372 EVOLUTION OF THE FLOWER 



yoke-form. Such flowers can be cut into two similar halves in 

 but one plane. 



It must not be understood that evolution of the flower has 

 progressed steadily through the various changes outlined above 

 and that consequently in the following lessons we can begin with 

 the most primitive type and proceed by regular steps to the 

 highest forms. The various orders of angiosperms have doubt- 

 less been derived from several distinct stocks and they have 

 not only varied in different degrees but especially will it be noted 

 that some orders have a tendency to emphasize certain forms of 

 these variations, while in other alliances, the variations will 

 proceed along quite different lines. These various modifications 

 of the simple type of flower have been retained very largely 

 because they are mutually beneficial to the plant and to insect 

 life. In the lower forms, the microspores are scattered by the 

 wind as in the gymnosperms. Such flowers are called anemoph- 

 ilous, meaning wind loving, and are small, very simple in struc- 

 ture and produced in large numbers to ensure the formation 

 of seed. In higher types of flowers, odor and nectar glands 

 appear and bright colors which serve to attract insects. Such 

 flowers are termed entomophilous, meaning insect loving. In 

 this way the spores are carried from one flower to the other 

 by the insect visitors with greater certainty than in the case of 

 the anemophilous plants. The variation in the form of the 

 flower and insect went on hand in hand and so it came about 

 that types of flowers finally appeared that were adapted to special 

 kinds of insects, sometimes to a single species, all other forms 

 being unable to enter the flower owing to its peculiar shape. 

 This is of the greatest benefit to both plant and insect because 

 the insect will confine its visits to such flowers owing to the 

 food which it alone can secure and the plant is equally fortu- 

 nate, since the microspores will only be carried to flowers of the 

 same kind. So we will expect to find in the following lessons 

 that the highest types of flowers are adapted to special kinds of 

 insects and it would be a natural inference that such plants will 

 be very abundant provided of course that they are also adapted 

 to the present conditions upon earth. 



