PREFACE vii 



A brief chapter on plant associations is appended to the 

 studies of families and species, in order to stimulate thought 

 and observation along these lines. No attempt has been made 

 to present plant ecology in a formal manner, since the entire 

 treatment of families and species in Part III is ecological in its 

 nature and thus presents the elements of ecology in a concrete 

 mariner in connection with field studies. 



Part III is therefore not primarily taxonomic, but rather 

 biological, economic, and ecological in its point of view, and is 

 thus in harmony with the treatment of plants in the first two 

 parts of the text. 



Distinctive features. Plants are presented throughout the 

 text as living, active organisms, comparable to animals and with 

 similar general physiological life functions. 



The purely scientific and descriptive portions of the text are 

 directly linked with the theoretical and economic aspects of 

 biology which are of immediate human interest. Thus, the 

 cellular structure of organisms is directly related to growth, 

 >to the structure and life of trees, and to the economic value of 

 wood ; the chapter on vegetative and sexual reproduction is 

 followed by a presentation of the essential facts and theories 

 relating to hybridization, breeding, and evolution. The topic 

 of evolution, however, is not presented as a theory by itself, 

 but rather as nature's method of plant breeding and improve- 

 ment, closely related to similar processes carried on by man. 



The chapters on plant physiology are summarized and closely 

 correlated with the seasonal life of such common plants as the 

 bean, clover, and locust. Physiological processes are thus made 

 directly and concretely applicable to the seasonal life of well- 

 known plant species. More space is devoted to the cell, mitosis, 

 and cell structure than is usually accorded such topics in an 

 elementary textbook. 



Two reasons seem to the author to justify giving this larger 

 space to cellular biology. The first is derived from experience 

 in presenting these subjects to beginning students, who usually 

 manifest more interest in them than in most other aspects of 



