THE WORK OF FLOWERS 113 



If we place the pollen of the sweet pea blossom in a 

 very small glass dish containing shghtly sweetened water, 

 in less than an hour we can see with the aid of a microscope 

 the pollen grains beginning to germinate. In perhaps two 

 hours a long slender tube will extend far out from the 

 original pollen grain. 



When a pollen grain falls upon a stigma, it absorbs 

 moisture and nourishment from the latter and sends out 

 a tube, just as do those of the sweet pea in sweetened water. 

 This tube pushes its way down the style into the ovary. 

 Here in a small organ, called the ovule, a sperm cell from 

 the pollen tube and an egg cell within the ovule unite, and 

 a seed begins to form. This union of sperm and egg cells, 

 which is the second step in seed formation, is called 

 fertilization. 



90. Cross-Fertilization the Rule. — As a rule, cross- 

 fertilization, in which the pollen of one plant fertilizes the 

 ovules of another plant, produces the greatest number of 

 seeds capable of germination. Self-fertilization, in which 

 the pollen of a plant fertilizes its own ovules, tends to 

 produce no seeds at all or small seeds of which few or none 

 are capable of germination. 



There are a few important exceptions to this rule, some 

 notable ones being the oat, wheat, barley and cotton 

 plants. As might be expected, the seed from a self- 

 fertilized plant is less subject to variation than the seed 

 from one which is cross-fertilized. 



91. Cross-Fertilization by Hand. — It is possible for 

 us to secure the pollen from a plant, place it on the stigma 

 of another plant, and thus know both parents of the seed 

 we secure. If the flowers of the plant which is to receive 

 the pollen contain both stamens and pistils, the stamens 

 must be removed with a small pair of scissors and the 



