THE PINE 129 



embryo, although very rarely two or more are found, 

 which have resulted from the fertilization of more than 

 one of the eggs of a female plant. When the seed sepa- 

 rates from the macrospore leaf on which it is borne, a thin 

 layer of the outer tissue of the leaf remains attached to it 

 (Fig. 76, Q. This wing helps in the carrying of the seed 

 by the wind. 



154. Times of Pollination, Fertilization, and Seed Development. 



Small buds which will grow into staminate and carpellate cones are 

 formed in the summer or fall. The next spring (in the northern United 

 States, late in May or early in June), the pollen ripens and pollina- 

 tion occurs. At this time the carpellate cone, the macrospore leaf, 

 and the macrospore sac are still small, and the female plant has just 

 begun to develop from the macrospore. Fertilization of course cannot 

 occur until the female plant has formed its eggs. So the pollen tube, 

 after it begins to develop, grows slowly in the tissues of the macrospore 

 sac while the female plant also continues to grow. It is not untiljune~] ' 

 or July of the following year, when the pollen tube has been growing 

 for about thirteen months, that the eggs are ripe and fertilization 

 occurs. In the summer following fertilization, the zygote develops /-.. 

 into an embryo and the seed is ripened ; this seed is shed in the fall 

 or in the following spring. Thus the history of a single carpellate cone 

 covers parts of three years. 



155. Germination of the Seed. The seed may germinate 

 in the spring following its ripening, or, if conditions then are 

 not favorable, it may remain apparently unchanged, but still 

 alive and capable of germination, for some years. When 

 conditions are right, the embryo becomes larger, at first 

 because its cells take in water ; the endosperm also absorbs 

 water ; and the pressure caused by the swelling of the embryo 

 and of the endosperm breaks the seed coats, which have been 

 softened by the water that has soaked into and through them. 

 The growing embryo digests and uses the food stored in the 

 endosperm, which thus helps to start the embryo in its devel- 

 opment. The radicle is the first part of the embryo to push 

 out of the seed coats. It turns downward and develops into 

 the primary root, except that, as in the squash, the upper end 



