CH. VI, 3] FERTILIZATION IN FLOWERS 



277 



unon 



Their function consists in effecting 

 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 subsidL 

 Rry_J,n thfi third fl/nd 



crucial^ one, viz. ^pol- 

 lination, growth of the 

 jjollen tube, and fusktn 

 ofjthe-sex-eells'. 



The pollen grains are 

 developed in the anthers, 

 and develop within them- 

 selves the male, OT^PERM, 

 cells (Fig. 188X 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 liny power 

 within the plant, but by 

 some external agency, 

 mostly by wind in the 

 inconspicuous flowers, 

 and by insects in rrvn-_ 

 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 nuclei near its tip, grows down through 

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



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.) 



