PHYSIOLOGY OF PLANTS 



CHAPTER I. 



INTRODUCTION. 

 SECTION I. General. 



ALL terrestrial life follows a continual cycle of development and decay, 

 each individual existence being of limited duration. The largest trees, 

 which may reach the age of more than a thousand years, are doomed 

 to final death and disintegration, with the same certainty as the tiny 

 fungus, which completes its life-cycle in a few days. Every living 

 organism has the power of producing offspring which inherit the charac- 

 teristics of the parent stock : the fact that the acorn always produces an 

 oak, a fungus spore the same specific fungus, is sufficient proof that it is 

 the inherent properties of the living substance of the embryonic organism 

 which primarily determine the shape, character, and individual peculiarities 

 of the adult organism. 



In the young plant the full development of such inherited characters 

 takes place only through interaction with the external world, and not 

 unless certain necessary conditions are fulfilled. Thus, if the plant is 

 deprived of nourishment, it finally dies of hunger, while vital activity 

 and growth are only possible in the presence of water and within certain 

 limits of temperature. It is evident that the absence of any one of the 

 necessary conditions must invalidate the remaining ones, so that, if the 

 amount of water is insufficient, or if the temperature sinks too low, the vital 

 activity of the organism is depressed or completely arrested, and similarly 

 at a high temperature death ultimately supervenes. 



The conditions of life are not identical for all plants, but differ in 

 different cases ; no plant, however, can live unless supplied with nourish- 

 ment and kept at a sufficiently high temperature. Light, calcium, and 

 free oxygen are necessary to most plants, but not to all, and without an 

 appropriate food supply, the continuance of life is in all cases inconceivable. 

 It will be shown subsequently how and by what means plants obtain 

 and make use of their food, but what is at once clear is that nutrient 



