Sexual Reproduction in Flowering Plants 247 



yucca flower, and carries it to the top of the pistil and pushes it 

 down into the tubular stigma. The eggs of the moths are then 

 laid in the ovulary by piercing the pistil wall. As a result of 

 pollination the ovules develop and furnish food for the young 

 Pronuba larvae. But the larvae eat only a small percentage of 

 the ovules. So the larvae of the moth are fed on the ovules that 

 resulted from pollination. The yucca matures many undisturbed 

 seeds in every pod where none are produced in the absence of the 

 moth. How the relation became established we do not know, 

 for the Pronuba moth never sees her offspring and they never see 

 her. 



Germination of the pollen. Further steps in the production of 

 seed are the germination of the pollen, the formation of the 

 pollen tube, and the fertilization of the egg. The details of these 

 processes vary in different plant groups, but the account here 

 given is representative of what is found in many flowering plants. 

 At the time of shedding, the pollen grains of many plants contain 

 three cells. One of the three cells is active in the formation 

 of the pollen tube ; the other two are the sperms, or male gam- 

 etes. The stigma, as we have learned, secretes a sticky fluid 

 containing sugars, acids, and other substances. Pollen ger- 

 minates best in fluid secreted by the stigmas of the same kind 

 of plant, and it usually germinates imperfectly, or not at all, on 

 the stigmas of other kinds of plants. Germination results in the 

 formation of a microscopic tube that grows downward among the 

 cells of the stigma and style into the ovulary and into an ovule. 

 Usually this is but a short distance. In corn, however, the silk 

 is the style and stigma, and the pollen tube must grow several 

 inches, or a foot, down the silk before reaching the ovules below. 

 As it grows downward, the two sperms move along near the end 

 of the tube. 



To summarize the steps in the formation and movement of the 

 male gametes preceding fertilization, there is (i) formation of 

 pollen in the pollen sacs, (2) opening of the pollen sacs and shed- 



