WA TER-PLANTS 
163 
Heterophylly (p. 50) is a frequent phenomenon in water- 
plants; the most common case is the production of swimming 
and submerged leaves of different patterns, e.g. in Cabomba, 
Callitriche, Potamogeton sp., Ranunculus sp., Sagittaria, 
Salvinia, Trapa, &c. 
Most water-plants live in comparatively quiet water, but 
in the tropics there is a very interesting family, the Podo- 
stemaceae, living in swiftly moving water, in rapids and 
water-falls, where they cling to the rocks like seaweeds on a 
rocky coast. As might be expected, they show a very 
peculiar structure ; they differ from other water-plants in the 
absence of intercellular spaces, and in their possession of a 
thallus. The seeds lie about on the rocks in the dry season, 
and germinate in the rainy season when submerged ; the 
thallus arises from the primary axis and is sometimes of 
shoot, sometimes of adventitious root nature. It is fastened 
to the rocks by root-hairs and by special holdfasts or haptera , 
and upon it arise numerous secondary shoots, upon which 
the flowers are ultimately developed, opening when the fall 
of the water in the dry season exposes them to the air. The 
exposed plants soon die. These plants show the most 
remarkable structural dorsiventrality among the higher 
plants. 
For further details reference should be made to Part II., 
articles Aldrovanda, Alismaceae, Aponogeton, Azolla, Bi- 
dens, Cabomba, Callitriche, Ceratophyllum, Eichhornia, 
Elodea, Jussieua, Lemna, Littorella, Limnanthemum, Myrio- 
phyllum, Naias, Nelumbium, Nuphar, Nymphaea, Nym- 
phaeaceae, Peplis, Pistia, Podostemaceae, Potamogeton, 
Potamogetonaceae, Ranunculus, Rumex, Ruppia, Sagittaria, 
Salvinia, Sesbania, Stratiotes, Subularia, Trapa, Vallisneria, 
Victoria, Zannichellia, Zostera, &c. 
Xerophytes 1 . In such a climate as that of the low- 
lands of western Europe, everything in the structure of the 
leaf, the phyllotaxy, branching, &c., is so arranged as to 
favour transpiration to the utmost, and thus to cause a 
rapid current of water from the roots to the leaves. So 
long as the roots can absorb plenty of water this form of 
1 Schimper , Pjlanzengeographie \ Warming, Oekologische Pflanzen- 
geographie ; Volkens, Flora der aegyptisch-arabischen Wiiste ; Goebel, 
Pflanzenbiologiscke Schilderurtgen . 
I I 2 
