Many persons have exclaimed with delight over a brilliant patch of the little red-crested lichen, or British soldiers, growing on sterile soil, and have gathered it for dish-gardens and table decorations; or have noted the widely-spreading, grayish green tufts of the so-called reindeer “moss,” one of the commonest of the soil lichens.
But the soil lichens are not generally so attractive nor so prominent as the rock-lichens. These rock-rosettes, or “doilies,” form an adornment on bare rocks and cliffs which are visible for great distances. Many of these attractive species are within easy reach of one who walks afield, except in the region of cities, for lichens cannot grow where the air is tainted with smoke or with the fumes of industry. The rock-lichens are especially noticeable on humid, damp, or rainy days.
Just what is a lichen? A lichen is a partnership between a fungus and an alga; a close association between a mass of fine fungal threads, entangling a multitude of little green cells of an alga. The algae most commonly present in this alga-fungus combination belong usually to the Blue-green Group, Cyanophyeeae or to the Green Group, Chlorophyeeae. The algae are able to live “wild” without the fungus; but in general the fungus threads soon perish if they do not come across the appropriate algae.
The body of many of the rock-lichens, generally of a rosette-form, consists of a particularly tough and resistant sheet of tissue made up of closely-felted fungal threads that completely protect the delicate little algal cells enmeshed within. The fungal threads afford shelter to the algal cells, and the algal cells manufacture food for the fungal threads, so “they live happily ever after.”
Of all the members of the plant world, the lichens possess the greatest capabilities of adapting themselves to the most widely divergent conditions of climate, altitude, moisture, drought, heat, and cold. They spread from the tropics to the poles, and their numbers increase from the equator northwards and southwards.
These are the first growths to appear on naked rocks and bare cliffs that offer neither foothold nor nourishment for other kinds of plants. With their acids, the lichens are able to dissolve the rock-forming minerals and thus break up the rock itself and secure a firm footing. They are, therefore, the pioneers which prepare the substratum upon which mosses and other minute growths can secure an anchorage and food. They live partly upon mineral solutes, and partly upon microscopic air-borne particles, but chiefly upon the products of the photosynthesis of their entrapped algal cells. After a. period of growth, rock-decomposition, and decay, these hardy pioneers have thus prepared the surface of the rock for the succession of higher plants which follow, and which further carry forward the transformation of the rock into soil.
Lichens are the most widely disseminated of the larger plant forms. One species, of interest in this respect, is the green map lichen, Rhizocarpon geographieum, or Leddea geographica, (shown in the drawing) said to be the most universally distributed larger organism of any kind. It is found literally to the ends of the earth. It is the highest-growing of any of the plants of the Alps, and was the only plant growth found by Agassiz near the summit of Mt. Blane. It has been collected at the elevation of 19,000 feet in the Himalayas, where it occupied the very last outpost of vegetation. The same species has been reported from Mt. Chimborazo in the Andes. This is the little lichen that so attractively colors and makes more cheerful the forbidding bare summits of our Adirondacks, Green Mountains, White Mountains, and others; splashing its little rosettes of bright apple-green thickly over the naked rocks. But other species also lavishly decorate the austere cliffs and ledges of our northern mountains.
It was reserved for the Second Byrd Antarctic Expedition to discover the most southerly existence of plant life. This was in the form of tiny lichens (not yet, I believe, determined as to genus or species) – lichens no bigger than the heads of pins, growing on the northern exposure of a mountain. Here for only a week or so in midsummer does the temperature rise above the freezing point. This short time, then, is the only growing-season for these minute atoms of life.
As to our known rock-lichens, it has long been supposed that some of them may attain to a very great age, though on this subject the records are fragmentary and scattered. One fmds such statements as these, for example: “Some species growing on the primitive rocks of the highest mountain ranges of the world are estimated to have attained the age of at least 1000 years.” (Lindsay). “One crust lichen, Vadolaria, has been known to increase one-half millimeter in size between the end of February and the end of September.” (Elliot). Some lichens “grow two or three years rapidly. Some have been known to he 45-years-old before beginning to fruit.” (Creevey). “We have no data from which to ascribe the age of tartareous species which adhere almost inseparably to the stones. Some of them are probably as old as any living organisms on earth.” (MacMillan). This authority also hazards the opinion that some rock-lichens may date back to the retreat of the last glacier. In this country this would mean something between 25,000 and 50,000 years.
Our longest and most careful study was made of three plants of the Parmelia genus, the commonest genus of rock-rosettes on mountains. The plants selected were three closely associated individuals of Parmelia centrifuga, a tightly adherent, thin, resistant species, that grows at a higher altitude than most other members of the genus. The botanical and mathematical details of this study were reported in the Bulletin of the Torrey Botanical Club in January, 1948. The gist of this study is that the average growth-rate of all these plants together, measured over a seven-year period, was only 0.85 millimeters a year. This was at an altitude of 2,100 feet, where growing-seasons are much longer. Higher, above 3,200 feet, on Mt. Chocorua, New Hampshire, we found large rosettes whose age must have been 1,500 years or more. Through binoculars on inaccessible cliffs some 10,000 feet or more in altitude, in the Alps, we have recorded far larger patches which we judged may have begun growing some 10,000 years or so ago. So it seems reasonable to suppose that lichens represent the oldest living beings on our earth. Of course, this does not mean that the plant tissue we see now has been in existence over that period of time, for the central point of original growth in these great rosettes has disappeared, and only a ring of younger growth may persist. The inner portion of the rosette is often occupied with subsequent concentric frilly lobes, or mosses which have lodged upon the decomposing tissue of the original earlier zones of lichen. But, however that may be, these plants carry downward the succession of an age-long chain of constant growth.
The life of such lichens is maintained tinder the most trying of conditions. On the very lofty mountains, growth must be exceedingly slow, half, quarter, or probably much less than a quarter of the rate we found by measuring the New Hampshire lichens already described. Moreover, it is known that lichens are able to live for long periods, probably many years, in a static condition, with no growth. Hence the notion advanced by some botanists, and now upheld by the present investigations, that some lichens may date back for their beginning of growth to the time of the retreat of the last glacier is not fantastic.
The life of the rock-clinging lichens of mountains consists of a very short growing period. For part of the time they are frozen solid, and again are baked to a fierce hot state of desiccation by the sun blazing down upon the rocks through the rarified atmosphere. We measured the temperature of the surface of the rocks next to several lichen patches we were studying in the mountains, and found it to be in excess of 144 degrees Fahrenheit – a temperature almost unbearable to the hand. The lichens in the vicinity were baked into a hard, apparently absolutely moistureless condition. They crumbled in the hand into a dry powder! And yet there must have been viable, soft, living protoplasm within the cells, for when clouds enveloped the mountain for a few hours and soaked all the rocks, out the lichens came, bright and soft and fresh.
by H Hausman
