726 



A TEXTBOOK OF THEORETICAL BOTANY 



stage the integument is fleshy throughout and the nucellus is united to it, 

 except near the apex. When the ovule is ready for polHnation, the gameto- 

 phyte being still undeveloped, the apex of the nucellus grows up into the 

 micropyle as a beak, and some of its cells break down to form a flask-like 

 hollow called the pollen chamber, which plays an important part in the 

 preliminary stages of fertilization (Fig. 731). 



'Micropyle 



Integument 



Pollen chamber 



tNucellar beak 



-^ 



-^ Embryo sac 





- Integumental trace 

 Prothallus tissue 



Chalazal trace 



Fig. 73 1 . — Stangeria paradoxa. Longitudinal 

 section of the ovule. 



By the time the archesporial cell is distinguishable the nucellus is well 

 grown. This cell divides to form a row of three or four cells, of which all 

 abort except the lowest, which becomes the megaspore. As it enlarges the 

 surrounding cells of the nucellus take on the character of " spongy tissue," 

 as described in Pinus. 



The Female Gametophyte 



In the formation of the prothallus there is a prolonged period of free 

 nuclear division in the young megaspore, the nuclei being forced outwards 

 by the growth of a large central vacuole to form a peripheral layer around 

 the wall of the cell. More than 1,000 nuclei may be formed before walls 

 begin to appear. The development of the prothallial cells is centripetal 

 and probably similar to that in Pinus. When the whole space inside the 

 megaspore has become filled with tissue the cells develop numerous starch 

 grains, except the outermost layer of small cells. The outer wall of the 

 megaspore thickens and persists as a solid megaspore membrane around the 

 prothallus. It consists of two layers, the outer of which is suberized and 

 appears finely papillate. 



