THE STEM AND THE LEAF 79 
trees grown in woodlands, where they stand moderately close 
together. In some trees — such as the so-called snap willows, 
the cottonwood, and the large-toothed aspen — live twigs fall 
very freely during windstorms and snowstorms, and when the 
tree is loaded with sleet. These twigs may be blown over crusted 
snow or floated along by brooks or rivers near by. and often 
lodge in spots where they take root and grow into new trees. 
77. Leaves of water plants. Water plants with aérial leaves, 
like the cat-tails and pickerel weeds, are perhaps the commonest 
type. Others, like the pond 
lilies (fig. 62). have floating 
leaves. with only the upper 
surface exposed to the air. 
Still others, like some pond- 
weeds, have all their leaves 
submerged. There are only 
a few common plants which 
have two types of leaf, like 
the water crowfoot (fig. 63), 
one set growing wholly in 
the air and the other set 
wholly under water. 
Floating leaves have sto- 
mata only on the upper sur- 
face; on the lower surface 
they could serve no useful purpose. Submerged leaves often 
have the thread-like form shown in figure 63. This form ren- 
ders them much less liable to injury from waves or currents 
of water, and also allows the freest exposure of the whole leaf 
surface to the surrounding water. This offers the best possi- 
ble opportunity for exchange of gases between the water in 
which they are dissolved and the interior of the leaf. 
78. Size and shape of leaves in relation to water supply. 
Plants which grow in earth (that is, neither aquatics nor air 
plants) often show a decided relation between the abundance 
of the water supply and the amount of leaf surface. Those 
Fig. 63. A shoot of water crowfoot 
_d, air leaves; B, thread-like water leaves. 
After Askenasy 
