COLOR IN PLANTS 155 



or nectar, as they go from plant to plant, will carry with 

 them pollen dust clinging to their heads and legs and bodies 

 (Fig. 45), and by means of the pollen thus carried the later 

 plants visited will secure cross-fertilization. One might per- 

 haps think that the insect visitor would scatter pollen from 

 one plant on the pistils of the same plant, and thus cause 

 self-fertilization as a general rule, but there are three chief 

 ways in which this is commonly prevented. 



Frequently the pollen and the ovules of a single plant 

 do not mature at the same time, so that self-fertilization 

 is prevented. 



Many plants have the parts of their blossoms so ar- 

 ranged that the visiting insect will go to the nectar first, 

 without coming into contact with the pollen until he is 

 about to depart, when he will become dusted with the 

 pollen and carry it away on his visit to the next blossom. 

 Here, on the way to the nectar, he will brush against the 

 tip of the pistil and give to it some of the pollen he has 

 brought from the first plant, thus providing a means of 

 cross-fertilization. Blossoms are often remarkably modified 

 in form and structure to prevent in this way self-fertiliza- 

 tion. In a moment we will consider a few instances of 

 such modification. 



The third and almost universal method of preventing 

 self-fertilization is a physiological one, the pollen from any 

 given plant being considerably slower to sprout on a pistil 

 of the same plant than it is upon the pistil of another 

 plant; thus, even though the pistil of any blossom be 

 dusted first with pollen from the same plant, if, later, pollen 

 from another plant be brought to the blossom, the later- 

 received pollen is likely to be that which will effect fertili- 



