22O Plant Life and Evolution 



animals should often show evident adaptations to 

 plant structures. The mouth parts and the digestive 

 organs of herbivorous animals, the beak and tongue 

 of humming-birds, the mouth part of many insects, 

 are a few of the most common examples. But it is 

 equally clear that certain modifications of plants 

 have also been induced by their relation to animals. 

 The most remarkable of these adaptations are asso- 

 ciated with the employment of animals as agents in 

 pollination and seed distribution. 



Of course it is impossible to say just how far the 

 relation of animals to plants has acted directly upon 

 the structures of the latter, but it is very evident 

 that whatever may have been the inducement of cer- 

 tain structures, the preservation and perfecting 

 of these characters have been very potent factors in 

 aiding the plants in the struggle for existence. Thus 

 the grasses have been extraordinarily successful in 

 holding their own in competition with other plants 

 in nearly all parts of the world, and at the same 

 time they form perhaps the most important of all 

 food plants for the higher animals. In common 

 with many other monocotyledons, the leaves fre- 

 quently possess an unlimited power of basal growth, 

 and may be cropped repeatedly without injury; 

 moreover, many grasses are characterized by an ex- 

 traordinary power of rapidly spreading by means 

 of underground stems or runners. It would per- 

 haps be rash to assert that these habits have arisen 

 in response to a need for protection against the rav- 



