2IO 



MORPHOLOGY. 



prothallium, and is called the nucellar cap. Only one end of the endosperm 



(prothallium) is shown in fig. 271. 



423. Archegonia. In the upper end of the endosperm (prothallium) are 



several archegonia, and they aid us in determining what portion is the female 



prothallium. The nucellus is of 

 course formed before the prothallium. 

 The latter arises from a cell (macro- 

 spore) near the center of the nucellus. 

 This cell is larger, and has a larger 

 nucleus than its fellows (see fig. 268). 

 The prothallium here is formed much 

 in the same way as in selaginella, 

 where we recollect it begins to de- 

 velop before the macrospore has 



Fig. 269. Fig. 270. 



Section of ovule of white pine, int, integ- Upper portion of nucellus of white pine, 



ument; f>c, pollen chamber; ft, pollen tube; A', pollen-grain remains ; spc, sperm cells; 



n, nucellus; /, macrospore cavity. vn, vegetative nucleus; pt, pollen tube. 



reached its full size, and where the archegonia begin to form before it leaves 

 the macrosporangium. 



421. Male prcthallia. By the time the pollen is mature the male pro- 

 thallum is already partly formed. In fig. 255 we can see two well-formed 

 cells. Other cells are said to be formed earlier, but they become so flattened 

 that it is difficult to make them out when the pollen grain is mature. At this 

 stage of development the pollen grain is lodged at the mouth of the ovule, 

 and is drawn up into the pollen chamber. 



425. Farther growth of the male prothallium. During the summer and 

 autumn the male prothallium makes some farther growth, but this is slow. 

 The larger cell, called the vegetative cell, elongates by the formation of a 

 tube, forming a sac, known as the pollen tube. It is either simple or branched. 

 Jnside Qf this sac the cells of the prothallium are protected, and farther 



