158 VEGETABLE PHYSIOLOGY 



the percentage rises, though the plant does not gain in 

 weight proportionately. The optimum quantity appears to 

 be about 10 per cent, with light of the ordinary intensity. 

 More than this gradually exerts a paralysing influence on 

 the chloroplast, and sets up consequently an inhibition of 

 the apparatus. The optimum amount of carbon dioxide 

 varies, however, considerably with the intensity of the 

 illumination and the temperature. Inhibition can be 

 caused also by the accumulation of the products of the 

 activity of the plastids, a concentration of the sugar 

 amounting to 8 per cent, being sufficient to bring it about. 



The mechanism is an exceedingly delicate one and can 

 be thrown out of gear by various external agencies. 

 Ewart has shown that it can be inhibited by heat, cold, 

 desiccation, partial asphyxiation, prolonged insolation, and 

 by the action of dilute alkalis or mineral acids. 



We mentioned at the commencement of this chapter 

 that the chlorophyll apparatus is concerned in the manu- 

 facture of almost the whole of the organic material of the 

 globe. In a few humble organisms the construction of 

 such material can proceed without its help. These are 

 certain bacteria which can transform ammonia compounds 

 into salts of nitrous and nitric acids, growing and multi- 

 plying at the expense of the products they thus obtain 

 together with carbon dioxide. There are two kinds of 

 these bacteria, one of which oxidises ammonia to nitrous 

 acid and the other converts this into nitric acid. They 

 grow freely in the soil and multiply with considerable 

 rapidity, the result being the formation of certain quanti- 

 ties of organic substance. They cause the carbon dioxide to 

 enter into combination, this gas being normally the only 

 source of their supply of carbon. They possess no chlorophyll 

 and consequently cannot utilise directly the radiant energy 

 of the sun. Their energy is apparently derived from the 

 oxidation of the nitrogenous compounds which they attack. 

 Nothing is known at present of the steps by which the 

 synthesis of the organic matter takes place. 



