302 PRINCIPLES OF BOTANY, ETC. 
torrid zone is very great. ‘The second, or St John’s 
shoot is, properly speaking, only a continuation of 
the first. ‘The first shoot is pushed forward by the 
old stock of sap which had been collected, the se« 
cond, by the sap which is still forming during fa- 
vourable weather. | 
§ 281. 
The green colour with which all the vegetable 
creation is invested, is a most cheerful sight, and it 
is but natural to suppose, that the investigation of its 
cause has always and long ago attracted the atten- 
tion of philosophers, and given rise to many hypo- 
theses. When phlogiston still had a number of ad- 
herents, the explanation of the green colour was 
very easy, it was considered as an effect of this prin- 
ciple. Since, however, the idea of its existence has 
been given up, different ways of explaining the na- 
ture of this green colour have been devised. We 
shall not at present enumerate them all, but merely 
notice the opinions and observations of late philoso- 
phers. Berthollet says, that the green of plants 
does not consist of blue and yellow, as the prism 
does not separate their green, like that of other 
bodies, into yellow and blue rays. 
After extracting with alcohol the green colour 
from the leaves, and exposing this mixture to the 
sun or atmosphere, the green colour disappears en- 
tirely. The oxygen of the atmosphere combines 
with the mixture, and banishes the colour. If a 
solution of ammonia, which consists of hydrogen 
and azote, be dropped into it, the oxygen parts with 
the 
