Application of the Micro-spectroscope, &c. By Thos. Palmer. 227 
from each other in the quantity which their leaves absorb during 
the night. Fleshy-leaved plants absorb the least, probably because, 
as has been previously shown, they emit no carbonic acid gas. 
Hence they can vegetate in high situations, where the surrounding 
atmosphere is more or less rarefied. In the case also of those 
plants, such as evergreens, of which we have principally to deal, 
though the oxygen absorbed is greater than in the case of fleshy- 
leaved plants, still it is much less than those which fall in winter. 
That all plants of every species and order absorb the moisture 
which is contained in the atmosphere in which they grow is too 
well known to require anything but simple mention. Bonnet 
showed in his researches concerning the use of leaves, that they 
continue to live for weeks if one of their surfaces be applied to 
water, and that they not only vegetate themselves, but imbibe 
enough water to support the vegetation of a whole branch and the 
leaves belonging to it ; though, according to Duchartre, plants 
such as Hortensia, Helianthus annuus, which wither in the 
evening in consequence of the dryness of the earth in the pot, 
did not recover or become turgid if copiously moistened by dew 
during a whole night, the pots, and therefore the roots being 
covered. It is, moreover, well known that Epidendral Orchids, 
Tillandsias, &c., behave in the same way in this respect ; they also 
absorb neither water nor aqueous vapour through their leaves, nor 
even in any considerable quantity through the roots. The water 
which they require for their transpiration and growth must be con- 
veyed to them in the form of rain, or dew, which moistens the root 
envelope or wounded surfaces. 
It would seem, therefore, in the case of land plants, which 
wither on a hot day and revive again in the evening, that it is the 
result of diminished transpiration with the decrease of heat and 
crease of the moisture in the air, the activity of the roots continuing, 
and not of any absorption of aqueous vapour or dew through the 
leaves. Bain again revives withered plants, not by penetrating the 
leaves, but by moistening them, and thus hindering further tran- 
spiration and conveying water to the roots, which they then 
conduct to the leaves. 
After all that has been said with regard to the functions of 
leaves, light is, without doubt, the most essential element, as the 
entire life of the plant depends upon its action on the cells that 
contain chlorophyll, this being the essential condition under which 
new organic compounds are formed out of the elements of carbon 
dioxide and water. The amount of oxygen evolved in this process 
is nearly the same as that required for the combustion of the 
substance of the plant, and the amount of work equivalent to the 
heat produced by this combustion gives a measure for the amount 
of work performed by light in the chlorophyll-containing cells of 
