CORROSION FIGURES. 263 



similar corrosion figures are to be obtained also on polished surfaces of Dolo- 

 mite (a mixture of calcium carbonate and magnesium carbonate), of Magnesite 

 (crystalline magnesium carbonate), and of Osteolite (earthy apatite, chiefly tribasic 

 calcium phosphate) when the roots of Beans, Maize, Wheat, Gourd, TropcBolum, 

 Peas, and others are caused to apply themselves closely, and grow over the 

 polished surfaces of these stones. The experiments were made by laying the 

 plate of stone, with the polished surface turned upwards, at the bottom of 

 a vessel 10-15 cm. deep; this was then filled with clean washed sand, and 

 several seeds placed in the latter. With sufficient moisture the radicles grow 

 vertically downwards until they abut upon the polished surface ; there they 

 bend to one side, grow on horizontally, clinging to the polished surface, and 

 form lateral roots which likewise become closely attached. When after 8-10 

 days, in summer, the vessel is upturned, and the stone plate carefully washed 

 and dried, the corrosion figure of the roots is seen on the polished surface. 

 The experunent succeeds with such certainty that'it may be used for demonstration 

 in lectures. 



It follows from these facts that the root-hairs, when they meet in the 

 soil with small pieces of stone which contain carbonate of lime, compounds 

 of phosphoric acid, magnesite and dolomite, dissolve small quantities of these 

 salts at their surfaces, and carry them into the plant. Since now the soil of 

 vegetation is a very various mixture of small particles of stone with organic 

 remains, so-called humus, diflferent hairs of the same root, and likewise the hairs 

 of different roots of a plant, find opportunity here and there of taking up sometimes 

 chiefly phosphates, at others potassium compounds, &c., and conveying them into 

 the sap of the plant. In this, each individual root-hair of course accomplishes 

 but very little ; the quantity which it is able to take up will perhaps only amount 

 to the millionth part of a milligram. As we know already from previous study, 

 each root-hair remains capable of action but a few days ; however, on a plant 

 even of inconsiderable size there are millions of active root-hairs, and as the 

 older of these die, millions of new ones appear in their place, to use up portions 

 of the soil not yet laid under contribution. We must here remember what 

 was said in the organography of the roots, that behind each root-apex as it 

 grows forward in the soil, new root-hairs are continually arising, while the older 

 ones further behind on the root perish (Fig. 207). Since now, at the same time, 

 new root-fibres spring in various directions from those already existing, and 

 extend in the soil, new portions of the latter are continually being placed in 

 requisition by the root-hairs; and thus the whole of the soil, permeated by the 

 thousands and hundreds of thousands of fine root-fibres of a plant, is gradually 

 used up. 



The roots thus evidently contribute to the chemical decomposition, of the solid 

 particles of stone in the soil of vegetation, — a process which proceeds at the 

 same time slowly by means of the carbonic acid contained in the soil, and of 

 the nitric acid contained in the rain water-; and if the rotting remains of 

 vegetation are again incorporated in the soil, the vegetable mould must in this 

 way continually become richer in soluble and absorbed nutritive matters. This 

 influence of plants on their mineral substratum is clearly evident where Lichens 



