form a row of two to four cells, the 

 lowest of which in functional. 



There is no central vacuole in the free-nuclear condition of the 

 embryo-sac. 



The embryo-sac, as in Gnetum, at first grows towards the mi- 

 cropyle. Its later increase is almost entirely in the lower half. In 

 consequence the chalazal end becomes broader Ihan the other. 



Many of the cells of the axial core of the nucellar cap coUapse. 

 A loosening of the tissue in the region into which the prothallial 

 tubes later penetrate is no doubt thus caused. 



No pollen-chamber is formed. The pollen-grains rest on the flat 

 top of the nucellus after the disorganisation of the cells of the nar- 

 row tip. 



Before cell-wals appear in the embryo-sac the nuclei are more 

 crowded and a little smaller in the chalazal end than in the upper 

 part. This is possibly the first sign of the differenciation of the sac 

 into fertile and sterile regions. 



The formation of cell-walls occurs throughout the embryo-sac. 

 In the youngest stage seen the cells of the micropylar quarter ot 

 the sac contain 1 — 2 nuclei, which are quite different in their cha- 

 racters from those of the lower three-quarters; these may number 

 12 or more in each cell. 



As the septation of the lower part of the prothallus becomes 

 more complete, nuclear division occurs in many, probably in the 

 majority of the micropylar cells, in each of which from two to tour 

 or five nuclei are eventually present. 



There is evidence that these nunclear divisions are direct. 



Each two-to-five-nucleate cells produces a tubulär outgrowth 

 (prothallial tube), which grows up into and at the expense of the 

 nucellar cap like a pollen-tube. The early course of the tube lies 

 within the axial core of the cap. 



The cells of the micropylar region in which nuclear division 

 has not occurred are crushed b)'^ the more actively growing tube- 

 producing cells. All the nuclei of prothallial tubes pass upwards as 

 the tube advances, and have travelled a considerable distance in 

 the nucellar cap before pollination occurs. The nuclei in each tube 

 are alike, except that a difference of size is sometimes apparent; 

 they remain close together in an irregulär mass or in a row one 

 behind the other. Their appearance and behaviour, up to the latest 

 stage seen, strongly favour the view that they are all potentially 

 equal in function. 



The development of the microsporangium proceeds along very 

 similar lines in the three genera Ephedra, Gnetum and Welwitschia. 

 In the germination of the microspore, Ephedra reveals its gym- 

 nospermous affinity, while Gnetum and Welwitschia show a greatly 

 reduced prothallus. While in the unicellular archesporium of the 

 macrosporangium Welwitschia agrees with Ephedra, in the later 

 stages of the embryo-sac and in the early condition of the prothal- 

 lus closer affinity between Gnetum Gnemon and Welwitschia is indi- 

 cated. But in the septation of the micropylar end of the sac and in 

 the production of the prothallial tubes, Welwitschia possesses cha- 

 racters which are quite distinct from anj'^thing known in Gnetum. 



Strasburger, whose material was insufficient for a study of 

 the Contents of the prothallial tubes, called the micropylar cells, 

 which produce them, corpuscula and suggested that the tube itself 



