LEAVES, BUDS AND FLOWERS 93 



moistened by the secretion of a viscid liquid, to which 

 the pollen grains readily adhere. Fertile pollen grains 

 (pollen that is capable of fecundating female cells of its 

 own species) alighting on the stigma of sufficiently near- 

 related plants during this period, undergo a process com- 

 parable 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 fecunda- 

 tion. 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 nor- 

 mally developed, and pollination occurs. Pollen is prob- 

 ably more profuse and more potent some seasons than 

 others. 



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. 



151. Cross-pollination occurs when the stigma receives 

 pollen from a plant of a different variety or species (21). 

 The offspring resulting is a hybrid (23) . Cross-pollination 

 is often performed artificially (440). 



Close- or self-pollination occurs when the stigma re- 

 ceives pollen from its own flower or plant. 



