204 Science of Plant Life 



mechanisms that favor cross-pollination, but it should be done 

 in the field or with the flowers in hand. In the white lily the 

 stigma is out of reach of the insects when the pollen is shed. 

 In other plants the pistillate and staminate flowers may 

 occur on different individuals, or on different branches of the 

 same plant. In primroses and bluets the stigmas and 

 stamens each have two different lengths ; the flowers on one 

 plant have long styles and short stamens, while the flowers 

 on another plant have short styles and long stamens. 



The most remarkable cases of cross-pollination by insects 

 are those in which a particular species of insect is necessary 

 for the pollination of a plant. Such relations exist in the 

 yuccas and in some orchids. In the absence of the particular 

 insect, pollination and seed production fail. Yuccas may be 

 grown in our Northern states, but in certain localities they 

 fail to produce seeds because the moth (Pronuba) needed to 

 pollinate the flowers does not live there. 



Formation and growth of the pollen tube. A second step 

 essential to the production of seed is the germination of the 

 pollen and the formation of the pollen tube. After a grain of 

 pollen is placed on the stigma, a microscopic tube develops 

 from its side and grows downward among the cells of the 

 stigma and style. At the time of shedding, the pollen grain 

 of most flowering plants contains three cells. One of the three 

 is active in the formation of the pollen tube ; the other two are 

 the sperms or male cells. 



The stigma, as we have seen, secretes a sticky fluid con- 

 taining sugar; acids, and other substances. The pollen 

 germinates best in the fluid secreted by the stigmas of the 

 same kind of plant, and it usually germinates imperfectly or 

 not at all on the stigma of a different kind of plant. Perfect 



