LECT. II.] VITAL FUNCTIONS OF PLANTS. 59 



pieces, each part, as happens with the slips or 

 cuttings of plants, becomes a perfect animal; and 

 jf part of a bone, or of a muscle, be taken away, 

 it is reproduced by the vital powers "of the animal; 

 but the cutis, like the leaves of plants, when once 

 destroyed, is never again restored. 



Plants absorb and transpire watery fluids by 

 their surface. If a plant, or part of one, as the 

 branch of a tree, be placed in a humid air, as long 

 as either of them lives, it will absorb the moisture 

 from the air by its surface, and augment in weight. 

 On the contrary, in a dry air of an increased tem- 

 perature, it transpires fluid in the form of invi- 

 sible vapour, in the same manner as animals part 

 with what is termed their insensible perspiration. 

 The intention of this function of plants is evidently 

 to throw off the superabundant water, which is 

 necessary for keeping the nutriment absorbed by 

 the roots in a state of extreme solution ; but 

 which is no longer useful when the sap is about to 

 be exposed to the action of the air in the leaf, and 

 returned for the purposes of secretion and assimi- 

 lation. The powers of absorption of the roots, 

 therefore, must be regulated by the quantity of 

 perspiration thrown off by the leaves and surface 

 of the stems and branches ; and Dr. Hales found 

 by experiment that the quantity of transpired fluid 

 was rather more than equal to the weight of the 

 water imbibed by the roots. Plants., however, which 



