Ch. VI, 3] FERTILIZATION IN FLOWERS 



277 



Their function consists in effecting fertilization, the union 

 of male and female sex cells, without which process seed 

 does not form. The accomplishment of fertilization is the 

 primary function of the flower. 



Fertilization in flowers involves three stages, two of them 

 preliminary and subsidi- 

 ary to the third and 

 crucial one, — viz. pol- 

 lination, growth of the 

 pollen tube, and fusion 

 of the sex cells. 



The pollen grains are 

 developed in the anthers, 

 and develop within them- 

 selves the male, or sperm, 

 cells (Fig. 188). The 

 transport of the ripe 

 pollen from anthers to 

 stigmas, through a space 

 sometimes small but fre- 

 quently great, is called 



POLLINATION. It IS not 



effected by any power 

 within the plant, but by 

 some external agency, — 

 mostly by wind in the 

 inconspicuous flowers, 

 and by insects in con- 

 spicuous ones. 



Pollination accomplished, the growth of the pollen tube 

 begins. Into the roughened, sugary-adhesive, epidermless 

 surface of the stigma (Fig. 189) there grows from the pollen 

 grain a slender, delicate, thin-walled tube, in which can be 

 seen the distinctive living protoplasm. This tube, carry- 

 ing the two sperm nuclu near its tip, grows down through 

 the tissues of the style, dissolving for itself a way by aid .of 

 (£)/>- 2 7 « 



Fig. 190. — A typical ovule, of Narcissus, 

 ready for fertilization, in section; much 

 magnified. 



Near the upper end of the embryo sac 

 lie three cells, of which the larger is the egg 

 cell. The pollen tube is shown entering 

 the micropyle. (Drawn from a wall chart 

 by Dodel-Port.) 



