PART III 

 TRANSFORMATION OF ENERGY 



LECTURE XXXI 

 FORMS OF ENERGY IN THE PLANT 



Side by side with the law of conservation of matter, one aspect of which we 

 have learned to recognize in the circulation of elements in the organic world, 

 we place the law of conservation of energy. According to this law the sum of 

 the energies of the universe is a constant quantity ; energy cannot be created or 

 destroyed, it can only be transformed. Thus, for example, heat energy can be 

 transformed into mechanical energy, or electrical energy can be transformed into 

 radiant energy. No special evidence need be adduced in proof of the fact that the 

 laws of conservation of matter and of energy are applicable alike to the living 

 and to the non-living world. The task before us now is to follow the transforma- 

 tion of energy, just as, in Part I, we studied the transformation of matter ; that 

 is to say, we must attempt to answer this fundamental question, viz. ' Whence 

 does the plant obtain its energy, and what does it do with it ? ' Of course, this 

 is not the first time we have encountered this problem, for in speaking of meta- 

 bolism it was impossible to avoid mentioning the question of the transformation 

 of energy which is inseparable from it. Separation of the treatment of the 

 subject of transformation of matter from that of transformation of energy is 

 merely a matter of practical convenience ; in nature they are bound together in 

 the most intimate manner. 



Thus we had to draw attention to the fact that light was essential to the 

 assimilation of carbon in the green plant, or, more exactly, that light energy 

 becomes transformed into chemical energy, which latter reappears as potential 

 energy in the products of assimilation. It has also been noted that sunlight 

 is the most important source of energy for all living things, since those plants 

 and animals which are unable to make use of sunlight directly with the aid of 

 chlorophyll, are compelled to absorb the products of assimilation of green plants, 

 and thus acquire the energy of sunlight indirectly. Further, it has been already 

 shown that not only light energy but also, in many cases, chemical energy, 

 obtained by the absorption of materials (nutriment), is a primary source of 

 energy in the plant. Nutritive substances are employed, as we have already 

 seen, only to a small extent in the actual manufacture of the organism ; the 

 greater part is again broken down in the course of respiration or in other related 

 processes, and thus the chemical energy released in the decomposition of more 

 complex compounds is turned to advantage in the general vital phenomena of 

 plant life. In the process of nutrition, however, the plant acquires other forms 

 of energy as well, which operate quite independently of the energy evolved in 

 metabolism. Thus electric energy, osmotic and surface-tension energy or energy 

 of crystallization of the included substances must be recognized, and these, with 

 the exception of electricity, play, as everyone knows, important parts in theplant 

 economy. It is not at all likely that the plant is able to make any use of 

 electrical or mechanical energy, absorbed as such from the environment and 

 not associated with actual materials, at least we are quite ignorant of such 

 cases ; on the contrary, we know that the plant thrives perfectly well without 



