162 
WATER-PLANTS 
mucilage by glands or hairs upon the surface. Its presence 
renders diffusion more slow and is probably advantageous in 
this way, by preventing the too rapid escape of the substances 
dissolved in the cell-sap into the surrounding water. 
Most water-plants are perennials ; annuals occur in the 
genera Naias, Subularia, Marsilea. Many Podostemaceae, 
though usually annual, may become perennial if not exposed 
in the dry season. 
The great vegetative growth and multiplication of water- 
plants is accompanied as usual (p. 113) by a reduction of 
the flowering activity (but cf. Podostemaceae which combine 
the two). Many species flower rarely, and few flower as 
profusely as land-plants. Most species display their flowers 
above the water for pollination by wind or insects. The 
mechanisms resemble those of ordinary land-plants, but the 
flowers are rarely successful in setting much cross-fertilised 
seed, for they are so close to the water that much of the 
pollen of the anemophilous species falls into the water and 
is wasted, and insects are few and far between so that the 
entomophilous species are not much visited. A few plants, 
e.g . Elodea and Vallisneria, display a modification of their 
floral structure adapted to pollination at the surface of the 
water, and in Ruppia, Zostera, &c., pollination takes place 
under water and the floral structure is highly modified. 
The fruits are usually achenes, nuts, or schizocarps, and 
are nearly always ripened under water; some plants, e.g. 
Vallisneria, have special arrangements for drawing them 
down to ripen ( cf. Cyclamen). The seeds (or fruits) usually 
sink in water, but some plants have arrangements by which 
they may be kept afloat for a time .and thus dispersed to a 
distance (see Nymphaeaceae, &c.). The germination of the 
seeds often furnishes interesting features — leaves differing 
from those of the mature plant, special adaptations, &c. 
The same plant may often grow at different times under 
different conditions and exhibit different structural features 
and habit. Many plants which usually grow in marshy 
places are able to grow submerged in water, and frequently 
the land and water forms differ, e.g. in Littorella, Polygonum 
sp., &c. Such plants may be termed amphibious. The 
most fully adapted water-plants such as Myriophyllum, 
Vallisneria, Zostera, &c., are not capable of living upon land. 
