CROSS- AND SELF-FERTILIZATION 



25 



stigma and down into the ovary. This is called fertilization, and 

 causes the ovary to immediately develop into a seed (Figs. 11 

 and 12). 



Cross- and Self-Fertilization. When the pollen produced by a 

 plant fertilizes its own ovaries, we call it self-fertilization. When 

 the pollen is carried by the air or by insects to other plants, we call 

 this cross-fertilization. 



In the cereals we find barley is always self- fertilized ; in fact, the 

 fertilization takes place before the heads emerge from the sheath. 

 Wheat and oats are also considered self-fertilized, but as the head 



Fia. 11. Diagram of a wheat flower, 

 showing the ovary, with feathery stigmas, 

 and the stamens or pollen sacs dropping 

 pollen. 



FIG. 12. Ovary of wheat grain. A pollen 

 grain caught on the stigma has germinated 

 and penetrated to the egg-cell. The con- 

 tents of the pollen grain pass to the egg- 

 cell, causing fertilization. The egg at once 

 grows into a wheat kernel. 



is fully exposed when the fertilization takes place there occasionally 

 happens a natural cross. In barley, wheat, and oats each ovary 

 receives pollen from its own stamens before the flower opens. 



Rye and corn are cross-fertilized. In rye the pollen in each 

 flower ripens before the stigma is ready and is, therefore, scattered in 

 the air, the stigma receiving pollen from another plant a day or two 

 later. In corn the pollen is in the tassel and is carried away from 

 its own ear by air currents, but in both rye and corn at least a few 

 seeds are usually self-fertilized. By experiment, it has been shown 

 that self-fertilized seeds in corn or rye will not produce as strong 



