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 toa 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 anembryo. 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. I, pp. 401-420. 
