190 Transactions of tee American Institute, 



juice or latex by the combined action of exhalation, absorption, 

 respiration and digestion. By exhalation, the superabundant water 

 of the sap is given off to the atmosphere, and the remaining sap 

 thus reduced, as it were, by concentration. It is analogous to per- 

 spiration in animals. Although absorption is chiefly performed by 

 the roots, the leaves also absorb in a less degree, and thus upon 

 occasion may, for a limited time, supply the want of full action on ; 

 the part of the roots. Respiration is analogous to breathing in ' 

 animals, and though performed chiefly by the leaves, consisting of 

 the absorption of oxj^gen from the atmosphere, and the evolution 

 of carbonic acid, is also partially performed by other parts, even 

 by the roots. It is carried on with peculiar activity during germi- 

 nation and flowering, and its eflect appears to be: 1st, the ridding 

 the plant of a portion of carbonic acid, as respiration in animals 

 removes the same acid from the blood; and 2d, the conversion of 

 starch into sugar, in germination for the nourishment of the young 

 plant, and in flowering for the nutrition of the pollen and the 

 ovules. Digestion consists in the decomposition of carbonic acid 

 by the green tissues of the leaves, under the stimulus of light, the 

 fixation of solid carbon, and the evolution of pure oxygen. The 

 crude sap then comes to the leaves a thin watery fluid, and by 

 the elaboration spoken of, is converted into true sap, and descends 

 by the lactiferous tissue of the liber, a part carried inward by 

 the medullary rays, is difiused through the whole stem, and the 

 remainder descends to and is in like manner difi'used throug^h 

 the roots, both for their nourishishment and to maintain the condi- 

 tions needed to insure the eudosmose current. At the end of 

 spring, a portion of the sap, now transformed into a viscid, glu- 

 tinous matter, called cambium, is deposited between the liber and 

 the wood, becomes organized into cells, and forms a new layer upon 

 each. After the fall of the leaves, and the consequent cessation 

 of all elaboration of sap, the roots still continue to act, and an 

 accumulation of sap takes place, and the plant becomes surcharged 

 with sap, so that instead of the sap retiring to the roots, there to 

 hybernate during the winter, like a bear in his den, leaving the 

 upper portions of the plant in a measure free from sap, these por- 

 tions generallj'- have an unusual quantity, and upon cutting through ; 

 the sapwood, this superabundance will cause a flow of sap, as in 

 the sugar maple, but after the leaves begin to act again, the flow 

 will cease. The sap ascends with extraordinary copiousness at two 

 periods of the year, spring and autumn, especially spring, and most 



