260 ACTION OF PLANTS ON THE SOIL. 



lime is to be found, that these snails again are to be reckoned as accumulators, and 

 that their shells, which consist almost entirely of lime, remain after the animals' 

 deaths in the top layer of soil, it is not surprising to find that the earth-mould on a 

 granite plateau contains a proportion of lime not much less than that yielded by 

 mould resting on argillaceous limestone. 



Still more striking than the influence of rock plants and land plants in trans- 

 posing and accumulating lime is the agency of hydrophytes in causing the same 

 results. In the trickling springs of mountainous regions as well as in the standing 

 pools of level country and no less in the depths of the sea, plants occur which 

 obtain part of the carbonic acid they require by the decomposition of the bicar- 

 bonate of lime dissolved in the surrounding water. The monocarbonate of lime, 

 which is insoluble in water, is then precipitated in the form of incrustations upon 

 the leaves and stems of the plants in question. Many of these hydrophytes take up 

 carbonate of lime into the substance of their cell-membranes; and in other cases both 

 phenomena occur, that is to say, not only are they incrusted externally with calcium 

 carbonate, but the cell-walls are also thoroughly impregnated by the same salt. In 

 the streams arising from springs loaded with bicarbonate of lime in solution derived 

 from the heart of a mountain, a number of mosses regularly occur — Gymnostomum 

 eurvirostre, Trichostomum tophaceum, Hypnum falcatum, and others besides. 

 These mosses and also several species of Nostocinese belonging to the genera 

 Dasyactis and Eiiactis become completely incrusted with lime, in the manner 

 referred to, but go on growing at the apical end as the older and lower parts 

 imbedded in lime die off. In consequence, the bed of the stream itself becomes 

 calcified and elevated, and, in course of time, banks of calcareous tufa are formed, 

 which may attain to considerable dimensions. Banks raised in this manner are 

 known which are no less than 16 meters in height; to construct them mosses must 

 have worked for more than 2000 years. 



Numerous Stoneworts (species of Chara or Nitella), the Water-milfoil and Horn- 

 wort (Myriophyllum and Ceratophyllum), Water-crowfoots (Banunculus divari- 

 catus and R. aquatilis), and more especially many Pond-weeds {Potamogeton), 

 which grow in continuous masses in still, inland waters, incrust their delicate stems 

 and leaves with lime during the summer, but in autumn shrink away, that is to say, 

 their stems and leaves fall and decay, leaving scarcely any trace of the mass of 

 vegetation till the advent of the following spring. The calcareous deposits, how- 

 ever, are preserved, and, sinking to the bottom of the water where the incrusted 

 plants lived, form a layer which year by year increases in thickness. Anyone who 

 undertakes the investigation of the sequestered wastes of water in the shallow 

 lakes of lowland districts will be convinced of the magnitude of the scale on which 

 this kind of accumulation must take place. As one's boat glides over places where 

 there is a luxuriant growth of the lime-incrusted Chara rudis and G. ceratophylla, 

 there is a crepitating sound in the water like the snapping of dry sticks of birch- 

 wood. Great numbers of stoneworts are fractured by the boat as it strikes against 

 them, and if one takes hold of the fragments they feel like a heap of brittle glass 



