212 STRUCTURAL BOTANY 



greater, but rather less, than that of the tuber from 

 which it has developed. It has grown entirely at the 

 expense of the food-substances ready stored in the 

 tuber ; a great deal of water has been absorbed, but 

 no new organic substance has been formed ; on the 

 contrary, some has been lost, for a reason which we 

 shall explain presently. The plant, in fact, has earned 

 nothing, but has been living on its capital, and has 

 naturally become poorer by doing so. 



The same thing would happen if a plant were 

 allowed to grow in the light, but in an atmosphere 

 destitute of carbon dioxide. For the purpose of this 

 experiment the specimen is grown under a bell-glass, 

 into which air is admitted through a tube, containing 

 pieces of pumice-stone soaked in a solution of caustic 

 potash (KOH). The potash absorbs the carbon 

 dioxide, so that none reaches the plant. Here also 

 the result is that no fresh organic material is formed ; 

 if the plant grows at all, it does so at the expense 

 of the food - substances which it had previously 

 stored up. 



We may sum up our conclusions up to this point 

 as follows : 



Green plants, as a rule, obtain all their carbon by 

 the decomposition of the carbon dioxide of the 

 atmosphere. 



Consequently, in the absence of carbon dioxide, no 

 fresh organic substance is formed. The decomposition 

 of carbon dioxide can only take place under the 

 action of light (the red rays being the most effective) 

 and at a sufficient temperature. 



