6 PLANT PHYSIOLOGY 



activities— special manifestations of life — according to species 

 and individuals as well as "kingdoms," there are also 

 general vital activities common to all living things. Before 

 passing to any consideration of these in detail, it will be 

 well to enumerate the conditions essential to life, to con- 

 sider the material and the structure in which life is resident 

 and which manifest it, and to realize that these conditions, 

 this material and structure, are the same for all living 

 things. 



I. THE CONDITIONS ESSENTIAL TO LIFE 



These may be comprehended under five headings : — 



1. Proper Food — (a) the source of the materials of which 



the body is built, and 

 (b) of the energy by which the body is built and 

 operated. 



2. Water — (a) the vehicle of the food-materials and of the 



foods absorbed into the body and transferred from 

 part to part, and also 

 (b) an indispensable component of actively living 

 protoplasm. 



3. Proper Temperature — which makes possible the vital, i.e. 



the chemical and phj'sical, changes which must go 

 on within the body, and in all of its parts, lest in- 

 action and death ensue. 



4. Proper Illumination — which furnishes the organism with 



the forms of energy — physical and chemical — thermal, 

 luminous, and actinic — of which it is directly or in- 

 directly in need. 



5. Proper Freedom — freedom from mechanical and other 



disturbances which would interfere with its supply of 

 food, water, warmth, and light, and prevent it from 

 carrying on its natural functions. 

 (To this list some might wish to add Oxygen, but 

 this is included under the first heading.) 



The many forces and matters included in this brief sum- 

 mary are not merely passively essential to life ; they actively 



