THE ECOLOGY OF PLANTS. 417 



more by the edaphic factors than by the climatic conditions, 

 and of the edaphic factors the most important are the physical 

 properties (especially the porosity) of the soil and the presence 

 in it of humus. 



401. Water- Plants. The submerged leaves of a water- 

 plant are usually either long and strap-like, as in Glyceria 

 fluitans, a water-grass ; or narrow and arranged in whorls, as 

 in Water Starwort (Callitriche) ; or divided into numerous 

 fine threads, as in Water Buttercups. These submerged 

 leaves have chlorophyll in the epidermis, which bears no 

 cuticle, so that water containing dissolved salts and gases can 

 pass in freely. Since the submerged parts get their salts, 

 oxygen, and carbon dioxide directly from the water, there are 

 no stomates, nor does the stem contain many wood- vessels, 

 and the roots serve chiefly to fix the plants to the bottom, 

 having few or no root-hairs, since root-absorption is not 

 required. 



As we have seen, a land-plant requires to have its stem and 

 leaves strengthened by hard tissue, arranged so as to resist 

 the strains caused by weight and by wind. In a water-plant 

 the weight is, of course, supported by the water, and the only 

 strain to which fixed submerged plants are subject is a 

 pulling-strain due to movement of the water, especially in 

 fast streams. For these reasons there is little or no special 

 mechanical tissue outside of the vascular bundles, and the 

 latter are placed in the centre of the submerged stem, as in 

 the root of a land-plant. 



Leaves which float on the surface of the water are entire 

 and rounded or slightly lobed (Water-lilies, Pondweeds, Duck- 

 weed, and some Water Buttercups), and bear stomates on 

 their upper surface, which is covered with cuticle or wax so 

 as to prevent wetting. These floating leaves have the same 

 general structure as those of land-plants, but the air-spaces 

 are very large and are continuous, with air-passages running 

 down the leaf -stalk to the submerged stem and roots. Air- 

 spaces are also present in the stems of plants which grow 

 with only their lower parts in water, e.g. Eushes, Sedges, 

 Mare's-tail, Horsetails. Besides helping to make the plant 

 buoyant;, the air-spaces store up air absorbed from the water 

 and aerate the lower parts of the plant which grow in deep 



8.B. 27 



