GYMNOSPERMS : WHITE PINE. 



505 



So it would appear that these 



divisions of a mother cell in the nucellu 



three or four cells are all 



spores. 



Only one of them, however, 

 the lower one, develops; the 

 others are disorganized and 

 disappear. The nucleus of 

 the macrospore now divides 

 several times to form several 

 free nuclei in the now enlarg- 

 ing cavity, much as the nu- 

 cleus of the macrospore in Pig. 357. 



Selaginella and Isoetes divides . P°"«n g'f "5 °[ P'"e. One of them genriinat- 

 ° mg. p' and p^, the two disintegrated prothalhal 



within the spore. The de- cells, = sterile part of male gametophyte; a.c, 

 , , ,1 r , 1 1 central cell of antheridium ; v.n., vegetative nu- 



velopment thus far takes place ^.leus or tube nucleus of the single-wall cell of 

 during the first summer, and antheridium; s.g., starch grains, (After Ferguson.) 



now with the approach of winter the very young female prothallium goes 



into rest about the stage shown in 

 fig. 358. The conical portion of 

 the nucellus which lies above is the 

 nucellar cap. 



621. Male prothallia. — By the 

 time the pollen is mature the male 

 prothallium is already partly 

 formed. In fig. 343 we can see 

 two well-formed cells. Two other 

 cells are formed earlier, but they 

 become so flattened that it is diffi- 

 cult to make them out when the 

 pollen grain is mature. These are 

 shown in fig. 357, />' and p-, and 

 they are the only sterile cells of the 

 male prothallium in the pines. The 

 large cell is the antheridium wall, 

 its nucleus v.n. in fig. 357. The 

 smaller cell, a.c, is the central cell 

 of the antheridium. During the 

 summer and autumn the male 

 prothallium makes some farther 

 growth, but this is slow. The 

 larger cell, called the vegetative 

 cell or tube cell, which is in reality 

 the wall of the antheridium, elon- 



Fig. 3 

 Section of ovule of white pine, 

 tegument; pc . pollen chamber; 

 tube 



in/, m- 

 pt, pollen 

 nucleus; m, macrospore cavity. 



