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How to Know the Lichens and Mosses — What They Are Doing on Rocks and Trees — Part 3

It is true that all green plants do a similar work, but they do not work under such primitive conditions as do the mosses and lichens.

Aquatic mosses possess, perhaps to a greater degree, the power of arresting and retaining mud and fine sand hurried along by a violent rush of water. The plants of Hypnum rusciforme and Amblystegium riparium, which cling to rocks in streams, are so conglomerated by mud and sand that they cannot be freed from it until the plants have become dried and shrivelled. Limnobium molle, which grows in the turbid waters from glaciers, has such an abundance of earthy particles adhering to it that only the green tips of the leaf-bearing stems are visible above the gray-coloured cushions imbedded in the mud. It is the dead parts alone which retain in their thick felt of interwoven filaments, the firmly divided mud and sand. That they are able to do this is due to the fact that the cell-membranes swell up and become slightly mucilaginous. This mechanical retention and storage of dust by rock-plants, and of mud by aquatic plants, is of the greatest importance in determining the development of the earth's covering of vegetation. The first settlers are crustaceous lichens, minute mosses, and algae. Larger lichens and mosses are able to gain a footing on the substratum prepared by them.

" 'Tis spring-time on the eastern hills!

Like torrents gush the summer rills,

Through winter's moss and dry dead leaves

The bladed grass revives and lives,

Pushes the mouldering waste away,

And glimpses to the April day."

Whittier-Mogg Megone, Pt. III

The dead filaments, stems, and leaves of this second generation arrest dust in the air and mud in the water, and thus prepare a soft bed for the germs of a third generation, which on rocks consists of grasses, composites, pinks, and other small herbs, and in the water of pond-weeds, water-crowfoots, hornwort, and related plants. The second generation is produced in greater abundance than the first, and the third develops more luxuriantly than the second. The third may be followed by a fourth, fifth, and sixth, each successive generation crushing out and supplanting the one preceding it.

Another marked and important change results from these small beginnings. Streams on rather flat lands are turned from their courses by the accumulation of debris made possible by the arrested sand and mud, ponds have their outlets choked so that often new outlets must be cut, and small lakes are often cut in two by a natural divide which is due to the accumulation of sand and silt bound together, first by water plants and later by shrubs and trees.

The fact that at the present time the lichens and mosses are the first plants to appear on the soil, leads one to think that in ages gone by these little plants may have been the first to appear on the earth, and that they may have reigned supreme for a time in the plant world. This view is not sustained by positive testimony from the rocks, as there is no fossil evidence that mosses existed in Paleozoic times, nor has any certain trace of a moss been found in the coal-measures. Fossil mosses have been obtained almost entirely from tertiary and quaternary deposits.

Notwithstanding that there is no fossil evidence that mosses did exist, there is no evidence that they did not exist, as their absence from the plant records written in the older formations is probably to be accounted for by reason of their insignificant size and the difficulty of their preservation.

Another use the lichens and mosses subserve in the economy of Nature is illustrated by their habit of retaining great quantities of water in their spongy mass both on lofty mountain heights and in the forests of the valleys.

In many parts of the world it is principally the moss-covered soil of the forests which, by collecting the rainfall, prevents the pouring down from mountains of violent and excessive torrents of water.

Above the tree-line, in slight depressions on the sloping, rocky mountain sides, one may often find extensive patches of Sphagnum-moss and Reindeer-lichens which are crisp and dry on the surface, and yet retain so much water in their matted bases as to render it possible for one to obtain a supply of clear water. From areas of moss more extensive and of greater depth, tiny rills often trickle on their way to join other rills of similar origin. The sources of many a babbling brook or purling spring in the valley may be traced to the supersaturated moss-bed of a mountain forest.

"Desolate ledges, frost-riven and bare,

A tiny rivulet bore on their breast;

Cloud-gray mosses and lichens fair

Mutely besought her to slumber and rest."

Willis Boyd Allen

"Thou hastenest down between the hills to meet me at the road,

The secret scarcely lisping of thy beautiful abode

Among the pines and mosses of yonder shadowy height,

Where thou dost sparkle into song, and fill the woods with light"

Lucy Larcom.