ABSORPTION OF FOOD-SALTS BY LITHOPHTTES. 81 



with millions of a species of Micrococcus, which lent a rosy hue to vast expanses 

 of snow. 



Most of the dust in the atmosphere originates, doubtless, from our earth. The 

 air that blows in waves over the earth can carry along with it not only dead and 

 detached portions of plants, but also loose particles of rock, sand, earth, and dried 

 mud. If one draws one's palm across the weather side of a dry rock composed of 

 dolomitic limestone, gneiss, trachyte, or mica-schist, the surface of the stone always 

 feels dusty, and the slightest movement of the hand is sufficient to detach a number 

 of particles which were already separate from the rock and only held in loose con- 

 nection with it. This dust is liable to be detached and carried away by any strong 

 gust of wind. Larger and heavier particles are not, it is true, lifted much above the 

 ground; they are rolled and pounded along and thereby reduced to a still finer 

 powder. This finer dust may then be scattered afar by gales blowing horizontally, 

 or even ascend into higher atmospheric strata. The finest dust in particular, how- 

 ever, is carried up into the higher layers of the air by the currents which ascend 

 from the earth in calm weather; and this applies not only to the tropics but to the 

 temperate zones as well, and even to the frigid regions of the arctic zone. When, 

 therefore, this dust is brought back by rain or snow from the upper aerial strata to 

 the earth, it but completes a circuit. Indeed it is highly probable that the particles 

 of dust restored to earth by means of atmospheric deposits recommence their aerial 

 travels as soon as they are thoroughly dry again, and that there is thus a circulation 

 of dust analogous to that of water. 



There is of course no inconsistency in the fact that meteoric dust, which is 

 often drifted along in surprisingly large quantities, may originate quite suddenly 

 during volcanic eruptions; nay, it is even possible that cosmic dust reaches our 

 atmosphere and thence falls to the earth. Chemical investigation of aerial dust 

 has, no doubt, yielded in most cases only sulphuric and phosphoric acids, lime, mag- 

 nesia, oxide of iron, alumina, silica, and traces of potash and soda, that is to say, the 

 most widely distributed constituents of the solid crust of our earth; but cobalt and 

 copper have also been found in it, over and over again, and it has hence been 

 inferred that the dust in these cases was of cosmic origin. 



Tn relation to the question which we have here to answer the above is, after all, 



almost a matter of indifference. The only important facts are that dust in a state of 



extremely fine division is blown about in the air, that this dust contains the salts 



required by plants for their food, that it is carried for the most part mechanically 



by drops of water and flakes of snow, condensed in the atmosphere, and is partially 



dissolved, that the atmospheric deposits supply lithophytic plants with a sufficient 



quantity of nutrient salts, and that the aqueous solution so supplied is rapidly 



absorbed by the whole surface of the plants in question. We must not omit to 



mention here that the demand of lithophytes for mineral food-salts is not very great. 



In particular the protonemse and even the leafy shoots of Grimmice, Rhacomitrice, 



And/receacece and other rock mosses, and the Collemaceae and most crustaceous 



lichens only contain very minute quantities of these substances. Water containing 

 Vol. I. " 6 



