80 VEGETABLE RESPIRATION. 
tables growing upon them, that soils can at last become able 
to support the growth of more perfect vegetables. Za 
* 276. Light, it is also known, is absolutely necessary for 
the healthy existence of the plant, and its operation, when it 
is in full power, seems to consist in causing carbon to be 
deposited, as plants grown in the dark are tender, feeble, 
and insipid, wanting many of those properties which, it is 
probable, must depend on the presence of carbon. if 
plant be kept in the dark or etiolated, the green parts be- 
come of a sickly white colour, and indeed the whole plant 
becomes soft and feeble. There is a deficiency of carbon, 
which is necessary to the firmness and stability of the plant 
and the development of the green colour. This is well 
seen in the Garden or Heading Cabbage, the internal leaves 
of which are white and tender, while the external ones are — 
strong and fibrous, and have the green colour properly de- 
veloped. When vegetation proceeds in the dark, the plant 
loses its peculiar virtues, and all the fluids are nearly alike, 
possessing a mild sweetish taste. »,Too much light, or great 
intensity of light, by increasing the evaporation and the 
accumulation of carbon, renders all the parts of the plant 
more solid, stiff, and hard, and thus impedes its free growth. 
277. These considerations, then, viewed by themselves, 
and in the absence of any decisive experiments,would see™ 
to lead to the opinion, that plants do gain carbon by their 
~ action on the air. “It may be observed, on the other hand, 
that vegetables can, in most cases, acquire a sufficient quaD- 
tity of carbon in the soil, and that, when there is a sufficient 
"provision for any end ead one source, it is not likely that 
there should be another means constantly: in operation con- 
ducing to the same end. This objection gains still more 
weight when we consider that leaves actually do, in certain 
_ circumstances, give out carbon (in the form of carbonic acid) 
rather an anomaly if they also take it in. Did we find that 
vegetables only take in carbon from the air when they cam- 
