130 CHAP. XVI. PHOTOSYNTHESIS WITHOUT CARBON DIOXIDE 



Similar results were obtained with oxalic acid, the 

 solutions of which had, however, to be made ten times more 

 dilute to guard against any poisonous effect. 



I have stated that all the Hydrilla plants became strongly 

 acid in the course of a few weeks of April, during transition 

 from spring to summer. The heat was greatly modified by 

 the rainy season, which commenced in July. The result 

 was that, by the third week of July, the Hydrilla plants 

 reverted to the normal non-acid condition. It was also 

 most interesting to find that their photosynthetic activity 

 had also returned to the normal — that is to say, that the 

 plant no longer exhibited any photosynthesis in the absence 

 of an external supply of C0 2 . The relation of the acid 

 condition of the plant to its power of assimilation of organic 

 acid is therefore fully established. 



Though land-plants obtain their main supply of carbon 

 dioxide for assimilation from the air, it is not impossible 

 that a portion may also be derived from the carbonic 

 acid dissolved in water in the soil. There is yet another 

 possible source of supply of carbon dioxide for assimilation. 

 In my work on the ' Physiology of the Ascent of Sap ' I have 

 shown that the ascent of water is maintained by the pump- 

 ing action of actively pulsating cells situated in the inner 

 cortex. The work performed in raising large masses of 

 water to the top of the tree is considerable, and necessi- 

 tates a corresponding consumption of energy. It is obvious 

 that organic substances in the actively pulsating cells will 

 be rapidly oxidised in order to supply the necessary energy. 

 The carbon dioxide thereby produced will be carried in 

 the same direction as the ascending sap along aeriferous 

 systems, reaching the large intercellular spaces in the 

 leaves. Here two streams of carbon dioxide will be avail- 

 able for assimilation, from the exterior as well as from the 

 interior. Since plants can exploit their own organic acids 

 in assimilation, it would be surprising if they did not 

 utilise the carbon dioxide produced in their tissues by 

 internal respiration. 



