The Flower. 99 



of fecundation. During a certain period, the surface of 

 the stigma is moistioned by the secretion of a viscid 

 liquid, to which the pollen grains readily adhere. Fer- 

 tile pollen grains,* alighting on the stigina of sufficiently 

 near-related plants during this period, undergo a process 

 comparable to germination, in which a slender projection 

 from the pollen cell penetrates the stigma, passes length- 

 wise through the center of the style and enters the ovule, 

 where fecundation occurs. 



Pollination is not necessarily followed by fecundation. 

 In young plants, and in older plants that are lacking in 

 vigor (9), flowers often fail to produce seed or fruit, even 

 when pistil and stamens appear to be normally devel- 

 oped, and pollination occurs. 



In some flowers, as in the pea, the stigma is brought 

 into direct contact with the pollen by the elongation of 

 the style, but in most plants the pollen must be trans- 

 ferred to the stigma by some outside influence, as by in- 

 sects, the wind, or gravity. Most flowers which have a 

 showy corolla or calyx, or secrete nectar, or yield a fra- 

 grant perfume, depend largely upon the visits of pollen- 

 loving or nectar-loving insects for pollination. The 

 showy parts and the perfume serve as signboards to- 

 direct the wandering insects to the flowers. Pollination 

 is favored by a warm, dry atmosphere. 



152. Cross-Pollination occurs when the stigma receives 

 pollen from a different plant, especially from a plant of 

 a different variety or species (21). The fecundation 

 resulting constitutes a cross or hybrid, as the case may be 



* Fertile pollen is pollen that is capable of fecundating female cells of 

 its own species. 



