ACTION OF PLANTS ON THE SOIL. 261 



fibres. What a quantity of carbonate of lime must be deposited yearly at the 

 bottom of these lakes and ponds! Amongst pond-weeds, Potamogeton lucens, in 

 particular, clothes its large shining leaves with a very stout, uniform crust, 

 which drops off in scales as the plant dries, the weight of which can be exactly 

 determined in the case of each separate leaf. The result of careful weighing showed 

 that a single leaf equal in weight to 0*492 grm. was covered with a calcareous crust 

 weighing 1-040 grm. Now, supposing one shoot of this pond- weed, having five 

 leaves, and covering an area of 1 square decimeter, decays in the autumn, and lets 

 its lime sink to the bottom of the pond, the approximate weight of lime deposited 

 each year on a square decimeter of the ground at the bottom is 5 grms., and, if 

 this process is repeated every year, a layer is deposited in ten years which weighs 

 50 grms., and consists of calcium carbonate and traces of iron, manganese, and 

 j silicic acid.^ 



There is no doubt that it is possible for calcareous strata of great depth to be 



j produced in this way in fresh water. That also in times past lacustrine deposits of 



I lime have had a similar origin is inferred from the fact that the spore-fruits of 



I stoneworts (Characese) and the nutlets of pond-weeds have been found over and 



j over again inclosed in these formations of lime. Calcareous deposits originating 



! in this manner are, at present at least, less frequent in the sea. Still, the Aceta- 



: bularioe undergo similar changes there, and may be the cause of an elevation of 



! the sea bottom and of an accumulation of lime. On the other hand, in the sea, 



the Lithothamnia and Corallinas play a predominant part, and form — just like 



true corals, and often indeed in conjunction with these and other marine animals — 



lime reefs of great magnitude. 



The agency of plants may occasion accumulations of iron hydroxide, silicic acid, 

 and salts of potassium and sodium at particular places besides lime. The formation 

 of meadow iron-ore, spring iron-ore, and bog iron-ore, the construction of tripoli, 

 agate, and flint, by the conglomeration of siliceous-coated Diatomaceae, and the 

 accumulation of potassium and sodium salts in the superficial strata of salt steppes 

 are processes which take place essentially in the same manner as the piling up of 

 carbonate of lime, although upon a more modest scale. 



The question now arises, why it is that the substances which are stored in pre- 

 ponderant quantities in the vegetable frame, which are the main constituents of the 

 living part of plants, and represent the alpha and omega of plant life, are not pre- 

 j served as well as the mineral food-salts in question. Why do not carbon and 

 nitrogen, materials so eagerly appropriated by the living plant, compounded by it 

 with the elements of water, secured in some measure in organic compounds, and 

 constituting the fundamental mass of the vegetable structure, remain behind in the 

 same condition after the death of the plant ? W^hen autumn comes and the lime- 

 laden pond-weed dies, only the calcareous crust falls to the ground, and, at the 

 bottom of the pond, enters upon a period of quiescence. The tissue of the plant 



^In the case investigated 96 per cent calcium carbonate, 28 per cent iron oxide, 1'51 manganese oxide, and 1'51 

 per cent silicic acid ; the last, from the Diatomacese, settled on the calcareous crust. 



