4 PLANT PHYSIOLOGY 



expressed, for who can tell what life is? Just as we say 

 that walking consists in the rapid restoration of the body 

 to a position of equilibrium after falling, so we may say that 

 living consists in the maintenance of the equilibrium between 

 constructive and destructive influences and processes. But 

 living is the evidence of life, just as thinking is the evidence 

 of brain ; living is not life itself. However, in studying the 

 means by which the equilibrium between constructive and 

 destructive influences is maintained — the means by which 

 living is attained — we are approaching the eternal question : 

 What is life itself? 



In determining that the means of maintaining the equi- 

 librium between constructive and destructive influences are 

 physical and chemical, and that the influences themselves 

 are physical and chemical, not peculiar "vital" forces, not 

 occult or supernatural, one question regarding life is an- 

 swered. Whatever maj- be our views regarding the origin of 

 life, there is no scientific or other heresy in accepting the 

 present evidence that life maintains itself, and is maintained, 

 by physical and chemical means only. In the following 

 pages these means will be examined and discussed. 



Pfefter's statement, quoted above, of the aim of physiology 

 does not limit the study to the manifestations of life in 

 either "kingdom" of organisms. One evidence of the wis- 

 dom of such a broad view is the value which Pfeffer's own 

 investigations, conducted mainly on plants — particularly 

 perhaps those on osmosis and on irritability — possess for 

 the animal physiologist. When physiologists, not satisfied 

 with the observation and description of the phenomena dis- 

 played by the different kinds of living organisms, began to 

 seek for the means by which these phenomena are executed 

 and for their causes, comparison revealed that the causes, 

 the means, and even the phenomena themselves, are alike 

 in all organisms. Physiology is now, therefore, a much 

 broader as well as deeper science than it was formerly con- 

 ceived to be, and though there are now and always must be 

 animal and plant physiologists, they are studying common 

 problems and contributing to the common mass of human 

 knowledge of living organisms. 



