92 The Living Plant 



others which have done the normal photosynthetic work, the ex- 

 tra bulk being nothing but water. Thus, the two sets of plants 

 in the accompanying picture (figure 31), were started by the water- 

 culture method, (later to be explained), from two sets of seeds of 

 exactly the same weight. But one set (that on the left) was grown 

 in the light and was able, therefore, to make up its loss by photo- 

 synthesis, while the other was grown in the dark and could not. 

 Yet the latter, owing to the habit of plants to spindle out greatly 

 in length in darkness, actually look larger than the former. 

 When, however, I weighed these two sets after all of the water 

 has been dried out, leaving only dry substance behind, the smaller 

 lighted plants weighed a good deal more than the larger ones 

 from the dark. It can always be accepted as true that respiration 

 entails loss of weight through the loss of carbon from the plant. 



We can now gather up the facts set forth in the preceding pages 

 in another of our generalizations, or verities, the energy indis- 

 pensable to the work of plants is principally provided by the oxida- 

 tion of carbonaceous food, and this is the essential feature of respira- 

 tion. 



In the statement of the foregoing verity the reader will notice 

 that I have used the word " principally," thus implying that 

 some other source of energy is available. In fact, while respiration 

 supplies by far the larger part of the energy used by organisms, 

 and especially by animals, they do derive some small part from 

 other sources, notably the heat of the surroundings. But this 

 part of the subject will all be elucidated later in this book. 



We are now face to face with a question of a very fundamental 

 sort, namely, what is the source of that energy which is thus 

 released from food in respiration? For everybody knows that 

 energy is not created upon the spot, but originates only by 

 transformation of pre-existing energy. In all science there is no 

 principle better established, or more important, than that of the 

 conservation of energy and matter, which teaches that the sum 

 total of both energy and matter in nature is constant, and that 



