FIG. 343. CABOMBA. 



A submerged leaf. 



THE LEAF IN RELATION TO ITS ENVIRONMENT 281 



a probable explanation indeed, it is only 

 when we turn to aquatic plants that the special 

 usefulness of heterophyllous leaves becomes 

 apparent. Mention has been made of the 

 Water-crowfoot (Ranunculus aquatilis), whose 

 submerged leaves are so different from the 

 floating ones, the former consisting merely of 

 narrow thread-like segments, while the latter 

 are three-lobed with dentate margins. This 

 difference may be partly accounted for by the 

 fact that the submerged leaves, being less 

 favourably situated for light than the others, make the most of the rays 

 that visit them by assuming the shredded form. It has been further 

 remarked that aquatic plants which develop filiform leaves are usually, if 

 not always, found in running water; and how well are they adapted for 

 such environment! yielding readily to the current, and participating in 

 its movements without injury. These observations apply equally to the 

 Potamogetons (P. heterophyllus, rufescens, and spatkulatus), to the Water- 

 caltrops (Trapa natans), and to the Cabomba (Cabomba aquatica, figs. 343. 

 344). The latter may be studied to advantage in the Victoria Regia 

 House at Kew. 



r l*he buoyancy of floating leaves is, in not a few cases, secured by special 

 air-channels, which may be situated either in the blade or the leaf-stalk 

 more frequently the latter. In the Brazilian Pickerel-weed (Pontederia 

 crassipes), the swollen and hollow leaf-stalks act as floats to the whole plant, 

 which, as it does not root itself to the mad, is carried hither and thither by 

 wind and current like a rudderless ship. In Desmanthus natans, an aquatic 

 plant of the Leguminous order, the stem takes the form of " a large-celled, 

 spongy, air-containing mantle," which subserves the same purpose as 

 the leaf-stalks of the Pickerel-weed, and is, in 

 fact, a veritable swimming apparatus. ^^^^-, 



As a consequence of their situation, aquatic 

 plants imbibe much more water than land 

 plants, and the transpiration is proportionately 

 greater. One sees in this fact the advantage 

 of their broad, flat, floating leaves, which, ly- 

 ing side by side on the surface of the water, 

 present so large a field for the sun's opera- 

 tions ; for it will be remembered that transpira- 

 tion takes place through the stomata, and that 

 these organs, in aquatic plants, are placed on 

 the upper surface of the leaves. When it is 

 stated that a single Water-lily-leaf of very 

 ordinary size may contain as many as eleven 



FIG. 344. CABOMBA. 



An aerial leaf and flower. 



