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How to Know the Lichens and Mosses — What They Are Doing on Rocks and Trees — Part 2

Their destined purpose."

One of the most important sources of the nourishment of plants is carbon dioxide (C O2). It is the gas which bubbles up from "soda water" and it is the gas breathed out by animals. It is formed wherever a candle, lamp, or wood is burning or wherever vegetable or animal matter is decomposing. The gas is itself a compound of an elementary gas, oxygen (O) united with an elementary solid, carbon (C) known by the common names of charcoal and graphite. Stated in a general way, the carbon dioxide passes through the walls of the plant cells into the cell-contents and there by the leaf-green (chlorophyll) the oxygen gas (O) is set free to return to the atmosphere, and the solid carbon (C) is worked up with water into plant foods called carbohydrates, compounds of carbon and water, of which starch, sugar, and plant tissues are examples.

"A small sisterhood of plodding lichens

Wrought on the rock; the sun, the wind and rain,

Helping then gladly, till each fissure filled

And fit for planting, mosses came in haste

And strewed small seeds (spores) among them, destined they

To clothe the stern old rock with softest verdure

With ferns and flowers, where yet the labouring bee

May find pasture."

Certain lichens carried by the winds to places unsuitable for other plants, begin their work of dissolving the inhospitable rock to obtain mineral salts which the leaf-green may, together with water, manufacture into plant food; the delicate threads of the lichen work their way in and out among the particles of rock too small to be visible to the naked eye, and as they swell with water absorbed from the atmosphere, they pry off tiny particles of rock, thus slowly but surely preparing soil for higher forms.

The mosses also can take their start in life on bare and rugged rock, although not so generally as the lichens.

If a tuft of Grimmia apocarpa is lifted away from the limestone upon which it is growing, one may see corroded depressions in the neighbourhood of the place where the stemlets of the moss colony meet, and one may see the rhizoids of the moss imbedded in loose particles of limestone which have been separated from the main rock by a dissolving fluid which the rhizoids secreted upon the rock. In this way the moss obtains mineral salts which are necessary for its growth. The solid rock is crumbled to a dust which may be blown by the wind to other localities, or which may remain on the spot and furnish soil for higher plants. In addition to the chemical action which the moss exerts in dissolving the rock, it, as well as the lichen, exerts a purely mechanical influence, for a growing rhizoid penetrates wherever the merest particle of limestone has been dissolved and by mechanical pressure separates the particles of limestone which remain.

The mosses and lichens are truly efficient agents in rendering rocks available for plant life by retaining minute particles of soil but their work does not stop here, for as the older plants die and crumble and mingle with the disintegrated rock, an incredible amount of earth-mould is formed which is a favourable site for higher forms of mosses, ferns, and other spore-bearing plants.

That the leafy parts above arrest to a remarkable degree the dust which pervades the atmosphere, not only along dusty road-sides and open plains, but also in remote mountain valleys, in Arctic ice fields, and in most of the elevated parts of the earth's crust, will be evident to one who detaches and examines a small tuft of Barbula, which everywhere occurs on roadside walls. He will be surprised to learn the extent to which the road dust has been lodged in the older dead parts of the plants, and he will be equally surprised to learn with what tenacity the dust is held. The power the older parts of the plants have of holding the dust is due to certain alterations which take place in the lifeless cell-tissue. To be convinced that fine dust is also carried to the more remote and elevated regions, one must examine the lichens and dark Grimmias, Andraeas and other rock mosses which grow in small cushion-like tufts on weather-beaten mountain crags, when he will find that not much less dust has been arrested by them than by the Barbula living near the dusty roadside.

Old crumbled lichens, together with dust blown thither by the wind, accumulate under the thallus, or leaf-like expansion of the lichen, and soon form a suitable home in which moss spores may grow. The mosses in turn add their share to the accumulation of humus preparatory to the coming of the ferns, and the ferns in turn prepare for the trees with winged seeds, the evergreens and birches, which require no very great depth of soil, sturdy pioneers of mountain forests.