This has been frequently proved. In large tropical plants,
where an immense number of blossoms are crowded together, the
temperature has risen twenty to fifty degrees above that of the
surrounding air.
As most of the aquatic plants are cryptogamous, or producing by spores,
and not by flowers, it seems probable that the evolution of carbonic
acid and heat is much less in degree in them, and therefore less in the
water than in the air. We may, therefore, venture to lay it down as a
general principle, that plants evolve free oxygen in water, when in
the sunlight, and remove the carbonic acid added to the water by the
respiration of the animals.
But since this is a digestive or nutritive process, it follows that
aquatic plants may derive much or all of their food from the water
itself, or the carbon in it, in the same manner as the so-called
air-plant, which grows without soil, does from the air. It is true, at
any rate, that, in the fresh-water aquarium, the river and brook plants
need no soil but pebbles; and that the marine plants have no proper
root, but are attached by a sort of sucker or foot-stalk to stones and
masses of rock. It is very easy to see, then, how the aquarium may
be made entirely self-supporting; and that, excepting for the larger
carnivorous fish, who exhaust in a longer or shorter period the minute
creatures on which they live, no external food is required.
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