PLANT SOCIETIES 313 
damp air and soil. All of them transpire freely, and many 
of them cannot live at all under the moisture conditions 
which suit ordinary plants. 
Some aquatics have their leaves wholly submerged, 
others, such as the duckweed and the pond-lilies (Fig. 218), 
have them floating, and still others, like the sedges in the 
same picture, have their leaves freely exposed to the air. 
A few plants have both 
water-leaves and air-leaves 
(Fig. 219). Some aquatic 
plants are rooted in the mud, 
while others have no roots 
at all, or, like the duckweed, 
have only water-roots. 
The leaves of land-plants A A 
in very rainy, subtropical % yy, INS 
climates are exposed to the VAAN NSS 
attacks of parasitic spore- 
: FIG. 219.— Submerged and Aérial Leaves 
plants which flourish _ of a European Crowfoot (Ranunculus 
their surfaces. To ward off Purshii). The leaf with thread-like 
5 2 divisions is the submerged one. 
the attacks of these it is 
necessary to keep water from accumulating on the surfaces 
of the leaves. This result is secured by a waxy deposit on 
the epidermis and also by the slender prolongation to drain 
off surplus water, shown in Fig. 221. That this slender 
leaf tip is useful in the way suggested is proved by the fact 
that when it is cut squarely off the leaf no longer sheds 
water freely. 3 
385. Xerophytes.— A zerophyte is a plant which is 
capable of sustaining life with a very scanty supply of 
water. Since the first plants which existed were aquatics 
