CHAPTER VI 



THE SOURCES AND TRANSFORMATIONS OF ENERGY IN THE PLANT 



SECTION 87. General View. 



THE fact that all vital activity is bound up with a liberation of chemical 

 energy by respiration gives no indication of the mode in which the energy 

 is utilized, nor does this energy necessarily become immediately manifested 

 externally as movement, heat, light, or electricity. It may be stored as 

 potential energy in the form of food- materials, or as osmotic energy which, 

 together with surface-tension energy, form two physical factors of the 

 utmost importance to plants. 



During photosynthesis the plant stores up food-materials and energy 

 for future or immediate use, and the energy thus obtained may never enter 

 directly into metabolism. For instance, many substances present in the 

 plant exert a considerable osmotic action without ever being drawn into 

 metabolism, being absorbed directly from the soil and accumulated in the 

 cells by a purely physical process of selective absorption and passive secre- 

 tion. Transpiration affords another instance of the creation of a difference 

 of potential which aids the ascent of water in trees, and hence is of consider- 

 able importance in the vital economy without being a purely vital function. 



The action of any form of energy in the plant is largely dependent 

 upon the structural arrangement and physical properties of the cells and 

 tissues, so that the same form of energy may produce widely different 

 results in different plants, or in different parts of the same plant, or in 

 the same part at different times. Every physiological action is coupled 

 with a transformation of energy, and for a complete causal explanation 

 of any such action not only must the sources and transformations of 

 energy be known, but also the metabolic changes connected with them. 

 Locomotion, growth, translocation, the production of heat, light and 

 electricity, and constructive and destructive metabolism in general, all 

 involve transformations of energy which may become perceptible internally 

 or externally, and which are to be regarded as manifestations of vital 

 activity. 



Apart from the locomotory movements which are absent from most 

 plants, as many external manifestations of energy are shown in the 

 vegetable kingdom as among animals. A growing plant, for instance, 

 may exert considerable pressure against a resistance. The internal mani- 

 festations of energy during growth are probably very similar in both 



