Prof, LeConte on the Correlation of Forces. 139 
vol. xxiv. p, 317, will eventually furnish the means of solving 
this very important problem. I do not wish to anticipate the 
final results of these experiments; but it seems to me that the 
negative results thus far obtained rather support the view that 
the action of light is not thus limited. In all experiments on 
this subject the light and heat of the sun have been combined. 
Now heat we know is favourable to combustion. The fact, then, 
that combined light and heat produced no effect, would seem to 
indicate that light counteracted the effect of the heat of the sun, 
5th. Etiolated plants, or plants artificially blanched by exclu- 
sion of light, exhibit the same phznomena, and for the same 
reason. These plants cannot receive thew organizing force 
through the decomposition of CO* by sunlight, therefore they 
are obliged to obtain it from decomposition of organic matter, 
Hence these plants require organic food; hence also they evolve 
CO? instead of oxygen. In this case also decomposition of 
organic matter, with a separation of a portion of the carbon in 
the form of CO*, furnishes the organizing force, In the absence 
of any external organic matter in the form of humus or manure, 
etiolated plants, like germinating seeds, will feed for awhile upon 
organic matter previously accumulated in their tissues in the 
form of starch, and actually lose weight of solid matter*. 
6th. In a most interesting and suggestive article in the Biblio- 
théque Universelle (Archives des Sciencest) on the subject of 
humus, M. Risler shows in the most conclusive manner that 
organic matter in a soluble condition (soluble humus) is taken 
up by almost all plants. This fact had been previously proved 
experimentally by Th. de Saussure; but having been denied by 
Liebig, it has been yery generally neglected by vegetable physio- 
logists. The doctrine of Liebig and of physiologists generally 
is that, except in case of pale plants, organic matter is decom- 
posed into CO?, HO, and NH, i. e. must fall into the mineral 
kingdom before it can be absorbed and assimilated by plants, 
and therefore that organic manures only supply the same sub- 
stances, and in exactly the same form, which are already sup- 
plied, but in insufficient quantities, by the atmosphere, But 
M. Risler repeats with great care the experiments of De Saussure, 
and confirms the accuracy of his conclusions. Hyacinths and 
other bulbs were placed with their roots suspended in water co- 
loured with soluble extract of humus. When these plants were 
placed in the sun, the water became rapidly decolorized. Other 
roots, such as carrots, also germinating grains of wheat, were ob-~ 
served to produce the same effects, An extract of humus was 
exposed at a somewhat elevated temperature to sunlight under a 
* Carpenter, Comp. Phys. p. 285. 
+ Bibl. Univ., Arch. des Sci., New Series, vol. i. p. 305. 
