AND OF PLANTS UPON THE ATMOSPHERE. 
161 
tained in the previous experiment,) whilst that in connexion with the one covered 
with the tiles had only increased by 32 grains. 
Thus it would appear, that although heat assists the process, some degree of light 
is essential to its activity. 
I was desirous, likewise, of ascertaining whether the brightest kind of light attain- 
able by artificial means contributed in any degree to the process under consideration; 
Professor Decandolle having found that the leaves of plants placed in a cellar 
became green on exposure to a strong light from lamps, and that their flowers even 
reversed their natural periods of opening, when the cellar was illuminated by night, 
and kept dark during the day. 
In my own experiments, the light employed was that produced by a jet of mixed 
oxygen and hydrogen directed upon a ball of quick-lime, a kind of light, which I 
have found capable, like that from the snn, of passing through, and being concen- 
trated by, a lens. Nevertheless, in two or three experiments, each lasting nearly an 
hour, in which the rays proceeding from the incandescent lime were directed towards, 
and thrown back upon the plant, by concave metallic reflectors, no increase in the 
quantity of moisture exhaled could be detected, beyond what the same individual 
had given out whilst in the dark. 
The last function which it was left for me to consider, namely, the absorption of 
water by the roots^ is so related to the preceding one, that it might almost be inferred 
a priori to be subject to the same laws. 
It seems indeed evident, that, cceteris paribus, in proportion to the velocity with 
which the sap ascends, will the extremities of the roots absorb moisture from the 
ground ; since, unless the former operation continued, the latter organs would very 
soon become fully charged with humidity, and thus the absorption be put a stop to. 
In order to ascertain the quantity of water absorbed under different circumstances 
by the roots of plants, I made the following experiments. 
Two small plants of Helianthus annuus , in pots marked A and B, were immersed in 
tin vessels nearly full of water, the height of which within was measured by glass tubes 
cemented into them below, and rising on the outside nearly to the top. These vessels 
were severally provided with tin covers, each of which had a circular aperture at its 
centre, through which the stem of the plant passed, and another small one at the side, 
through which water might be introduced. Being elsewhere closely attached to the 
vessel, little or no evaporation could take place from the surface of the included water. 
Things being thus prepared, the two plants were placed for twenty-four hours in 
the open air, part of the time exposed to a bright sun, with the thermometer at 75° 
in the shade. 
At the expiration of that time it was found that the vessels had lost, as nearly as 
could be ascertained, the same quantity of water, which amounted in each case to 
four ounces. 
MDCCCXXXVI. 
Y 
