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. 
