SUMMARY AND CONCLUSION 295 



ited by insects, and thus of insuring cross-fertilization, 

 which appears to be distinctly advantageous to^ the 

 plant. From these probably accidental variations have 

 been developed the mechanical devices for insuring 

 cross-fertilization, as well as infinite varieties of color 

 and form, and the production of nectar and odors, serv- 

 ing as lures to attract insects. The extraordinary 

 development of the Angiosperms and Insects, the two 

 largest divisions of the vegetable and animal kingdoms 

 respectively, is to a very great degree correlated, the 

 two groups being largely dependent upon each other 

 for their existence. 



While provision for the development of seed is one 

 of the most important functions of the plant, their dis- 

 tribution is also necessary, and many arrangements for 

 this have been evolved. The development of edible 

 seeds and fruits, and of the numerous organs like the 

 wings of such fruits as those of the maple and ash, or 

 the down in the thistle or milkweed, the hooks and 

 prickles upon the fruits of many Composite and Bor- 

 raginese, are all devices for facilitating the distribution 

 of the seeds through the agency of the wind or by 

 animals. 



Extensive modifications have arisen in the plant by 

 which it adapts itself to a changed environment or pro- 

 tects itself against the attacks of animal enemies. The 

 earliest plants were probably aquatic, and their de- 

 scendants, but little changed, still exist in the low 

 green algse. The change from fresh to salt water has 

 no doubt changed the marine forms profoundly, this 

 being especially marked in the red and the brown algse, 

 which differ widely from their probably more primitive 



