1856.] The Absence of Trees from Prairies, 271 



and for transmitting chemical power. I have, therefore, been led to 

 conclude that the elaboration of the sap depends not only on solar light 

 but on the evap nation from the leaves ; and that this gives rise to°an 

 electric excitement which extends to all parts of the plants, and serves to 

 consolidate the organic materials which the sap holds in solution. 



This theory was first announced in two essays, which I published in 

 the year 1848, and my researches and observations since that time, have 

 shown that it is confirmed by numerous facts, of which, I can here only 

 notice a few. The change which evaporation from the leaves produces 

 on the sap of trees, may be traced with great precision. Before their 

 leaves are developed, the sap of the maple and hickory affords sugar, 

 while that of the peach, plum, and cherry trees, chiefly consist of a solu- 

 tion of gum. But as soon as the leaves are formed, the sugar, gum, and 

 other soluble matter composing vegetable juices, slowly change into wood. 

 If a tree be deprived of its foliage during summer, or if a long continu- 

 ance of dry weather cau^:-es evaporation to be interrupted, the°formation 

 of wood will be arrested, and the sap will return to its condition during 

 Winter and Spring. After a long absence of rain, the exudation of 

 gum from cherry trees takes place in the most copious quantities ; and 

 climates where such drouths are common, always furnish the most copious 

 supplies of this article. The exudation of gum is also caused by over- 

 pruning peach and cherry trees ; as a small amount of foliage is adequate 

 to the production of wood, only when rains are frequent, and evaporation 

 subject to no long interruption. The juice of the Maguey plant, which 

 grows in warm climates, undergoes fermentation if collected before the 

 leaves grow, and it is used by the Mexicans as a palatable beverage. 

 Its character is, however, altered on the formation "of the leaves, and to 

 render it fermentable during Summer, it is necessary to deprive the tree 

 of its leaves several times. This course is often pursued, though it is 

 found to cause the death of the plant in one or two years. 



The most hardy members of the forest might be destroyed by a similar 

 removal of their foliage. Even the health of every tree is impaired, and 

 Its age IS shortened by every interruption which the elaboration of the 

 sap experiences, cither from excessive pruning or from a long absence of 

 ram. The durability of wood depends, in a great measure, on the evap- 

 oration which takes place from the foliage of the livinir plant; and 

 when the lignifying process has been imperfectlv carried on^he tendency 

 to decay will be unusually great. Now the decomposition of vegetable 

 matter, like many diseases of animals, is contagious; and an Imper- 

 fectly elaborated layer of wood, produced during a dry season, will 

 not only decay prematurely itself, but will cause all contiguous parts of 



