76 ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 
aquatic plants to land-plants. When the water sinks and they are finally left lying 
exposed on the mud or wet sand, to which they appear to be firmly attached by 
their abundant roots, it is only the previously submerged leaves that dry up. That 
part of the foliage which floated on the surface and was consequently always in 
contact with the air continues to thrive, and any fresh leaves that may be developed 
adapt themselves completely to the new environment. Similar behaviour is ob- 
served in many of the plants which float freely on the surface of water. Such, for 
instance, is the case with some species of duckweed (Lemna minor and L. 
polyrrhiza), with Azolla, Pontederia and Pistia; they do not die when the water 
sinks, leaving them stranded, but absorb food-stuffs from the wet earth through 
their roots, and in this condition are not to be distinguished from land-plants. 
Hydrophytes in the narrow sense, 7.e. plants which are entirely submerged and 
die if they are surrounded by air instead of water for any length of time, are for 
the most part fixed to some support beneath the water. In many cases the 
characteristic method of reproduction consists in the separation of special cells, 
which then swim about for a time in the water. Sooner or later, however, they 
re-attach themselves to some seemingly suitable spot, and the further phases of their 
development are again stationary. Comparatively few permanently submerged 
species are freely suspended in the liquid medium in every stage of development. 
Such free plants are liable to be shifted by currents in the water, but the extent of 
their displacement is never very great, owing to the fact that submerged species of 
this kind occur almost exclusively in still water. As instances may be mentioned 
the ivy-leaved duckweed (Lemna trisulca), the water-violet (Hottonia palustris), 
the various species of hornwort (Ceratophyllum), in all of which roots are absent; 
and in addition amongst the lower or eryptogamie plants Riccia jluitans, and 
many of the Desmidiacez, Spirogyras and Nostocinez. 
Some of these aquatic plants periodically rest on the bottom of the pond or 
lake in which they live. An example is afforded by the remarkable plant known 
as the water-soldier (Stratiotes aloides), which, as is indicated by its Latin name, 
is not unlike an aloe in appearance. During the winter, this plant rests at the 
bottom of the pond it inhabits. As April draws near, the individual plants rise 
almost to the surface and remain floating there, producing fresh sword-shaped 
leaves and bunches of roots which arise from the abbreviated axis, and finally flowers 
which, when the summer is at its height, float upon the surface. When the time of 
flowering is over, the plant sinks again to mature its fruit and seeds, and develop 
buds for the production of young daughter-plants. Towards the end of August, 
it rises for the second time in one year. The young plants that have meantime 
grown up resemble their parent completely, except that their size is smaller. 
They grow at the end of long stalks springing from amongst the whorled leaves, 
and the stately mother-plant is now surrounded by them like a hen by her chickens. 
During the autumn, the shoots connecting the daughter-plants with their parent rot 
away, and, thus isolated, each little rosette, as well as the mother-plant, sinks once 
more to the bottom of the pond and there hibernates. 
