THE BEAN 



149 



171. The Mature Male Plant ; Fertilization. — The pollen tube, 

 in the bean as in the pine, is an outgrowth of the vegetative cell of 

 the pollen grain (Fig. 86, C). Soon after the tube begins to grow, the 

 generative cell moves into the tube and there divides to form two male 

 gametes. The mature male plant of the bean consists therefore of 

 only three cells — the vegetative cell which includes the pollen tube, 

 and the two male gametes. The pollen tube grows from the stigma 

 down through the canal of the style, feeding as it grows on substances 

 produced by the cells lining the canal ; it pushes into the ovary, reaches 

 an ovule, grows through the micropyle and into the tissues of the 

 ovule, and finally its tip penetrates the female plant itself (Fig. 89). 

 The end of the pollen tube now bursts, so that its contents are forced 

 into the interior of the female plant. Of these contents, the vegetative 

 nucleus and apparently the cjrtoplasm (includ- 

 ing the cytoplasm of the male gametes) are of 

 no further use, except perhaps as food. The 

 nucleus of one male gamete unites with the egg 

 nucleus, and the result of this union is the 

 zygote which will develop into a new asexual 

 plant. The other male gamete nucleus passes 

 into the large central cell of the female plant , 

 and there unites with the two nuclei already 

 present in this cell. « 



172. Formation of the Seed. — The 



central cell, containing the large nucleus 

 that has been formed by the union of 

 three, begins to develop into endosperm 

 before the zygote begins to grow into 

 an embryo; so that when the embryo is 

 formed a supply of food is ready for it 

 in the endosperm. The cells of the 

 female plant, other than the egg and the 

 large central cell, are crushed' and dis- 

 appear. The embryo feeds upon the 

 endosperm ; but the" endosperm also 

 continues to grow (Fig. 90), using the 

 surrounding tissue of the ovule for its 

 food. However, the developing endo- 



FiG. 90. — A young 

 bean embryo, almost 

 surrounded by endo- 

 sperm cells which will 

 be used as food by the 

 embryo ; a, embryo ; 

 b, endosperm ; c, inner- 

 most layer of cells of 

 the body of the ovule. 

 After a preparation by 

 Miss Mabel Brown. 



