Prof. LeConte on the Correlation of Forces. 141 
heaping earth around them, many buds which would otherwise 
form leafy branches develope into tubers. Hence also the expla- 
nation of the equally well-known fact, that the roots of plants 
seek and grow most rapidly in the direction of most abundant 
food. If the sap is elaborated entirely in the leaves, it is diffi- 
cult to understand why the descending sap should flow in greater 
abundance in one direction than another. But if sap is elabo- 
rated in the root itself, it is easy to see why growth is most rapid 
in the direction of most abundant manure. It is easy to see, 
too, why roots avoid the light ; since light decomposes CO2, and 
therefore must be unfavourable to the formation of this substance. 
7th. It is a well-known fact that the so-called respiration of 
plants consists of two distinct and apparently opposite processes : 
Ist, the absorption of CO? by the leaves, and also in solution by 
the roots, the decomposition of this CO*® by means of light, 
with the fixation of the carbon and the elimination of the oxy- 
gen; 2nd, the recomposition and evolution of CO?.. The decom- 
position of CO? undoubtedly takes place in the leaves ; but where 
the recomposition of CO* takes place is not so well ascertained. 
It is exhaled, however, like the oxygen, from the leaves. The 
process of decomposition of CO® takes place only during the day, 
as light is absolutely necessary for this process. The recomposi- 
tion of CO? takes place night and day, although its exhalation, 
according to some observers, seems to be more abundant during 
the night. The process of decomposition of CO? is well under- 
stood ; of that of recomposition our knowledge is very imperfect. 
M. Risler’s explanation of this latter process seems most probable. 
Plants, we have seen, undoubtedly absorb soluble organic mat- 
ter, 7. e. humus. Humus we know is a more highly carbonized 
substance than cellulose or starch. This humus is therefore 
oxidized in the roots and interior of the trunk, away from light, 
by means of oxygen, also absorbed by the roots, and thus forms 
CO*. This CO? then circulates in the sap to be exhaled by the 
leaves, or perhaps to be again decomposed by sunlight in these 
organs. In the absence of light the whole is exhaled undecom- 
osed. This readily accounts for the apparently greater exhala- 
tion of CO? during the night. A series of well-conducted ex- 
periments would test the truth of this view. If it is true, there 
should be a relation between the richness of the soil in organic 
manures and the amount of CO? exhaled. For a given amount 
of growth, the amount of CO* exhaled is the measure of the 
amount of food taken up in the form of organic matter, and the 
amount of oxygen exhaled is the measure of the amount of food 
taken in the form of minera] matter. Or if the exhaled CO? is 
decomposed in the leaves during the day, then of course the 
difference between the amount exhaled during the night and day 
