Reproduction in Flowering Plants 203 



to the plants, and in this way perform a service for the plants. 

 The perfumes of flowers aid the insects in finding them, and 

 conspicuous white or brightly colored parts of flowers may 

 serve the same purpose. The massing of many small flowers 

 in clusters and heads certaialy makes them more conspicuous. 



Cross-pollination. When a flower is pollinated with its 

 own pollen or with that from another flower on the same 

 plant, it is said to be self-pollinated. If the pollen comes from 

 another plant, a flower is said to be cross-pollinated. In 

 many plants it makes no difference whether the pollen comes 

 from the stamens of the same plant or from those of another 

 plant. In the common tobacco plant the poUen may be 

 transferred to the stigma of the same flower, and seeds wiU 

 be produced. In some plants, however, it is only when the 

 flowers are cross-pollinated that seeds are formed. The 

 sunflower is a good example of this kind of plant. In stiU 

 other plants seeds that are formed after self-pollination are 

 less vigorous than those formed after cross-poUination. 



From the above statements it wiU be seen that cross-pol- 

 lination is an advantage to some plants, and we find in flowers 

 many arrangements that help to bring this about and to pre- 

 vent self-pollination. Often the anthers do not shed their 

 pollen at the time when the adjoining stigma is in condition 

 to receive it. The pollen may be shed either before or after 

 the ripening of the stigma. In such plants there is Httle 

 possibility of the stigma's being pollinated from the stamens 

 of the same flower. So, as insects go from one flower to 

 another they transfer pollen from flowers in which the pollen 

 is ripe to flowers in which the stigmas are ripe. This favors 

 cross-poUination. 



It is exceedingly interesting to study the various other 



