PROCUMBENT AND FLOATING STEMS. 665 



necessarily limited. Again, the earth offers an insuperable barrier to the develop- 

 ment of foliage in a downward direction. In the dark bosom of the earth a green 

 leaf would be quite useless, and, as a matter of fact, there is not a single plant 

 whose green tissue is situated in the depths of the soil. 



With water it is otherwise. In it green cells and tissues can function as 

 far down as the light can peneti-ate. Since the water also maintains the stem 

 and leaves in a definite position, and the plants consequently are spared the 

 development of wood and bast and, generally, of masses of tissue for strength 

 and resistance to bending, and since, finally, a saving of material and work is 

 effected inasmuch as water-plants do not i-equire to construct organs for con- 

 duction of water and for transpiration, it might be suj^posed that water would 

 be an extremely favourable medium for green vegetation, and that, consequently, 

 stretches of water all over the world would be quite crowded with green 

 plants. That this is not the case is explained by the fact that light does not 

 penetrate far enough into the water. In the deep gloom, 200 metres below the 

 surface, green plant-life is as impossible in water as in the dark bosom of the 

 earth, and the bottom of the ocean over an enormous area is a plantless waste 

 shrouded in gloom. But as far as the water is illuminated, in all places where 

 it fills shallow basins, and also in a comparatively narrow girdle around the 

 coasts, an inexhaustible wealth of plants is to be found. Of course, spore-bearing 

 plants, which are built up of rows, nets, and plates of cells, have the preponderance, 

 whilst seed-plants are markedly in abeyance in relative number of species. But 

 the latter species are just the ones which claim our interest in a special degree 

 on account of the very peculiar conditions under which they live. 



The floating stems of water and marsh plants, as already repeatedly stated, 

 have no wood or bast, while, on the other hand, they are penetrated by remarkably 

 large air-canals, and are, in consequence, exceedingly light and buoyant. If the 

 erect stem of a water-plant growing at the bottom of a lake is cut through 

 close above its roots, it rises immediately to the surface of the water, there 

 assumes a horizontal position, and remains floating; under certain circumstances 

 it may continue to grow and may perhaps take root should it drift to a shallow 

 place. On the other hand, if a pond filled with Water Crowfoot, Myriophyllum, 

 Elodea, &c., be emptied, all these plants sink limp and withered on to the mud, 

 as their stems have not the strength to hold themselves erect. The water in 

 which they float supports and bears them, and in this respect they may be 

 likened to climbing stems which also require a support to enable them to rise 

 above the ground. The analogy between these plants is evident in so far as 

 the need for "more light" influences the direction of growth in both cases — in 

 the one case the stem gi-ows out from the gloom of the forest floor up to the 

 sunny tops of the trees, in the other, from the subdued light at the bottom of 

 the lake up to the surface of the water. In many cases, of course, the stem of 

 water-plants remains so short that it scarcely rises above the mud at the bottom 

 of the pond, but the leaves arising from it are shaped into long ribbons, wliose 



