CHARACTER OF WATER-PLANTS 45 
from which they obtain the necessary mineral salts. 
As regards the supply of oxygen and carbon dioxide 
which the air supplies to them, those with floating 
leaves absorb it from the atmosphere, while those 
whose leaves are submerged have to subsist on the 
small quantity of these gases which is dissolved in 
the water—no wonder that such plants are rare in 
stagnant waters where aeration is poor. To assist 
respiration and transpiration, abundant and often 
comparatively gigantic air-spaces are provided in 
roots or stems or leaves, giving them a cellular 
appearance, and making them singularly light and 
spongy in texture. The leaf system of those plants 
which possess floating leaves—such as Water Lily 
(Castalia and Nymphea) or Common Pondweed 
(Potamogeton natans), are well worth study. They 
are tough, to withstand battering by waves; the 
stomata are situated, not on the lower side of the leaf, 
as in land plants, but on the upper side, where they 
are in contact with the atmosphere; and the upper 
surface is waxy or oily, so that it is not wetted and 
the stomata are not blocked. Changes of water-level 
are met by means of long flexible stems, rising not 
vertically from the root, but at an angle, so that the 
. leaves can rise with a rise of water-level. But not all 
the plants are anchored to the bottom. Some, which 
favour especially ditches and quiet waters, float freely 
with roots hanging down in the water—the Frog-bit 
(Hydrocharis) and Duckweeds (Lemna) are familiar 
examples. In the Duckweeds true leaves are absent, 
but the tiny stems are flattened and green and serve 
the same purpose, the minute flowers being borne on 
their edges. A few plants, such as the smallest of the 
