CHAP. II. FUNCTION OP NUTRITION. 183 



a large quantity at its cut extremity. This " adfluxion " 

 of the sap, as it has been termed, is clearly the result 

 of a different cause from that of its propulsion, explained 

 in the last article. 



THIRD PERIOD OP NUTRITION. 



(168.) Exhalation. A large portion of the water 

 imbibed by the spongioles is afterwards discharged at 

 the surface of the leaves, in a manner analogous to the 

 insensible perspiration of animals. This discharge may 

 be attributed to the operation of two distinct causes. A 

 very small portion is carried off by the ordinary 

 effects of evaporation, but a far greater quantity by 

 a process which has been named " exhalation," and 

 which is ascribed to the immediate action of the 

 vital force. That a certain portion of the discharge 

 must be due to the evaporation of the contained 

 fluid through the membranous coats of the vesicles, is 

 proved by the gradual desiccation of the succulent 

 parts- of dead plants, and by the effects observed, in the 

 preservation of pulpy fruits. But still, the effects of 

 evaporation alone are scarcely perceptible, when com- 

 pared with the rapid manner in which the fluid is dis- 

 charged from the surface of the leaf. It has been 

 ascertained that a common sunflower of three feet in 

 height, will exhale about twenty ounces of water every 

 day ; and a common-sized cabbage discharges moisture 

 at the same rate : so that the surfaces of these plants 

 exhale at a rate which is seventeen times greater than 

 that at which the insensible perspiration is given off 

 from the surface of the human body. 



(169.) Exhaling Organs. By comparing the effects 

 produced by the leaves of different species, it has been 

 found that those exhale the most which possess the 

 greatest number of stomata ; whilst those surfaces which 

 possess none, produce very little or no effect beyond the 

 ordinary loss sustained by evaporation. It is quite as 

 evident therefore that the stomata are the true exhaling 

 N 4 



