IX] SHOOTS OF AQUATIC PLANTS 127 



wrist, and marked with the scales of several dozens of 

 leaves. 



The foregoing are prominent types of modified shoots 

 growing under or near the ground, and concern land-plants 

 especially. But similar modifications occur in aquatic 

 plants whose roots are in the soil, while the shoots are 

 wholly or for the most part in water. 



Thus, species of Sparganium, Zoster a, A cor us, Potamo- 

 geton, Typha, various Water-lilies, Myriophyllum, Marsilea, 

 &c., afford examples of rhizomes; while Ranunculus 

 aquatilis and R. hederaceus, Hottonia, Limnanthemum, 

 Mentha aquatica, A Us ma, Hydrocharis, Poa aquatica and 

 P. fluitans generally have stolon-like branches which creep 

 on the surface of the mud and emit roots into it. 



Many aquatic plants, however, have the majority of 

 their leafy shoots either wholly submerged and buoyed up 

 in the water, as Utricularia, Myriophyllum, Elodea, &c., 

 or with some at least of the leaves floating on the surface, 

 as some Potamogetons, Ranunculus hederaceus, Hydro- 

 charis, &c. 



In Lemna, the Duckweed, the whole shoot of the leaf- 

 less plant is represented by the floating discoid green 

 bodies so well known on ponds and ditches, from the 

 centre of which, below, the single root hangs vertically 

 down into the water. Species of Pistia and Pontederia 

 also float free on the water, their leaves emerging into the 

 air, while Aldrovandia, some Utricularice and others only 

 bring their free leafy shoots just to the surface. In the 

 cases of Salvinia and Azolla we have floating horizontal 

 shoots, creeping on the surface of the water as it were. 



Of all modified shoots the strangest is probably the 

 swollen fleshy form acquired by certain desert plants, or by 

 such as are at certain seasons of the year exposed periodi- 

 cally to excessive droughts. 



