FUNCTION. 185 



ated in their functions : by the roots, water and mineral 

 substances are absorbed ; while the leaves take in. and de- 

 compose, carbonic acid. Nevertheless, leaves retain a con- 

 siderable power of absorbing water : and in what are popu- 

 larly called " air-plants," the absorption of water is wholly 

 carried on by them and bv the stems. Converselv, the under- 



V / */ ' 



ground parts can partially assume the functions of leaves : 

 the exposed tuber of a potato develops chlorophyll on its 

 surface, and in other cases, roots, properly so called, do the 

 like. In trees, the trunks, which have in great measure 

 ceased to produce buds, recommence producing them if the 

 branches are cut off; and under such circumstances the 

 roots, though not in the habit of developing leaf-bearing 

 organs, send up numerous suckers. Much more 



various examples of vicarious function may be found among 

 animals. Starting with the extreme case of the common 

 hydra, which can live when the duties of skin and stomach 

 have been interchanged by turning it inside out, we find in 

 all grades, even up to the highest, that absorbent and excret- 

 ing organs can partially supply each others' places. Among 

 well-organized animals, the taking in of nutriment is ef- 

 fected exclusively by an internal membrane ; but the external 

 membrane is not wholly without the power to take in nutri- 

 ment : when food cannot be swallowed, life may be pro- 

 longed by immersing the body in nutritive fluids. The ex- 

 cretion of carbonic acid and absorption of oxvsren, are mainly 



JL v O * 



performed by the lungs, in creatures which have lungs ; but 

 in such creatures there continues a certain amount of cutane- 

 ous respiration, and in soft-skinned batrachians like the frog, 

 this cutaneous respiration is important. Again, when the 

 kidneys are not discharging their duties, a notable quantity 

 of urea is got rid of by perspiration. Other 



instances are supplied by the higher functions. In man, 

 the limbs, which among lower vertebrates are almost wholly 

 organs of locomotion, are specialized into organs of locomo- 

 tion and organs of manipulation. Nevertheless, the human 



