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, i.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 (Ceratophyllwm), in all of which roots are absent; 

 and in addition amongst the lower or cryptogamic plants Riccia fluitans, and 

 many of the Desmidiaceae, Spirogyras and Nostocineae. 



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. 



