no 
nished by plants proceeds from the decomposi- 
tion of carbonic acid-, and since this decomposition 
is effected by the joint action of the green parts 
of vegetables and of solar light ; why, it may be 
asked, are these parts so exclusively concerned in 
this operation, and what are those peculiarities of 
structure or of composition, which thus enable them 
to produce changes in the air, so different from those 
which all the other parts of the vegetable perform, 
even in sunshine, and so contrary to their own pro- 
per functions in the shade ? This operation has been 
shewn to be in no respect necessary to the life of 
plants ; for they live and grow in situations where they 
afford no oxygen gas. Neither can it be considered 
as a necessary result of vegetable organisation ; for 
the structure of white leaves, which dFord no oxy- 
gen, is as perfectly developed as that of green 
leaves, which yield it in abundance ; and when plants 
are successively rendered green, or white, by the al- 
ternate admission or exclusion of the solar rays, in 
which states they respectively furnish a pure or im- 
pure air, it cannot be" supposed that the vegetable or- 
ganisation has undergone such material changes as 
should qualify it thus rapidly to present such con- 
trasted results. 
352. This decomposition of carbonic acid, which 
thus gives birth to oxygen, we have likewise seen to be 
effected in the parenchymatous structure of the leaf, 
and the agency of the solar rays appears to be essen- 
tial to its completion. But it is in the same part of 
the plant that its colourable juices reside, and these 
juices, also, acquire their green colour from the di- 
