138 
391. We have already seen, that, in a very weak 
and diffused light, some plants appear to decompose 
carbonic acid, and to retain their verdure ; and many 
leaves also, which grow in the shade, are not less 
green than those which are fully exposed to the sun. 
All such facts may be easily explained, on the suppo- 
sition that alkaline matter naturally superabounds in 
the plant, and produces its specific effects on their 
colourable juices ; while, in other cases, the direct 
agency of light is more or less necessary to withdraw 
and decompose the acid, whose presence prevents 
the appearance of the green colour. It is, however, 
highly probable,, as M. de Saussure observes, that 
carbonic acid is decomposed in plants, even in a very 
weak light (312.), although, from the slowness of 
the operation, and the immediate consumption of the 
oxygen by vegetation, the fact cannot be easily de- 
tected. Such an opinion permits us to suppose a 
redundance of alkaline matter, without resorting to 
the less obvious supposition of an unusual supply of 
it, to counteract the effects of the retained acid. 
392. Besides those transitions from red to green, 
which the young leaves of certain plants have been 
shewn to exhibit, there are other leaves which pre- 
sent contrary characters, being at first nearly green, 
and becoming at length perfectly and permanently 
red. The red cabbage affords an example of this 
kind ; and a comparison of its peculiarities with those 
af the green variety may assist us in explaining its 
cause. In the common cabbage, all the leaves which 
are exposed to light, acquire a green colour, while 
those in the centre or heart of the plant have but lit- 
