140 Prof. LeConte on the Correlation of Forces. 
bell-glass. Microscopie plants developed in great abundance. 
As long as these plants continued to develope, the infusion was 
transparent and did not putrefy in the slightest degree ; and yet 
there was a constant evolution of CO?, as shown by analysis of 
the air in the bell-glass. ‘* Now the cellules formed in the liquid 
contained carbon. ‘This carbon did not come from the CO? of 
the air, for the liquid, far from absorbing, disengagedCO*. There- 
fore the soluble humus must have furnished the carbon directly 
to the vegetable cells.” It could not have furnished it indirectly 
in the form of CO? derived from decomposition of the organic 
matter, otherwise oxygen, instead of CO*, would have been elimi- 
nated. M. Risler thinks moreover that the embryo in germina- 
tion takes up soluble organic matter in the form of humus in 
addition to the soluble organic matter contained within the coty- 
ledons, and that the evolution of CO* by germinating seeds is 
due in part also to the oxidation of humus. Finally, according 
to the same author, the formation of roots in all plants, but par- 
ticularly those containing much starch or sugar, is due to the 
direct absorption of humus, and not, as is generally supposed, 
to the fixation of carbon by means of light. “In order,” says 
he, “that CO? of the air should form these substances, it is neces- 
sary, in the beet and the potato, that there should be a descend- 
ing sap, which there is not.” Moreover, if the carbon were taken 
from the soil in the form of CO?, there should be elimination of 
oxygen instead of evolution of CO?; but the converse is the fact, 
as has been proved in the most indisputable manner by De Saus- 
sure and Boussingault*. 
Mulder is equally explicit in affirming that plants absorb 
soluble organic matter, which is converted in the roots, by elimi- 
nation of a portion of the carbon, into starch and sugar (Mulder, 
pp. 620, 664, 682). Thus, according to these authors, sap is 
actually elaborated by the roots from organic manures. 
Now according to the theory which I propose, this change 
from humus into starch, sugar, or cellulose, furnishes an addi- 
tional life-foree. Humus is a more highly carbonized substance 
than either starch or cellulose. By the partial decomposition of 
humus in the tissues of the plant, with the elimination of a por- 
tion of its carbon (removed by oxidation), a chemical force is set 
free which serves to assimilate the remainder. Hence this pro- 
cess of evolution of CO?, as we have already said, is opposed by 
light, but favoured by darkness and heat. Light favours the 
formation of chlorophyll, of woody fibre, of essential oils, gums, 
&e.; darkness, heat, and organic manures favour the formation 
of sugar, starch, &c. Hence the explanation of the well-known 
fact, that by covering up the lower portions of potato plants by 
* Bibl. Univ., Arch. des Sci., New Series, vol. i. p. 5. 
