ABSORPTION OF FOOD-SALTS BY LITHOPHYTES. sl 
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 
dolomitie 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 lable 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 meteorie dust, which is 
often drifted along in surprisingly large quantities, may originate quite suddenly 
during volcanic eruptions; nay, it is even possible that eosmie dust reaches our 
atmosphere and thence falls to the earth. Chemical investigation of aerial dust 
has, no doubt, yielded in most cases only sulphurie and phosphorie 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. 
In 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 lithophytie 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 protonemz and even the leafy shoots of Grimmic, Rhacomitrie, 
Andrewacee and other rock mosses, and the Collemacece and most crustaceous 
lichens only contain very minute quantities of these substances. Water containing 
Vou. I. 6 
