THE CYCLE OF LIFE: FLOWERS 47 



The stigmas ripen ahead of the stamens in the common plantain, 

 in the potentilla, or cinquefoil, and in the Oriental grass known as 

 Job's tears. 



3. Physiological relations. In some species of plants it is 

 found that if the pollen gets from the stamen to the stigma of 

 the same flower, the pollen will not lead to fertilization. In 

 buckwheat, in most orchids, in certain species of day lily, and in 

 some members of the bean family the pollen will not even put 

 out a tube if placed on the stigma of the same flower. 



There are, then, many species of plants in which close-pollenation 

 cannot take place, or in which it is not very effective if it does take 

 place. How, then, do these plants produce seeds, or, rather, how do 

 they secure pollenation? In other words, how is pollen carried from 

 flower to flower? 



38. Cross-pollenation. Plants that cannot pollenate them- 

 selves simply depend upon outside moving bodies or moving 

 forces to transfer the pollen for them. 



1. Wind pollenation. The most common moving agency that 

 acts between plant and plant is the wind. The dryness and 

 abundance of the pollen produced by many of the common 

 trees, and the presence of pollen in the dust at certain seasons 

 of the year, would make us suspect that the wind must dis- 

 tribute a great deal of pollen (see Fig. 21). 



2. Water pollenation. Another agent that is effective in dis- 

 tributing pollen for plants is water. This is of course limited 

 to plants that live in the water (see Fig. 20). 



3. Bird pollenation. Next to the wind the most common 

 moving agents that go from flower to flower are flying animals, 

 like birds and insects. We know that not all birds or all in- 

 sects can serve plants as pollen carriers, but only those that do 

 regularly visit flowers (see Fig. 22). 



4. Insect pollenation. There are hundreds of species of plants 

 whose flowers are pollenated by insects, chiefly those of the bee 

 order and of the butterfly and moth order. All these insects 

 have sucking mouths, and many of them visit flowers that con- 



