144 PLANT LIFE. 



the way in which these are affected by external forces. In 

 its broadest sense it also treats of the relation of the plant as 

 a whole to external forces and to other living beings, both 

 plants and animals. But it is convenient to separate the 

 latter from physiology proper as ecology * (See Part IV.) 



The study of physiology proper necessitates methods of 

 controlling these external forces, carefully planned and re- 

 peated experiments, and cautious inferences. 



The study of ecology requires observation in the field of 

 the adaptations of plants to prevent injury by unfavorable 

 physical conditions and the attacks of other beings, and to 

 take advantage of the favorable forces and beneficent agents. 



178. Chemical and physical forces. The functions of a 

 plant may be divided for the sake of convenience into nu- 

 trition, respiration, growth, movement, and reproduction. 

 These are largely special modes of chemical and physical 

 action. Nutrition and respiration, for example, consist 

 chiefly of a series of chemical changes ; while movement is 

 mainly a result of physical alterations in certain organs. 

 But the action of chemical and physical forces does not suffice 

 at present to explain all the phenomena of the living plant. 

 Moreover, the peculiar manifestation of these forces which 

 we call life occurs only in connection with the substance 

 which we call protoplasm. 



179. The unit of function. The functions performed by 

 the entire plant are necessarily a sum of the functions per- 

 formed by the physiological units of which it is composed. 

 As the unit of structure is the plant cell, so the unit of 

 function is the protoplasmic body of that cell. Although 

 only a portion of any plant is composed of living matter, it 

 is to that living matter only that we are to look for the seat 

 of its powers. 



* Spelled in lexicons, oecology, but best usage drops the o ; sometimes 

 improperly called biology or plant biology. 



