THE CHLOROPHYLL APPARATUS 163 
the temperature. Inhibition can be caused also by the 
accumulation of the products of the activity of the plastide, 
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 mhibited by heat, cold, desiccation, 
partial asphyxiation, prolonged insolation, and by the action 
of dilute alkalies 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. 
A process which at first appeared to involve a mechanism 
resembling that of the chlorophyll apparatus was discovered 
some years ago by Engelmann. Certain bacteria which 
contain a purple pigment were found to possess the power 
of photosynthesis. The pigment was thought to be allied 
to chlorophyll and to possess the same power of absorbing 
and utilising the radiant energy of light. Recent researches 
LL 
