THE PINE 125 



150. Germination of the Microspore. We have seen that 

 many microspores (pollen grains) are formed in each pollen 

 sac (microspore sac) . The microspores also germinate in the 

 spore sac in which they were formed. But the male plant 

 that is formed from a microspore is not, like the female 

 plant, firmly held in the spore sac ; after it has developed for 

 a time within the sac, it becomes free. The germinating 

 microspore does not at first grow larger ; it merely divides 

 to form four cells, all lying inside the microspore wall (Fig. 



Of the four cells which now make up the young plant, three are 

 small and lie in a group at one side, close to the spore wall. The two 

 small cells nearest the wall seem to be of no use, and they, break down 

 and almost entirely disappear soon after they are formed. The third 

 small cell remains alive ; it is the generative cell. The fourth, a large 

 vegetative cell, fills the greater part of the space within the spore wall. 

 At this stage the development of a microspore into a male plant stops ; 

 the spore sac splits open, and the pollen grains escape. Thus we see 

 that a pollen grain is a microspore when it is first formed ; but by the 

 time that it escapes from the spore sac, the pollen grain is no longer a 

 spore, but a young male plant. A pollen grain has two bladder-like 

 projections, one at either side ; these are cavities formed by the ex- 

 pansion of the outer layer of the wall of the pollen grain ; these projec- 

 tions, by increasing the surface area of the pollen grain, help in its 

 scattering by the wind. 



151. Pollination. Since the female plant remains inside 

 the spore sac, the male plant (the pollen grain) must in some 

 way be brought to its neighborhood before a union of gam- 

 etes can occur. The wind is the agent that carries the 

 pollen of the pine. Since the wind scatters the pollen in 

 every direction, so that only one grain in many thousands will 

 land in the right place, the pine must produce much more 

 pollen than can actually be used. At about the time that the 

 pollen is being shed, the carpellate cone grows rather rapidly 

 in length ; this growth separates the macrospore leaves, and 

 some of the pollen grains which fall upon the cone sift down 

 between the spore leaves and so come near the opening of a 



