H. BOTANY AND HORTICULTURE. 369 



have been previously carried np into the tree, for it evidently 

 has not raised itself there. It is the power of the heat pro- 

 ceeding from the sun which has effected the development 

 and growth of the tree. It is, therefore, practicable to de- 

 termine the mechanical coefficient of growth, as Sausom has 

 determined the mechanical coefficient of nourishment for 

 foods. It is the property of vegetable organisms to utilize 

 the heat received by them from any source, for the conver- 

 sion of crude material into such forms as are needed by them 

 for their own growth. In this process, however, force is 

 neither created nor lost, although much of it is secreted 

 within the body of the plant or mineral. Other things be- 

 ing equal, the quantity of carbon fixed in any plant varies 

 with the average elevation of the height of its centre of 



CD CD 



gravity. Morren, on the Energy of Vegetation, Brussels, 

 1873. 



THE RESPIRATION OP LEAVES IN THE DARK. 



An important paper by Deherain and Moissan upon the 

 respiration of leaves in the dark has lately been published 

 in Comptes Renclus. Among the more important conclu- 

 sions reached by the authors in their researches are : First, 

 that the quantity of carbonic acid which is thrown off by 

 leaves in the dark increases with the increase of tempera- 

 ture; second, that the quantity of carbonic acid thrown off 

 is comparable to that yielded by the cold-blooded animals ; 

 third, that leaves kept in the dark absorb more oxygen than 

 they throw off carbonic acid ; fourth, that leaves continue 

 to throw out carbonic acid in an atmosphere deprived of 

 oxygen. 



The authors present the following hypothesis upon the 

 physiological uses of this internal combustion which takes 

 place in the leaves, as the result of their numerous experi- 

 ments. The immediate constituents which are necessary to 

 the growth of the plants, and to the formation of new or- 

 gans, are in part formed in the leaves. This growth is es- 

 pecially favored by warmth in the dark, as a principle well 

 known to gardeners who cover plants, the development of 

 which they wish to accelerate, under glass, in which case a 

 part of the light necessary for the composition of the car- 

 bonic acid is reflected, but an elevated temperature is se- 



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