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ABSORPTION AND EXHALATION OF GASES BY LEAVES. 799 
plants, water is also constantly excreted in drops, at certain 
periods of vegetation, through the water-pores there situated. 
But the most remarkable plant of this kind is the Caladiwm dis- 
tillatoriwm, from which half a pint of fluid has been noticed to 
drop away during asingle night, from orifices (water- pores) placed 
at the extremities of the leaves, and communicating freely with 
internal passages. In those Mosses which have no trace of vascular 
bundles, Oltmanns points out that the rise of water does not take 
place within the stem, but by capillarity externally, and that in 
these plants transpiration does not take place. 
(2) Absorption of Fluids by Leaves.—Hales, Bonnet, and 
others, inferred that leaves were capable of absorbing moisture, 
though De Candolle and others subsequently asserted positively 
that such was not the case, and that leaves remained fresh for 
some tiie when exposed to the influence of moisture, solely be- 
cause transpiration was hindered or arrested. The more recent 
researches of Henslow, however, as already noticed (page 787), 
seem to prove conclusively that both leaves and green internodes 
are capable of absorbing a large amount of moisture, and that 
probably the quantity absorbed is independent of the presence 
or absence of stomata. The experiments of Darwin and others 
with Carnivorous Plants seem to prove this also. 
(5) Absorption and Hxhalution of Gases by Leaves.—We have 
already noticed (page 790) the property possessed by the roots 
of absorbing liquid food from the medium in which they grow, 
and also their power of excretion (page 792). Whilst plants 
are thus intimately connected by their roots with the soil or 
medium in which they are placed, they have also important re- 
lations with the atmosphere by their leaves and other external 
organs, which are constantly absorbing from, or exhaling into 
it, certain gases. The atmosphere, it should he remembered, is 
brought into communication with the interior of the leaves by 
the stomata : it indeed fills the whole intercellular structure of 
these organs much in the same way as the air fills the lungs of 
a mammal, or the lungs, bones, &c. of a bird, to which in func- 
tion they bear some sort of resemblance. The gases which are 
thus absorbed and exhaled by the leaves and other green organs 
and parts of plants have been proved, by a vast number and 
variety of experiments, to be essentially carbon dioxide and 
oxygen. The experiments of Boussingault would also indicate 
that, in some cases at least, carbon oxide is evolved with the 
free oxygen. Draper, Mulder, Cloez, Gratiolet, and others, like- 
wise believe that leaves and other parts exhale nitrogen when 
exposed to sunlight. Plants, under certain circumstances, may 
also absorb nitrogen from the air, though it does not then serve 
for nutrition ; but the investigations of Lawes, Gilbert, Daubeny, 
and Pugh tend, on the contrary, to negative this statement. 
Sir J. B. Lawes has recently confirmed his old opinion that the 
source of nitrogen is the soil, of carbon dioxide the air. 
