372 
NATURAL HISTORY. 
cut transversely, a large quantity of fluids is discharged, as 
has been demonstrated by Dr. llales. 
Plants, as well as animals, perspire ; and in both cases this 
function is essential to health. By the experiments ol Di. 
Hales, and M. Guettard, it appears, that the perspirable mat- 
ter of vegetables dill’ers in no respect from puie watet, ex- 
cepting that it becomes rather sooner putrid. I he quantity 
perspired varies, according to the extent ot the suiface bom 
which it is emitted, the temperature of the air, the time of the 
dav, and the humidity ot the atmosphere. As the leaves tot m 
the greatest part of the surface, it is natural to suppose that 
the quantity of these will very materially affect the quantity ot 
the perspiration. Accordingly, the experiments ot Di. Hales 
have ascertained, that the perspiration of vegetables is in- 
creased or diminished, chiefly in proportion to the increase 
or diminution of their foliage. The degree of heat in which 
the plant was kept, according to the same author, varied the 
quantity of matter perspired, this being greater in propor- 
tion to die greater heat of the surrounding atinospheie. I he 
decree ot lmht has likewise considerable influence in this 
respect: fo? Mr. Philip Miller’s experiments prove that 
plants uniformly perspire most in the forenoon, though the 
temperature of the air in which they are placed should be 
unvaried. M. Guettard likewise informs us, that a plant ex- 
posed to the rays of the sun has its perspiration increased to 
a much greater degree than if it had been exposed to the 
same heat under the shade, finally, the perspiiation of 
vegetables is increased in proportion as the atmosphere is 
dry, or in other words, diminished in proportion as the 
atmosphere is humid. 
The more vigorousandhealtliy the plants, the more copious 
the perspiration ; this function, like the rest, depending much 
on the vital energy. Excessive perspiration seems to hurt, and 
even sometimes to destroy vegetables ; defective perspiration 
is equally injurious. It is also found, that this function is 
performed chiefly, if not altogether, b v the leaves and young 
shoots. That it may be properly carried on, all leaves are 
deciduous; in those trees called ever-greens, there being a 
constant succession of leaves, to prevent the organ of per- 
spiration from becoming rigid. 
Dr. Hales first observed, that a quantity of moisture is ab- 
sorbed by plants, when exposed to a humid atmosphere, 
this absorption, as well as the perspiration, is performed by 
the leaves; but in what manner has not yet been ascertained 
Experiments made by M. Guettard shew, that perspiration is 
more considerable from the upper than from the under surface 
