THE LARGER AQUATIC VEGETATION 



l8l 



great variety in leaf form as is noticeable in land plants. The 

 floating leaves are usually borne on a stalk which in most cases is 

 flexible, so that the leaf blade may rise or fall with the fluctuating 

 level of the water. The exposed surface of the floating leaf is 

 usually waterproof. This is provided for in a variety of ways. 

 In some cases a waxy coating renders the skin nearly impermeable. 



This is true with some of the Potamo- ^i^ 



gctons. In some cases a coating of 

 very dehcate hairs so abundant as to 

 enclose an envelope of air prevents 

 the water from actually touching the 

 epidermis proper. This is to be ob- 

 served in the case of Nelumbo. Some- 

 times one may see drops of water 

 standing on the surface of such leaves 

 and when the leaf is submerged and 

 then allowed to emerge the water rolls 

 off leaving the leaf apparently dry. 



In striking contrast to the floating 

 leaves the submerged ones seldom 

 have a distinct blade and stalk. This 

 is consistent with the general tendency 

 to uniformity of structure under a 

 uniform environment. Vallisneria 

 (Fig. 254) may be regarded as show- 

 ing a typical ribbon form which is well 

 adapted to life under water, because 

 it is so flexible and is thus able to 

 endure swiftly flowing currents or 

 wave movements. In some species, as that of Potamogeton pcr- 

 foliatus, the submerged leaves are expanded into blades but are 

 sessile on the stem, that is, are without a leaf stalk. The latter 

 would be of no advantage to leaves which are not intended 

 to reach the surface. They would tend to make the plant top- 

 heavy and easily uprooted by a sudden rush of water. Moreover, 

 it is quite probable that a greater exposure of leaf surface is nec- 

 essary because of the diminished hght under water. Linear or 



Fig. 254. Vallisneria spiralis. Staminate 

 and pistillate plants, showing the long rib- 

 bon leaves which are all blade and have 

 no apparent stalk. (After Kerner and 

 Oliver.) 



