178 
THE FLORIST’S JOURNAL. 
sort of pulp. This pulp, like the animal blood, is destined to 
suppty fresh materials for the increase of the body of which it 
forms part. Here exists the difference: the animal is con¬ 
stantly consuming oxygen, and giving off carbon, while the 
vegetable, on the contrary, in bright sun-light, parts with a 
large proportion of pure air or oxygen and assimilates the con- 
trary gas; thus rendering the atmosphere more pure and fit for 
man’s use. Were it not for this wise provision, it is more than 
probable that thickly wooded countries would be uninhabitable ; 
as during the night the action of leaves upon the atmosphere is 
the same as that of the animal lungs, namely, a consuming or 
inspiration of oxygen and an expiration of carbonic acid, though 
in a less degree than that of the first. 
The pulp before mentioned is contained in minute tubes or 
cells; and Sennebier explains the causes of green being the 
predominant colour in the foliage ot vegetables in this way. 
The sap tubes or cells being yellow, and the pulp itself, from 
the residue of carbon contained in it, of a dark blue, produce 
between them the green colour of the leaves and young shoots ; 
and it is with the nature of this pulp the chief portion of our 
present question rests. 
It has been correctly ascertained by various experiments, that 
the quantity of oxygen given off depends entirely on the in¬ 
tensity of light to which the leaves are subject; an increase of 
the one being followed by an increase of the other in a corre¬ 
sponding ratio ; and in like manner the amount of evaporation 
is determined by the amount of surrounding heat: so that it ap¬ 
pears evident that plants in a tropical climate must, from affect¬ 
ing circumstances, as the stronger light and more intense power 
of the sun than we experience in our more temperate latitudes, 
lose a much larger proportion of aqueous matter; the pulp, 
therefore, is rendered thicker; and, to afford it a passage, the 
vessels are by gradual distensions made larger and stronger, 
and are not so soon obstructed. This, it must be observed, is 
brought on by very gradual enlargements; and the same cause 
which operates thus on the sap increases also, in thickness 
and strength, the outer covering of the whole leaf: and it is 
from this cause that our oaks, and other deciduous plants, as¬ 
sume the character of evergreens when transplanted to a tropi¬ 
cal climate. 
