RETURN OF THE SAP. 33 



large quantity of water, after undergoing in the leaves, as in 

 a chemical laboratory, the double processes of exhalation and 

 aeration, has become much more highly charged with nutri- 

 ment; and that nutriment has been reduced to those particu- 

 lar forms and states of composition which render it applica- 

 ble to the growth of the organs, and the other purposes of 

 vegetable life. This fluid, therefore, corresponds to tlic 

 blood of animals, which, like the elaborated sap, may be re- 

 garded as fluid nutriment, perfectly assimilated to that par- 

 ticular kind of organization, with which it is to be afterwards 

 incorporated. From the circumstance of its being sent back 

 from the leaves for distribution to the several organs where 

 its presence is required, it has received the name of the re- 

 turning sap, that it might be distinguished from the crude 

 fluid which arrives at the leaves, and which is termed tlie 

 ascending sap. 



The returning sap still contains a considerable quantity 

 of water, in its simple liquid form; which was necessary in 

 order that it might still be the vehicle of various nutritive 

 materials that are dissolved in it. It appears, however, that 

 a large proportion of the water, which was not exhaled by 

 the leaves, has been actually decomposed, and that its sepa- 

 rated elements, the oxygen and the hydrogen, have been 

 combined with certain proportions of carbon, hydrogen, ni- 

 trogen and various earths, metals and salts, so as to form the 

 proximate vegetable products, which are found in the re- 

 turning sap. 



The simplest, and generally the most abundant of these 

 products, is that which is called Gum:^ From the universal 

 presence of this substance in the vegetable juices, and more 



* According- to the investig-ations of Dr. Prout, 1000 grains of gum are 

 composed of 586 grains of the elements of water, that is, of oxygen and 

 hydrogen, in the exact proportions in wliich they would have united to 

 form 586 grains of water; together witli 414 of carbon, or the base of carbo- 

 nic acid. This, according to the doctrine of chemical equivalents, corre- 

 sponds to one molecule of water, and one molecule of carbon. I'hil. Trans. 

 1827, 584. 



Vol. 11. 5 



