RESPIRATION. 99 
alone. But since these are opened by the influence of the light 
and closed in its absence, it follows that exhalation can proceed 
only in the presence of the light (155). 
a. If a plate of glass be held near the wnder surface of an active leaf of the 
Hydrangea, in a still air, it will soon be covered with dew; but if the experiment 
be repeated by holding the glass over the upper surface, it will remain dry. 
Again, if the light be suddenly excluded from the plant in a state of active 
growth, it will immediately cease to transpire, whatever be the temperature; and 
if the stomata be then examined they will be found closed. 
272. That exhalation and absorption by the roots are mutually dependent upon 
each other, has already been stated (155). The quantity of fluid discharged by 
the former may therefore be inferred from that of the latter. This has also been 
confirmed by experiment. A sunflower 3} feet high, was ascertained by Hales 
to transpire from 20 to 30 oz. of water daily; a cabbage from 15 to 25 oz., &e. 
Experiments have also been made upon single leaves, recently plucked, with the 
petiole immersed in water. Thus a leaf of the sunflower, weighing 31 grains, 
absorbed and exhaled its own weight of water in 6 hours. 
273. ABSORPTION is primarily the office of the roots (154), but 
in certain circumstances it is performed by the leaves also. 
a. When the roots are imperfect, or wanting, or serve merely to fix the plant 
in its position, as in some aerial parasites, and in some of the Orchidacew, it is 
evident that the plant must derive its nourishment chiefly from the absorption 
performed by the leaves. Experiment also proves that the leaves of plants in 
general are capable of this function. Every one knows how plants, when parched 
and withered by drought, are revived by a shower which does not reach their 
roots, but only moistens their leaves. 
274. ‘The lower surface of the leaf appears to be chiefly instrumental in absorp- 
tion. This is readily shown by experiment. Leaves with their lower surfaces in 
contact with the water, remain fresh much longer than others with their upper 
surfaces thus placed. Leaves of the white mulberry, with the upper surface only 
in contact with water, faded in six days, while others, reversed in position, lasted 
as many months. 
275. REspiraTion in plants is analogous to respiration, or 
breathing, in animals. In both it is equally constant and equally 
necessary. It is performed principally by the leaves, but is not 
confined to them, being partially performed by other parts also, 
even by the roots. 
276. Respiration consists of the absorption of oxygen from 
the atmosphere, accompanied by the evolution of carbonic acid. 
a. This process must not be confounded with another which occurs, of a con- 
trary nature, treated of under the head of digestion. 
mr: g* 
