THE GAMETOPHYTES OF THE PINE 373 



354. The gametophytes of the pine. The pine, like all seed 

 plants, is of course heterosporous because it has microspores 

 (pollen grains) and megaspores (embryo sacs) ; so there are two 

 gametophytes, male and female. 



The male gametophyte. The male gametophyte, as in most, if 

 not all, seed plants, begins to develop before the pollen is shed. 

 There are three nuclear divisions which cut off two small cells, 

 called prothallial cells, of which traces may sometimes be found 

 against the wall of the pollen grain (Figs. 298, F,p ; 300, -Z?,^>). 

 The third division leaves the pollen grain with a nucleus (the 

 tube nucleus) in the central region and a small lens-shaped cell 

 (the generative cell) at one side (Fig. 298. F, g). This is the 

 condition when the pollen is shed. 



Shortly after pollination the pollen tubes begin to develop 

 in the pollen chamber (Fig. 300, A, p c), but their development 

 is very slow until the following spring. Then the large tube 

 nucleus passes to the tip of the tube, which grows rapidly 

 towards the center of the nucellus (disorganizing the surround- 

 ing tissue as it does so), where the female gametophyte lies 

 within the embryo sac. The generative cell now divides into a 

 stalk and body cell which pass into the tube. The body cell 

 forms two sperm nuclei a few weeks later. Four nuclei are 

 then finally present at the end of the pollen tube (two sperm 

 nuclei, the tube nucleus, and that of the stalk cell). The pollen 

 tube has now reached the embryo sac and is ready to discharge 

 its contents into one of the eggs developed by the gametophyte 

 (Fig. 300, D). 



The female gametophyte. The embryo sac (megaspore) is a 

 one-nucleate cell at about the time of pollination. This nucleus 

 gives rise by repeated divisions to a large number of nuclei that 

 lie at first freely in the protoplasm as the embryo sac gradually 

 increases in size. Later, cell walls are formed around the free 

 nuclei, and the entire embryo sac becomes filled with a delicate 

 tissue called the endosperm (Fig. 300, A), which corresponds to 

 the vegetative portion of a prothalliuni. It takes almost a full 



