ACTION OF PLANTS ON THE SOIL. 257 



7. CHANGES IN THE SOIL INCIDENT TO THE NUTRITION 



OF PLANTS. 



Solution, displacement, and accumulation of particular mineral constituents of the soil owing to the 

 action of living plants. — Accumulation and decomposition of dead plants. — Mechanical changes 

 effected in the soil by plants. 



SOLUTION, DISPLACEMENT, AND ACCUMULATION OF PARTICULAR MINERAL 

 CONSTITUENTS OF THE SOIL RESULTING FROM THE ACTION OF PLANTS. 



Reference was made in the preceding section to a marble pillar on the faces of 

 which a dozen different lichens have settled in the course of centuries. I again 

 introduce to the reader's notice this unobtrusive monument in order to demonstrate 

 in its case the changes to which stone is subjected by the plants clinging to it or 

 nestling in its crevices. It may be premised, as a matter of course, that when the 

 marble column was erected two hundred years ago the eight sides were polished, 

 and presented perfectly even surfaces. But what is its appearance to-day? The 

 whole is rough and uneven; in parts it is as though corroded, and there are little 

 pits clustered together in places. The idea might arise that depressions have been 

 formed in course of time by the impact of drops of rain, but nearer inspection 

 shows that there can be no question that the inequalities have been produced in 

 this way; on the contrary, it is by the influence of the lichens adherent to the stone. 

 Especially on the two sides of the pillar facing south and south-west, one sees 

 clearly how each pit corresponds exactly in size to a species of grey lichen there 

 ensconced, and how this lichen, as it continues to grow and extends radially, 

 corrodes and etches the marble it touches in ever-widening circles. The expression 

 "to etch" may here be taken literally, for there is no doubt that the process, the 

 result of which is manifested in the formation of little pits, is mainly caused by the 

 excretion of carbonic acid from the lichen's hyphse, whereby the calcium carbonate 

 is converted into bicarbonate. The latter, being soluble in water, is, in part, taken 

 up by the lichen as nutriment, whilst part is washed away by the rain. 



In addition to this chemical action, the hyphal filaments exercise also a purely 

 mechanical influence. A growing hypha penetrates wherever the merest particle of 

 carbonate of lime has been dissolved and accomplishes regular mining operations at 

 the spot. Projecting particles of the carbonate not yet dissolved are separated by 

 mechanical pressure from the main mass; and at the places in question where a 

 lichen is in a state of energetic growth, tiny loose rhombohedral fragments of the 

 lime are to be seen, which are washed away by the next shower or else carried off 

 as dust by the wind. The same process as that which may be so clearly traced on 

 the marble pillar at Ambras takes place, of course, also on the limestone that has not 

 been carved or polished, in every locality where lichens exist at all. We notice it 

 in the case of other kinds of stone as well — in dolomite, felspar, and even in pure 

 quartz rock — for even quartz is not able to withstand the long-continued action of 



Vol. I. 17 



