258 ACTION OF PLANTS ON THE SOIL. 



carbonic acid and the mechanical operations above referred to in the performance 

 of which the hyphse act like levers. Some of the powerful iron bands belonging 

 to the great suspension bridge across the Danube at Budapest afford us the 

 opportunity of observing the mining operations of lichens on a substratum of pure 

 iron. Of course in these cases the decomposition and solution initiated by the 

 carbonic acid varies according to the nature of the substratum; the result is, 

 however, invariably the same; there is always a loss of substance on the part of the 

 substratum, and a part of the dissolved matter is always taken up by the adherent 

 plant, whilst another part is carried away either in solution or mechanically by 

 wind or rain. 



Mosses act in precisely the same manner as lichens. If a tuft of Grimmia 

 apocarpa is lifted away from the side of a block of limestone, it becomes evident 

 that in the neighbourhood of the place where all the stemlets of the little moss- 

 colony meet, the underlying stone is threaded through and through, and rendered 

 friable. There lie the rhizoids imbedded between isolated particles of lime, which 

 are as fine as dust, and have been disintegrated by the chemical and mechanical 

 activity of the organs in question. At spots where plants of Grimmia have died, 

 the limestone always exhibits an obvious loss of substance in the form of unevenly 

 corroded depressions. 



The fact that the roots of Phanerogams also alter the subjacent stone in a 

 similar manner may be proved by the following experiment. A polished slab of 

 marble is covered with a layer of sand, and seeds of plants caused to germinate in 

 this sand. The roots of the seedlings as they grow downwards come almost 

 immediately upon the marble slab, and, turning round, creep onward in close 

 contact with the stone. After a short time the parts of the slab against which the 

 roots are pressed become rough as though they had been etched; a solution of 

 individual particles of the carbonate of lime takes place under the influence of the 

 acid juice saturating the cell- walls of the root's cells, and this circumstance reveals 

 itself to the naked eye as a roughness which is readily perceptible. 



Whereas the loss of substance affecting the solid substratum of plants may thus 

 be at once detected by sight, the removal of constituents of the air and of water 

 eludes direct observation. The ingredients withdrawn by plants are instantly 

 replaced in water and still more in the air by influx from the environment, and 

 obviously no holes or pits are the outcome as in the case of a surface of limestone rock. 



In the discussions that follow it is important to retain the conception that in 

 the process of vegetable nutrition certain substances may undergo local displace- 

 ment, accumulation, and aggregation, and temporary consignment to a state of 

 quiescence. Ingredients of the earth's crust are borne upwards into atmospheric 

 regions, and constituent parts of the air are carried deep down into the ground. 

 Lime, potash, silicic acid, iron, &c, pass from disintegrated rocks into the realms 

 above ground — into stems and leaves, and to the tops of the highest trees, whilst 

 carbon and nitrogen pass from the aerial shoots and from the foliage spread out in 

 the sunshine into the deepest shafts which the roots have bored for themselves in 



