INTRODUCTORY 15 



moderate, the healthy plant is found to increase in size, 

 and gradually to show all the phenomena of growth. 

 Hence these various compounds have been regarded as 

 its food. This is not, however, a correct view, for they 

 should be considered only as raw materials from which 

 the green plant can make the food it needs. This is 

 effected by the agency of the green colouring matter, 

 the so-called chlorophyll, but only when it receives an 

 appropriate amount of light. In the absence of chloro- 

 phyll or in insufficient light the supply of all these 

 various compounds does not afford any nourishment to 

 the plant. Plants without chlorophyll are not far to 

 seek; we find them in the mushrooms, in the moulds 

 that grow so readily on decaying matter, the mildews of 

 corn and other crops, and so on. These cannot develop 

 at all when supplied only with the inorganic compounds 

 mentioned. 



The plant then in order to grow and to establish itself 

 has to be provided with suitable food. If it has chloro- 

 phyll and is properly illuminated it makes this food for 

 itself from the inorganic materials the soil and air pro- 

 vide. Plants which cannot make their food have to 

 obtain it from living or dead organic matter. Though 

 this is difficult it is not impossible, for such matter 

 abounds almost everywhere not only in the soil but in 

 the numerous manufactured products which we meet 

 with all around us. Living organisms also are often 

 made to yield food to these non-green plants. The 

 chlorophyll-containing plants are continuously making 

 the organic substance which constitutes their food as 

 long as light shines upon them. We find them growing 

 at its expense and accumulating large quantities of such 

 substances as sugar, starch, proteins, and fats in their 

 own bodies. As they in their turn, or many of them, 

 ultimately become the food of animals, we may see 

 their importance in the role of nature. The fact is that 



