904 EXPERIMENT STATION RECORD. 



extremities of infinitely ramified, flexible branches, which are agitated 

 by the slightest breeze, thus facilitating contact of the leaves with the 

 constantly renewed layers of air about them. That the absorption of 

 carbon dioxid is very rapid, may be shown by placing a leaf from which 

 the air has been exhausted by means of an air pump in an atmosphere 

 of carbon dioxid in an apparatus : which measures the change of the 

 volume. It will be observed that absorption begins instantly, but that 

 it is largely dependent upon the quantity of the water present in the 

 leaf. Thus, the coefficient of absorption of the carbon dioxid in old 

 leaves of Japanese Euonymus, containing 06.3 per cent of water, was 

 found to be 0.70 at 15°, while in young leaves of the same tree con- 

 taining 75.4 per cent of water the coefficient was 0.83. A comparison, 

 at different temperatures, of the coefficient of absorption of carbon 

 dioxid in the leaves with that in pure water, shows the absorption in 

 the leaves to be somewhat greater than in pure water. This indicates 

 that the carbon dioxid is not simply dissolved in the water in leaves, 

 but that it combines with the water to form a hydrate. It will be 

 shown later that this fact is of great importance. 



If a section of the epidermis of a leaf be examined under the micro- 

 scope, numerous little openings (stomata) will be observed which are 

 generally more abundant on the under side than on the upper side, 

 which is usually covered with a thick, smooth, glossy cuticle. The 

 experiments of Boussingault led to the conclusion, which was long 

 accepted, that the carbon dioxid diffused through the cuticle. F. Black- 

 man 2 has recently demonstrated, however, that the carbon dioxid 

 enters primarily through the stomata. 



DECOMPOSITION OF CARBON DIOXID IN LEAVES. 



The carbon dioxid which is absorbed by the leaves is decomposed and 

 the products of this decomposition are utilized in the formation of the 

 simplest primary organic compounds, from which the more complex 

 constituents of plants are derived. To accomplish this the principal 

 condition is that the leaf be perfectly healthy. If it does not contain 

 its normal proportion of water, L e., if the roots do not draw from the 

 soil as much water as is given off through the leaves, the'decoinposition 

 of carbon dioxid is checked. Assimilation has ceased when as at the 

 end of a summer day the leaves of the tobacco plant, for instance, are 

 hanging down the stem, or those of the beet lie flat on the soil. In fact 

 it has been found that the decomposition of carbon dioxid begins to 

 decline even before the leaves have lost their turgescence. 



Light is absolutely essential to the assimilation of carbon by the 

 leaves of plants. The principal source of this energy is, of course, the 

 sun, but attempts have been made to utilize artificial light, especially 



1 Deherain and Maquenne, Ann. Agron., 12 (1886), p. 525. 



2 Proc. Roy. Soc. [Loudon], 57 (1895), p. 165; Phil. Trans. Roy. Soc. [London], 

 186 (1895;, p. 48 (E. S. R., 6, p. 782). 



