ECOLOGY OF LEAVES 115 
leaf of an unknown plant whether it is an alpine, a desert, 
or a seaside species. This is because of the importance 
of leaves in disposing of the water taken into the plant 
(Chapter XIII). 
135. Leaves of Hydrophytes.—Not nearly all hydrophytes 
are aquatics, but some merely prefer very moist soil or 
moist air. Of the truly aquatic species some have their 
leaves wholly submerged ; 
others, such as the duck- 
weeds and pond-lilies, have 
them floating; and still 
others, like the sedges, the 
bur reeds, the cat-tails, and. 
the pickerel weeds, have 
their leaves freely exposed 
to the air. A few plants 
have both water leaves and 
air leaves (Fig. 79). It is 
generally supposed that the p,,. 79. Submerged and Aérial 
thread-like form of sub- Leaves of a European Crowfoot 
merged leaves in so many (Ranunculus Purshii). The leaf 
: ‘ with thread-like divisions is the 
species of aquatics gives mabIneeeA one, 
them greater capacity to 
absorb dissolved gases from the water which surrounds them. 
136. Leaves of Xerophytes. — In regions where the great- 
est dangers to vegetation arise from long droughts and 
the excessive heat of the sun, the leaves of plants usu- 
ally offer much less surface to the sun and air than is the 
case in temperate climates, as shown in the Australian 
blackberry (Fig. 80). Sometimes the blade of the leaf is 
absent and the expanded petiole answers the purpose of a 
blade, or, again, foliage leaves are altogether lacking, as in 
