2 THE FIELD OF PLANT PHYSIOLOGY 



Just as different species of plants differ in outward configuration and 

 internal anatomy, so do they also differ in physiology. The world of plants 

 includes a great number of very diverse kinds of organisms that range in 

 size from simple bacterial cells a few microns long to the enormous redwoods 

 of our Pacific coast and mountain forests. Their difference in size, although 

 striking enough, is not as fundamental as another distinction which exists 

 between redwoods and bacteria. The redwoods and all other green plants are 

 able to manufacture their own food, while the bacteria (with a very few excep- 

 tions) and all other non-green plants must obtain their nourishment from some 

 outside source. The physiology of the green and the non-green plants is there- 

 fore basically unlike. This book is essentially a discussion of the physiology 

 of the chlorophyllous (green) plants with the emphasis on the vascular green 

 plants. The physiological processes of the bacteria and fungi have been con- 

 sidered only when they have a direct bearing on the physiology of the green 

 plants. 



With few exceptions all vascular green plants carry on the same fun- 

 damental physiological processes, but there are many differences in subsidiary 

 processes. Most green plants, for example, synthesize starch, but many do 

 not. Numerous similar variations occur in the kinds of metabolic products 

 built up by various species of plants. Most of the physiological differences 

 among the species of green plants, however, are quantitative rather than quali- 

 tative. All chlorophyllous plants carry on photosynthesis, but when different 

 species are exposed to the same environmental conditions the rate at which the 

 process takes place may differ greatly from species to species. A similar situa- 

 tion holds with respect to practically all of the other processes occurring in 

 green plants. Even varieties of the same species often exhibit marked differ- 

 ences in physiological behavior when exposed to a given environmental complex. 

 Some varieties of wheat, for example, are markedly cold resistant while others 

 are not. 



The Relation of Physiology to the Physical Sciences. — Formerly the 

 opinion was almost universal that living organisms owe their distinctive prop- 

 erties to the possession of subtle and unknown forces which are peculiar to 

 "living matter." At the present time such "vitalistic" theories find very few 

 advocates. The contrary and now widely held assumption is that living 

 organisms operate in accordance with the same physico-chemical principles that 

 hold in the inanimate world. The complexity and elusiveness of living 

 processes are not assumed to be due to intangible unknown varieties of energ}', 

 but to the interplay of recognizable physico-chemical forces in the complex 

 organized system of the protoplasm. 



Adoption of this latter point of view has led to a widespread use of the 



