LECTURE XVIII. 



THE PRODUCTION OF THE ORGANIC SUBSTANCE OF 

 PLANTS— ASSIMILATION. 



Having learnt in the last lecture what are the materials which a normal green 

 plant must absorb from without, in order to build up its substance from 

 them, we will now examine the question how and under what conditions the 

 organic substance is manufactured. Since we know almost nothing as to the ways and 

 means by which the constituents of the ash co-operate in these processes, we may limit 

 our considerations to four points. In the first place, the decomposition of carbon 

 dioxide may be treated of This furnishes, as we know already, the whole of the 

 carbon necessary for the manufacture of the substance of normal plants with green 

 leaves. In the second place, proof is to be given that the cells containing chloro- 

 phyll — or, more properly speaking, the chlorophyll-corpuscles only — are the organs 

 by means of which organic substance is manufactured from carbon dioxide. Thirdly, 

 it is to be observed that the chlorophyll is only effective as an instrument of assimila- 

 tion when the vibrations of the ether which are perceptible to our eyes as light 

 penetrate into its substance, and impart to it the forces necessary to produce organic 

 substance from carbon dioxide and water. Finally, we wish to know what is the 

 immediate result of the 'decomposition of the carbon dioxide in the chlorophyll 

 under the influence of light. 



Concerning ourselves first with the decomposition of carbon dioxide, we may 

 again confine our remarks, for the sake of clearness, to the artificially nourished 

 plant referred to in the previous lecture. The substance of this plant is combustible, 

 and after combustion it leaves behind a small quantity of ash, consisting of the 

 salts taken up by the roots, and which we know to be indispensable for the 

 production of organic substance. The fact that the organic substance of the plant 

 is combustible, however, essentially implies that it consists of chemical compounds 

 poor in oxygen, which are in a condition to become oxidised on the entrance of 

 atmospheric oxygen at a red heat, and hence to be transformed for the most part 

 into carbon dioxide and water. Just as organic plant-substance is produced from 

 compounds rich in oxygen — carbon dioxide and water (with the co-operation of 

 other compounds rich in oxygen) — so also they may be brought back again into 

 the form of the same compounds by oxidation. The fact that plant-substance is 

 combustible, itself implies that in its production from non-combustible compounds 



