292 
PHYSIOLOGY. 
BOOK II. 
leaf, it rarely sets or arrives at maturity, but falls off soon 
after beginning to swell, from want of an accumulation of 
food for its support ; while, if the same plant is not allowed 
to bear fruit until it has provided a considerable supply of 
food, as will be the case after the leaves are fully formed, and 
have been some little time in action, the fruit which may then 
set swells rapidly, and speedily arrives at the highest degree 
of perfection of w^iich it may be susceptible. And in woody 
trees, also, a similar phenomenon occurs : it is well known to 
gardenei^, that, if a season occurs in which trees in a state of 
maturity are prevented bearing their usual crops, the succeed- 
ing year their fruit is unusually fine and abundant ; owing to 
their having a whole year's extra stock of accumulated sap to 
feed upon. 
The cause of the fruit attracting food from surrounding 
parts is probably to be sought in the phenomenon called endos- 
mose. All the sap that may be at first impelled into the fruit by 
the action of vegetation, not being able to find an exit, collects 
within the fruit, and, in consequence of evaporation, becomes 
gradually more dense than that in the surrounding tissue : it 
will then begin to attract to itself all the more aqueous fluid 
that is in communication with it ; and the impulse, once given 
in this way to the concentration of the sap in particular points, 
will continue until the growth of the fruit is completed, and 
its tissue so much gorged as to be incapable of receiving 
any more food, when it usually falls off. 
No one has studied the effects of fruit upon the atmosphere, 
and the nature of the chemical changes it undergoes, with 
more success than Theodore de Saussure and Berard, an 
account of whose discoveries I partly translate and partly con- 
dense from De Candolle. According to the first of these 
original observers, " Fruits, while green, whether leafy or 
fleshy, act much as leaves either in the sun or in shade, and 
differ from those organs principally in the intensity of their 
action. In the night they destroy the oxygen of their atmos- 
phere, and replace it with carbonic acid, w^hich they partially 
absorb again. This absorption is generally less in the open 
air than under a receiver ; and, their volume remaining the 
same, they consume more oxygen in darkness when distant 
