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 which it may be susceptible. And in woody 

 trees, also, a similar phenomenon occurs : it is well known to 

 gardeners, that, if a season occurs in which trees in a state of 

 maturity ai'e 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 accumidated 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, which they partially 

 absoi'b 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 



