140 LECTURE IX. 



one ; it is simply the expression of the fact that the gain of 

 the plant is greater than its loss. Further, another portion of 

 the organic substance is in all cases stored-up in various 

 forms as reserve-material, in connexion either with the pro- 

 duction of new individuals or with the renewed growth of 

 the plant itself at a subsequent period. For example, reserve- 

 material is deposited in all seeds and spores to serve as food 

 to the plant to be developed from them during the early 

 stages of its growth. Again, the most various parts of 

 perennial plants, stems, rhizomes, bulbs, roots, buds, etc., 

 contain during the winter, in our climate, stores of reserve- 

 material, which are deposited there whilst the plant is actively 

 assimilating in the summer, and which will supply it with the 

 means of recommencing its growth in the spring. 



In the last lecture we ascertained what substances consti- 

 tute the food of plants ; we have now to enquire into the 

 nature of the constructively metabolic processes by which 

 the comparatively simple food-materials are converted into 

 complex organic substances. 



I . The Formation of Non-nitrogenous Organic Stibstance. 



We will take as our starting-point the fact mentioned 

 in a previous lecture (p. 81), that green plants and parts of 

 plants absorb carbon dioxide when exposed to bright light, 

 and that the absorption of carbon dioxide is accompanied 

 by an evolution of oxygen. We will endeavour to ascertain 

 the nature of the chemical processes of which this inter- 

 change of gases is an expression, and to determine the con- 

 nexion of these processes with the presence of chlorophyll 

 and with the action of light. 



We shall find it convenient in discussing this subject, to 

 trace step by step the historical development of our know- 

 ledge concerning it. Priestley was the first to observe that 

 green plants absorb carbon dioxide and evolve oxygen under 

 the influence of sunlight, or, as he put it, " dephlogisticate " 

 the air. Then Ingenhousz drew special attention to the 

 influence of sunlight upon this process, proving that it is 

 not the heat of the sun but its light which is of importance. 



