INTRODUCTION xiii 



a place to live in, to obtain its nourishment, to develop a 

 healthy, vigorous body ; all in order that it may produce a 

 crop of good sound seeds. Every plant, we may say, is 

 struggling against every other and against the animals, great 

 and small, against certain unfavorable conditions of temper- 

 ature, moisture, dryness, etc., and only the stronger and 

 better equipped survive, while the weaker perish. This 

 struggle, while not so evident, is fully as real as that going 

 on between the different animals, and many peculiarities of 

 plant structure are more or less readily explained upon this 

 basis. What we must look for in our different laboratory 

 studies, then, is to see how different plants have changed or 

 modified very different parts to accomplish practically the 

 same object. 



5. Individuals, Species, and Genera. Each separate 

 plant is an individual. It is easy to distinguish the individ- 

 ual in almost all the higher plants, but occasionally the 

 technical distinction may be somewhat difficult to establish. 

 These technical difficulties need not, however, disturb us in 

 our work at all. 



We recognize that many individuals are of the same kind 

 or species. They resemble one another so very closely, that 

 there are no essential differences. We further recognize 

 that there are groups of kinds or species more nearly resem- 

 bling one another than they do other species ; such as the 

 different kinds of violet, the different kinds of roses, etc. 

 These species we group under the head of genus (plural 

 genera} ; e.g. we speak of the genus violet as including the 



