On the Use of Air=Vessels in Plants. 83 
mation, and the different mechanism of the two leaves, will point 
out the use of air-vessels. The little influence the water has on 
the essential properties of the plant will be shown, since it in a 
very trifling degree alters either seed, flower, or fruit. I gave 
before a letter on Water Plants*. 1 then showed how the water 
grass, Festuca fluitans, is supported on the stream by the same 
means (a layer of atr-vessels under the leaf) and that almost all 
water plants have ¢wo sorts of leaves; Ist, floating ones to yield 
oxygen, and form the different compound juices required by the 
plant; 2dly, the under leaves to support and invest it, and for 
many other purposes yet to be discovered; but in the Potamo- 
getons, the lower leaves are too narrow to cover the bud, and this 
is wholly left to the vails. In the Potamogeton natans, when 
the water happens to dry up, the air-vessels by degrees lose all — 
their beauty; they contract in such a manner as soon to become 
merely common vessels, and then the plant carries the appear- 
ance of a Plantago. Waving found one last summer, I should 
not have known it to be a Potwmogeton, but for the observation 
of Linneus, who had seen and mentioned the alterations that 
sometimes took place in the species, ‘This is remarkably the 
case in many of the semi-water plants, which, instead of decaying 
for the want of water, become land plants, by losing or contracting 
their vessels, particularly the Veronica scutellaria, and some- 
times the Beccabunga. 
The strange idea that water plants perspired, I think I 
should not here have shown to be false, as I flatter myself that 
what I have already in former letters proved, is so plain and evi- 
dent to common sense, as to carry conviction to every breast that 
seeks it: but as the mistake originated in land plants, by figures 
(which in reality resembled any thing rather than a bubble of 
water), and as water plants are almost divested of hairs, retorts, 
or any appearance resembling them, it 1s proper that I should 
show what was taken for perspiration in water plants. A very 
light species of Conferva, hardly to be distinguished by the naked 
eye, very frequently covers the floating leaves of water plants, and 
between its pellucid and slender hairs the water is necessarily 
detained; the bubbles of oxygen, continually flowing from the 
leaves, are caught in its diminutive meshes; and the plant, thus 
situated, appears covered with diamonds, which when swept off 
by the hand, the no longer imprisoned air disappears, but the 
water (the cause of the phenomenon) still remains with the Con- 
ferva, as an apparent proof of the perspiration ; it being supposed 
to proceed from the bubbles. But I have so often examined the 
whole matter, so often placed a large magnifier over the leaves 
* Philosophical Journal. 
F2 and 
