461 



437. Hydrophytes. Some of these are herbaceous aquatic 

 plants, like the duckweed, the pickerel weed, the pond lily, 

 and the water crowfoot ; others, such as the cultivated " calla " 

 (Richardia), the buck bean, the cat-tail, and the sweet flag, many 

 ferns, mosses, and liverworts, prefer damp air and soil. All of 

 them transpire freely, and many of them cannot live at all 

 under the moisture conditions which suit ordinary plants. 



FIG. 353. Submerged and aerial 

 leaves of a crowfoot (Ranunculus 

 aquatilis) 



The leaf with thread-like divisions is 

 the submerged one. After Giesen- 

 hagen 



FIG. 354. Cross sections of 

 leaves of arrowhead (Sag- 

 ittaria) 



A, aerial leaf; B, submerged 

 leaf. The submerged leaf 

 has no ordinary epidermis 

 and no palisade layer, but 

 large air spaces. Much 

 magnified. After Bonnier 

 and Sablon 



Some aquatics have their leaves wholly submerged, others, 

 such as the duckweed (Fig. 355) and the pond lilies (Fig. 352), 

 have them floating, aud still others, like the sedges in the same 

 illustration, have their leaves freely exposed to the air. A few 

 plants have both water leaves and air leaves (Fig. 353). Some 

 aquatic plants are rooted in the mud, while others have no roots 

 at all, or, like the duckweed, have only water roots. 1 

 1 See grouping in Sec. 454. 



