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Origin and Nature of Lichens — Part 1

STRANGE opinions were entertained in regard to the origin of lichens. The belief was general that they were spontaneously generated. In them the philosopher found the origin of plant life. "Spontaneously, inorganic stone became living plant!" Dr. Hornschuch wrote in 1819, "Algae, lichens, and mosses may develop without seed from decomposing water. The decomposition of water induced by warmth and sunlight gives rise to the common ancestral type of algae, lichens, and mosses. This ancestral type is a vegetable infusorium known as monas lens which, when acted upon by light and air, undergoes an evolutionary transformation into algae, lichens, and moss."

Nees Von Esenbeck, in 1820, was wont to lead his pupils to an old castle in order to demonstrate ad oculos, how the green substance when occurring on rocks will develop into lichens.

De Bary was the first author to hint at the true nature of lichens (1866). His conception of the lichen as a dual organism composed of a fungus and an alga, was upheld by the researches of Schwendener and Bornet in 1868.

Further investigation seems to prove that the lichen is not an individual plant, but that it is the result of an alliance perhaps for mutual benefit between two forms of plant life, an alga and a fungus. The alga gives the green colour to the lichen and is a relative of the simple plants which make damp stone or woodwork green on the shady sides of streets and houses and trees. The fungus is a relative of the toadstools and moulds. If one look at a piece of white mouldy bread, or in the ground at the base of a toadstool, one can see a true fungus plant which is simply a network of fine white treads (hyphae) stealing their food instead of manufacturing it for themselves. They have lost their leaf-green granules, the tools with which plant-food is manufactured from air and water and mineral salts, but they have acquired the power of absorbing great quantities of water and of resisting alternate drying and wetting.

The alga will perish if exposed to dry air, but when kept moist is capable of taking elements from the air and or manufacturing them into plant-food by means of little granules of leaf-green it has in its cells.

In the alliance the fungus is entirely dependent upon the food manufactured by the green alga and in return keeps the sun's rays from the alga and absorbs water for its work.

The Alga-fungus company, or lichen, is perhaps one of the earliest instances of division of labour, a little community in which one party manufactures and supplies food to the other which serves as protector.

The gray-green of a lichen is then due to the fact that a bright-green plant is covered over by a translucent white plant, and the brighter green of the wet lichen is due to the fact that the wet strands of the fungus are rendered transparent by the absorbed moisture, and permit the colour of the imbedded green to be seen.

A magnified portion of a dissected lichen very much resembles a tangle of fine white threads in which are scattered bits of green.

The white threads of the fungus creep around in search of moisture and as a rule determine the shape the lichen is to be, While the green cells or threads of the alga follow their protecting fungus. However little moisture there may be in the surrounding air, the fungus threads absorb it for their working companion, and so the lichen can live in places too dry and parched for other plants.

"Strong in loveliness, they neither blanch in heat nor pine in frost."

On account of this dual nature it has been difficult to decide where to place the lichens in the plant kingdom; to decide whether they belong with the algae, with the fungi or have a place as individual plants.

It is claimed that with the microscope one may often determine the species of the associated fungus, as well as that of the associated alga and that this alga freed from the lichen-fungus pursues its normal mode of life and can then be identified.

It is also claimed that lichens have been formed from the spores of a fungus partner allowed to germinate on free-growing algae, and that a variety of lichens have thus been developed and that the same alga will produce different kinds of lichens if associated with different fungi, and that spores of the fungus-partner have been grown on nutrient solutions and have produced a fungus. One instance is known of a fungus-partner (Cora pavonia) which can lead an existence independent of the alga-partner.