INFLUENCE OF LIGHT AND HEAT ON EXHALATION. 171 



any protection, the substance of plants habitually living under 

 water; and this evaporation will be the more considerable, as 

 the surface exposed in these passages is much greater than that 

 of the leaf itself. The exhalation of fluid from the living plant, 

 then, may be regarded in part as a kind of evaporation from its 

 interior, and will be promoted by the warmth and dryness of the 

 air around ; but it is entirely controlled by the stomata, which, 

 by admitting or excluding the air, permit or check it in accord- 

 ance with the influence of light upon them. Still it cannot be 

 regarded as entirely due to evaporation ; as fluid is sometimes 

 exhaled more rapidly than it can be removed by that process 

 (. 265); and we are probably to regard this secretion as really 

 analogous to the cutaneous exhalation of Animals. (See ANIM. 

 PHYSIOL., . 371, 372.) 



259. Thus, then, we see one important mode, in which light 

 influences the growing Plant. No amount of heat can supply a 

 deficiency of this agent ; for, if it be excluded, exhalation is en- 

 tirely prevented ; and all the fluid that is transpired, has to pass 

 off' by the slow process of evaporation from the external surface 

 only, which is not nearly sufficient for the required concentra- 

 tion of the sap. Moreover, when the exhalation is checked, 

 absorption soon ceases ; for the tissues become gorged with 

 fluid, and are capable of containing no more. If a plant, ac- 

 customed to grow in open day, be kept for some time in the 

 dark, it becomes unhealthy, and, as it were, dropsical ; and 

 will generally die, if not restored to its usual condition. This 

 is not, however, the only process performed by the leaves, which 

 is checked by the want of light ; and therefore the unhealthim ss 

 which results cannot be imputed to it alone (. 288). 



260. We are now prepared to understand, then, the sliaie 

 which the leaves have, in promoting and maintaining the ab- 

 sorption of fluid by the roots. The exhalation which takes 

 place in the leaves, has a corresponding effect with the combus- 

 tion of oil at the top of the wick of a lamp. occasioning a con- 

 tinual demand for fluid from below. If the flame be extinguished, 

 the oil does not flow over the top of the wick, because the ab 

 sorption of it ceases also ; and so the action of the roots is 



