142 STRUCTURE OF FLORAL ORGANS; FERTILIZATION 



175. Pollination. The transference of pollen from anthers to 

 stigmas is called pollination. In the case of plants with dry, 

 dust-like pollen this is generally due to the action of the wind. 

 Moist, sticky pollen is generally carried by some kind of animal, 

 usually by insects. The subject of pollination is so important, 



especially in relation to the visits of 

 insects, that it needs a chapter by 

 itself (see Chapter xxxn). 



176. Fertilization. By fertiliza- 

 tion in seed plants the botanist 

 means the union of a male sexual 

 nucleus from a pollen grain with the 

 female nucleus of the egg cell at the 

 apex of the embryo sac (Fig. 157). 

 This process gives rise to a cell 

 which contains protoplasm derived 

 from the pollen tube and from the 

 egg cell. In many plants the pol- 

 len, in order best to secure fertiliza- 

 tion, must come from another plant 

 of the same kind, and not from the 

 individual which bears the ovules to 

 be fertilized. 



Pollen tubes (Fig. 156) begin to 

 form soon after pollen grains lodge on 

 the stigma. The time required for 

 the process to begin varies in differ- 

 ent 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 conditions. 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 (Fig. 157, m), called the micropyle (meaning little gate), 

 and transfers a male nucleus into the egg cell. 



FIG. 155. Pollen grains produ- 

 cing tubes, on stigma of a lily 



g, pollen grains ; t, pollen tubes ; 

 p, papillae of stigma; c, canal 

 or passage running toward 

 ovary. Much magnified. 

 After Dodel-Port 



