THE PIXE 



127 



The male plant is now fully formed. It includes at most 

 six ceUs, of which two were small and have almost or quite 

 disappeared ; the four remaining are the stalk cell, the two 

 gametes, and the vegetative cell of which the tube is a part. 

 Finally, the end of the tube makes its way through the tissue 

 of the macrospore sac into the neck of an archegone, and 

 there comes in contact with the egg. Then the end of the 

 poUen tube bursts, and its contents, including the male 

 gametes, are forced into the egg. One male gamete unites 

 with the egg. The result of 

 this union is a zygote, which 

 ven,' soon begins to develop into 

 a new asexual generation. The 

 second male gamete and the 

 other substances that pass from 

 the pollen tube into the egg 

 seem to have no further use, 

 imless it is to ser\'e as food for 

 the egg. 



153. The Seed (Fig. 



;V — 



The growth and division of the 

 zygote result in the formation 

 of a young asexual plant (the 

 embryo) which Hes in the center 



Fig. 75 . — A lengthwise section 



of the tissues of the female through a pine seed; a, micro- 

 plant, part of which the embr^'o ^l^^' *' '^"^^^^ °^ ^'^^ ^% °^ 

 i^ ' ^ - the ovule; c, suspensor (a part of 



has destroyed and used as food, the embrj-o that takes no part in 



The embryo develops the same the development of the mature 



, ^, ^ J- J • 4-1. plant); i/, endosperm; e, radicle; 



parts that we found m the em- P jj^^ ^ ^^^^. ^^ ^;^^^ ^^ 



bryo of the squash: a radicle coat; A, plumule; f, seed leaves, 

 whose growing point is turned 



toward the micropyle, several seed leaves, and a verA' small 

 plumule. When these parts have been developed, the 

 embryo ceases to grow for a time. This stopping of the 

 growth of the new plant at a very early age is another 



