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 wider 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 vaW 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 transpu-e from 20 to 30 oz. of water daily; a cabbage from 15 to 25 oz., &c. 

 Experiments have also been made upon single leaves, recently plucked, with the 

 petiole immersed in water. Thus a leaf of the sunflower, Aveighing 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 Orchidacea, it is 

 evident that the plant must derive its nomishment chiefly from the absorption 

 performed by the leaves. Experiment also proves that the leaves of plants in 

 general are capable of this function. Ever}' one knows hoAv 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 absoi-p- 

 tion. This is readily shown by experiment. Leaves with their lower surfaces in 

 contact with the Avater, remain fresli much longer than others Avith their upper 

 surfaces thus placed. Leaves of the white mulbeny, Avith the upper surface only 

 in contact with Avater, faded in six days, Avhile others, rcA^ersed in position, lasted 

 as many months. 



275. PvESPiRATioN 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 Avith another Avhich occurs, of a con- 

 trary nature, treated of under the licad of digestion. 

 9* 



