82 On the Use of Air-Vessels in Plants. 
increases in length with the depth of the water, and it is wholly 
filled with air-vessels which sustain it in a perpendicular direction 
to where the leaf hegins, and it has a layer of air-vessels also un- 
derneath the leaf, to support it on the stream, and keep its upper 
surface perfectly dry. But when the plant is to be kept above a 
foot under water, the leaves lose their accompaniment of air- 
vessels under, and retain only those which surround the leaves, 
and a bubble at the apex and bottom of the midrib, which is 
increased or diminished according to the necessities of the plant 
for rising or falling in the stream. But if leaves are to be main- 
tained in a perpendicular direction in the water, then more art 
is used for the purpose, which I shall now show in describing the 
whole formation of a water-lily, or the Nymphaea lutea; ob- 
serving that the dissection is the same in all the species I have 
been able to procure for the purpose. 
That the water-lily has two sorts of leaves, is a fact, I believe, 
not known even to the worthy Baronet who so well observed them 
in their native waters im Chima: that they wholly differ in thick- 
ness, and in form, as they do also in the various uses for which 
they are designed, I shall now show. The first I shall describe, 
is that which swims on the water. A double layer of air-vessels 
just covered by a skin perfectly impervious to water, forms its 
lower surface, that which is in contact with the stream; while 
the upper one has a treble net, instead of pabulum, covered by a 
double skin through which nowater can pass. I have given a sketch 
of the thickness of the leaf in (Plate 11. fig. 1.) that the difference 
might be well understood, and the use of air-vesseis fully exem- 
plified. I have said that the stem lengthened with the depth of the 
water, but the air-vessels in the midrib stop with the commence- 
ment of the leaf, so that it lies quite flat, and the upper surface 
perfectly dry. But the other leaf is of a very different descrip- 
tion: when the corolla has fallen off, and the seeds have dropped 
from the receptacle, the pericarpium decays, and a new germ, 
which 2s to contain the next year’s fruit and flower, just peeps 
above the mud, but is wholly covered with water. This is sur- 
rounded with leaves of a very peculiar form, flapped into a kind 
of scollop at the edges; they press the bud in every other part ; 
while the apex of the midrib just keeps aleve the water, enabled 
to do so by a large bubble of air which it encloses, and by all 
the midrib being full of air-vessels: of course there are none 
under the leaf, and fewer in the leaf-stalk; but the large opening 
for air between the skins of the leaf, at the top scollop, keeps it 
constantly upright. 
The (fig. 2.) will give an idea of the comparative thickness of 
the leaf, while (A, fig. 1.) and (B, fig. 2.) will show the shape of 
the leaf to. which each dissection belongs; and its interior for- 
mation, — 
