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 aerial 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 



A, air leaves ; B, thread-like water leaves. 

 Alter Askenasy 



