V 
ON TIIE AGENCY AND FUNCTIONS OF LEAVES. 177 
this method I have a profusion of very fine blooms, from Oc¬ 
tober until May. All the attention they require in the winter 
is protection from severe frost, all the air possible being given 
them in fine weather. The compost to use in the frame is, two 
parts turfy loam, one part leaf mould. 
Cheltenham. J. Green. 
ON THE AGENCY AND FUNCTIONS OF LEAVES. 
We are asked by a correspondent, Why are the greater pro¬ 
portion of exotic plants evergreen, and what is the cause of 
certain others being deciduous ? This question embraces a 
wide range of botanic theory, and to explain it we must endea¬ 
vour to show the agency and functions of leaves, in so far as 
they are connected with this phenomenon. 
Physiologists are pretty well agreed in considering the func¬ 
tions of leaves to be in several respects very nearly assimilated 
to those of the lungs of animals: in the latter the blood is there 
distributed and spread out, as it were, through the very thin 
membranes composing these organs, to the action of the fresh 
air taken in by the act of breathing— the object being to sup¬ 
ply an opportunity for parting with a quantity of carbonic acid 
gas with which the blood, in its then crude state, is loaded, and 
also for obtaining a portion of pure oxygen derived from the at¬ 
mosphere. 
A material difference, however, exists between the two, 
arising from their different positions. The animal lungs per¬ 
form all their offices in the dark, while those of the vegetable 
are alternately in the light and darkness. Hence the assimi¬ 
lation by leaves differs by day and night. In the day, or when 
subject to the influence of light, leaves, or rather the sap which 
is distributed over the upper surface of the leaf immediately 
beneath the epidermis or thin transparent covering of the 
same, gives off a considerable portion of the fluid matter it is 
composed of, and at the same time the oxygen contained in 
the carbonic acid gas previously taken in by the roots and also 
by the leaves themselves when in the dark: but, though the 
oxygen is given off, the carbon remains; and this, with the les¬ 
sened amount of water by evaporation, reduces the sap to a 
18 
VOL. IV. NO. X. 
