CHAPTER XVIII. 



Fertilization; Transfer of Pollen, Protection 

 of Pollen. 



193. Fertilization. — By fertilization in flowering plants the 

 botanist means the union of a nucleus from a pollen-grain with 

 that of a cell at the apex of the embryo sac (Fig. 142). This pro- 

 cess gives rise to a cell which contains material derived from the 

 pollen and from the ovule cell. In a great many plants the 

 pollen, in order to accomplish the most successful fertilization, 

 must come from another plant of the same kind, not from the 

 individual which bears the ovules that are being fertilized. 



Pollen tubes begin to form soon after pollen-grains lodge 

 on the stigma. The time required for the process to begin 

 varies in different kinds of plants, requiring in many cases 

 twenty-four hours or more. The length of time needed for 

 the pollen tube to make its way through the .style to the 

 ovary depends upon the length of the style and other condi- 

 tions. In the crocus, which has a style several inches long, 

 the descent takes from one to three days. 



Finally the tube penetrates the opening at the apex of the 

 ovule m, in Fig. 142, reaches one of the cells shown at e, and 

 transfers a nucleus into this egg-cell. The latter is thus 

 enabled to divide and grow rapidly into an embryo. This the 

 cell does by forming cell walls and then increasing by con- 

 tinued subdivision in much the same way in which the cells 

 at the growing point near the tip of the root, or those of the 

 cambium layer subdivide.' 



194. Nature of the Fertilizing Process. — The necessary 

 feature of the process of fertilization is the union of the essen- 

 tial contents of two cells to form a new one, from which the 



1 See Kerner and Oliver, vol. II, pp. 401^20. 



