WATER-PLANTS 
161 
plants are able to grow on dry land, usually in other forms 
more suited to land-existence (p. 20). The floating leaf, as 
seen in Nymphaeaceae, Trapa, Ranunculus sp., &c., is 
usually large, entire or nearly so (a divided leaf would be 
easily submerged, or at least wetted so as to interfere with 
its functions), of leathery consistency and usually of oval 
or circular outline. The stalk is capable of renewing its 
growth should the leaf be submerged, so as to bring it back 
to the surface. The upper surface of such a leaf is provided 
with cuticle (often waxy), palisade-tissue and stomata, and 
functions like a land-leaf. 
Another determining factor is light. Water absorbs it 
to such an extent that submerged plants are practically 
shade-plants (p. 142) and exhibit similar features. The 
internodes are long (etiolated ; cf climbing plants, below), 
the leaves are usually thin and have no palisade-tissue, and 
the cells of the epidermis contain chlorophyll. 
A very characteristic feature is the presence of enormous 
intercellular spaces, giving the tissues a spongy consistency 
easily visible to the naked eye. They are full of air and 
probably serve more than one function; they act as floats 
to the plant, they probably aid in the assimilatory functions 
by supplying gases to the cells, and they seem to act as 
channels by which oxygen can reach those parts of the 
plant which are in deep water or in mud, where there is 
little or no oxygen for respiration. In Podostemaceae, 
which live in rapids attached to rocks, and consequently 
have all their parts exposed to well-aerated water, the large 
air-spaces are entirely absent, while in marsh-plants they 
are usually strongly marked. Secondary respiratory tissue 
( aerenchyma ), formed by the phellogen layer, is frequently 
found in plants growing in mud or nearly stagnant water, 
e.g. in roots of Sesbania and Jussieua, stems of Neptunia, &c. 
It is also said to occur on submerged parts of Lycopus, 
Lythrum, and other mud-plants (cf. also Rumex). In 
many mangroves, Taxodium, &c., special erect roots are 
formed, with stout aerenchymatous tissue ; they appear to be 
respiratory organs, and are sometimes termed pneumatophores. 
One of the most common structural peculiarities of water- 
plants, whether Algae or higher plants, is their sliminess. 
In the flowering plants it is usually due to a secretion of 
w. 
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