WATER-PLANTS 49 
and the presence of stomata is rendered unnecessary. 
For this reason, stomata are either absent on the sub- 
merged parts, or, if present, they do not open and close. 
In floating leaves—e.g., water-lilies—normal self-regu- 
lating stomata occur on the upper surfaces which are in 
contact with the air. 
3. In submerged aquatics the leaves are thin, and the 
epidermal cells contain chlorophyll, and so take part in 
the work of assimilation. This is correlated with the 
weak light that reaches them under water. By catching 
it in the outermost cells, the leaves are able to utilize 
the light at its strongest, before it suffers further loss 
by penetrating tissues which do not assimilate. 
4. Diminution of the Vascular System.—Since water 
can be absorbed over almost the whole surface, the presence 
of a vascular system conveying water from the roots to 
the leaves is not required, and, like all useless structures, 
it tends to disappear. The part of the vascular system 
which conducts water is the wood, or aylem, and it 
consists of vessels or tubes whose walls have become 
lignified, or woody. In the oldest aquatics the wood has 
disappeared entirely, but the phloem—that part of the 
vascular system which is set apart for the conduction 
of food-material made in the leaves—remains as it was. 
The need for the distribution of water throughout the 
plant has disappeared, but not the need for the distribu- 
tion of food. 
5. Roots are also superfluous in aquatics, and tend 
to disappear. The British plants Wolffia (a duckweed), 
Utricularia (the bladderwort, Fig. 46), and Ceratophyllum 
(the hornwort), have no roots. In other cases roots are 
present, but they do not act as absorbing organs ; they 
bear no root-hairs, and merely serve to anchor the plants 
in the mud. In Lemna (duckweed) they still persist, 
although the plant is free-floating, but the chief purpose 
they seem to serve is to keep the plant right side up 
on the surface of the water. 
In dealing with xerophytes, we showed that the leaf 
is that part of the plant which expresses in the most 
striking manner the nature of the environment. The 
same is true in aquatics. 
