44 VITAL ACTIONS. 



should be in proportion to the quantity of light it 

 receives by day. 



68. But, while this is true as a general axiom, it is 

 necessary to observe that some plants are naturally 

 inhabitants of shady situations, and are so organised 

 as to be fit for such places and for no others : plants 

 of this description will not endure full exposure to the 

 sun ; not because an abundant decomposition of car- 

 bonic acid is otherwise than favourable to them, but 

 because their epidermis allows the escape of water 

 too freely by insensible perspiration, under the solar 

 stimulus. 



69. The mere fact of plants absorbing fluids from 

 the earth, would render it probable that they have 

 some means of parting with a portion of it by their 

 surface ; but that they do perspire is susceptible of 

 direct proof, and is by no means a mere matter of 

 inference. 



70. "We do not indeed see vapour flying off from 

 the surface of plants ; neither do we from that of ani- 

 mals, except when the air is so cold as to condense 

 the vapour ; yet we know that in both cases perspira- 

 tion is perpetually going on, and it would appear that 

 in plants it takes place more abundantly than in ani- 

 mals. If a plant covered with leaves is placed under 

 a glass vessel, and exposed to the sun, the sides of the 

 vessel are speedily covered with dew, produced by 

 the condensation of the insensible perspiration of the 

 plant. If the branch of a plant is placed in a bottle 

 of water, and the neck of the bottle is luted to the 

 branch, so that no evaporation can take place, never- 



