156 SOUTH AFRICAN BOTANY 
189. Hydrophytes or Water Plants are subject to 
less extremes of temperature than land plants, owing to 
the specific heat of water being so high. There is also 
more carbon di-oxide in water than in air, as this gas 
dissolves readily in water. Hence we find that water 
plants grow very rapidly and branch freely, especially 
in the tropics, where growth is not hampered by either 
a cold or a dry season. The leaves of a water plant, if 
submerged, are either ribbon-shaped or narrow, and 
arranged in whorls or finely divided. The epidermis 
contains chlorophyll and has no cuticle, and there are 
no stomata. The floating leaves are rounded or slightly 
lobed, bear stomata on their upper surface, and have a 
waxy cuticle so as to prevent wetting. Some Hydro- 
phytes bear two kinds of leaves, floating and submerged, 
others have only the floating leaves. The stems have 
very little mechanical tissue, as the water supports all 
the weight of the plant; if, however, the plant grows in 
running water it may be subjected to strong pulling 
strains due to the current; in this case the vascular 
bundles are placed centrally, as in the root of a land 
plant. There is little conducting tissue in stem or root, 
as the whole plant is able to absorb water. Tor the 
same reason there are few or no root-hairs, the root 
serving merely to fix the plant to the bottom. Air 
spaces are present in al) parts of a water plant. They 
help it to float, and also convey air to the lower parts 
growing in deep water or mud where there is little 
oxygen present for respiration. Most water plants 
reproduce themselves extensively by vegetative methods, 
and flower much less freely than land plants; when they 
