174 



General Botany 



these free-floating forms 

 are the duckweeds, the 

 Salvinia (a fern), and the 

 water hyacinth. These 

 plants float because of the 

 internal air cavities. In 

 the duckweeds stem and 

 leaf are not differentiated 

 — the plant consisting of 

 a flattened globular green 

 mass of cells with a pen- 

 dant root. In other float- 

 ing plants the stem extends 

 the plant by branching 

 and by forming new plants 

 at intervals. The duck- 

 weed plants that form in 

 cold water in late autumn 

 are constructed with more 

 compact tissues than those 

 formed earlier in the sea- 

 son. These later plants, being heavier than water, drop to the 

 bottom of the pond and remain there during the winter. Only 

 the late fall plants survive ; the earher ones that are Hghter re- 

 main afloat and are frozen. 



Submerged-rooted type. Other hydrophytes, like the pond- 

 weeds and water lihes, are rooted in the soil, and their stems bear 

 submerged or floating leaves. The stems have little or no me- 

 chanical tissue. As compared with land plants, the conductive 

 system is much reduced. Most hydrophytes develop horizontal 

 underground rootstocks and tubers. For this reason the plants 

 commonly grow in masses. Contrary to the usual opinion, even 

 whoUy submerged seed plants obtain their water and mineral 

 salts from the soil, and not from the water surrounding the upper 



Fig. ioo. Submerged-rooted plants. From left to 

 right: eelgrass {Vallisneria), naiad (Najas), water 

 weed (Elodea), and pond weed {Potamogcton). In 

 such plants the mechanical tissue and the conduc- 

 tive system are poorly developed. 



