MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 265 



By filling with water a glass which contains vegetable and animal remains in 

 a state of putrefaction and swarming with bacteria, one is enabled to follow this 

 process of mineralization from day to day. First, a decrease of the organic matter 

 clouding the liquid, accompanied by simultaneous increase of ammonia and nitrous 

 and nitric acids, is observed; then, after about two months, a complete clearing up 

 of the liquid. The water is now colourless and odourless, but a precipitate has 

 formed at the bottom, which contains, in addition to insoluble food-salts, bacteria in 

 a state of temporary quiescence on the termination of their task and waiting till 

 fresh prey becomes accessible. No doubt these processes occur in nature in just the 

 same manner as in the glass of water, and the so-called self-purification of rivers, 

 for example, has been rightly attributed to mineralization. It was long ago noticed 

 that the water of such rivers as flow through great towns and consequently take up 

 considerable quantities of animal and vegetable refuse contains no discoverable 

 trace of all these impurities a few miles below the mouths of the drainage pipes and 

 sewers. The water of the Elbe, which receives the refuse of the towns of Prague, 

 Dresden, and Magdeburg, is so pure at Hamburg that it is there used for drinking 

 purposes without protest 1 . The Seine, after taking up masses of rubbish in Paris, is 

 already by the time it reaches Meulan, a distance of 70 kilometers, clear and pure 

 again, and does not even exhibit there any traces of the organic matter received in 

 the great city. Were it not for the activity of the putrefactive bacteria, this 

 purification would never take place; and although the statement that putrefactive 

 bacteria are the best of purifiers sounds at first like a paradox, it must be 

 acknowledged to be consistent and based on experience. 



MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 



All the alterations hitherto spoken of as being brought about in earth and 

 under the influence of vegetation subsisting therein are reducible to chemical 

 transpositions. Added to these, there are always certain purely mechanical changes. 

 The penetration of the rhizoids of a rock-moss or the hyphae of a crustaceous lichen 

 into limestone is accompanied, as has been already stated, by a solution of part of 

 the substratum and a mechanical separation of. another part; the rhizoids or hyphse, 

 as the case may be, becoming imbedded amongst tiny detached fragments of the 

 underlying stone. When the hyphse and rhizoids die, the corresponding piece of 

 the substratum is left porous, and admits air and water, whilst other plants are 

 enabled to settle on it, although they may not perhaps possess the power of eating 

 into stone and pulverizing it in the same degree as their predecessors. This is also 

 true of the roots of Phanerogams. The food-seeking root-tips and their absorption- 

 cells displace particles of earth as they insinuate themselves, and when they decay 

 later on, the soil at those particular places is intersected by passages of varying size. 

 No doubt these passages mostly collapse like the abandoned shafts and galleries of a 

 mine, but some trace of root-action always remains behind in the shape of an 



1 This was written before the last outbreak of cholera. 



