34 SYLVA AMERICANA. 



passages through the cortical vessels into the cavities of the 

 cellular tissue for elaboration, such portions of it as are to be 

 applied to the purposes of nutrition, or to those secretions that 

 are necessary for the preservation of the plant. 



The functions of the leaves have been compared to the 

 respiration of animals, by which the blood parts with its superflu- 

 ous water, and acquires new principles from the atmosphere ; 

 and hence the leaves have been denominated the lungs of the 

 plant. But in this, as in every other instance, a strong line of 

 distinction may be drawn. The change in the leaf is simply 

 effected by exudation and absorption^ both of which are varied 

 according to the existing temperature and the time of the day. 

 That in the lungs is accomplished by muscular action, and is 

 uniformly the same at all periods, and under every atmospherical 

 change. Animal respiration destroys the purity of the atmosphere. 

 Vegetation restores it ; the deterioration of the night being amply 

 balanced by the renovating operations of the day. In the 

 winter, when foliation is suspended, the absence of vegetation is 

 supplied by the agitating storms of the season, bringing with 

 them purifying breezes from the ocean ; cr rendering less 

 stationary the deleterious exhalations of the land. 



Thus in the natural as in the moral world, occurrences, which 

 individually appear to be very striking evils, collectively are 

 productive of the greatest degree of good ; and the functions of 

 organic substances, which from their primary effects carry with 

 them strong features of similarity, when more closely investigated, 

 are found to be productive of very opposite consequences. 



Color of Leaves. The coloration of plants presents one of 

 the most interesting, and, at the same time, obscure branches' of 

 physiological research. Humboldt attributes the green color of 

 leaves to the agency of hydrogen, because he had observed some 

 plants retain their green color in mines. Saussure, however, 

 could not increase the green of plants by means of hydrogen. 

 Humboldt also ascribes the white color to oxygen, which seems 

 to be erroneous, as this oxygen existed in a state of combination 

 previous to its being made apparent, and cannot therefore be 

 proved to produce this white color. Senebier's phlogistic 



