The science of life : fully illustrated in tone and line and including many diagrams

THE LOWLY AND MINUTE

it needs than it would if it lived exposed on a rock or on the bark of a tree. It is noticeable that the alge that have been enslaved by lichen-fungi are larger than free-living individuals of the same species, much as our domesticated cattle and sheep and pigs are fatter than their wild relatives. As a result of this mutual assistance the lichens can grow in places which are too barren to support any other form of vegetable life.

The fungi are the dominant, active members of the association ; it is they that capture the alge that are enslaved. The specific characters of a lichen—its form and colour and so forth—depend on the particular kind of fungus that it includes. Moreover the lichen-making fungi are completely dependent upon their captives, and they cannot live by themselves (unless they are carefully nursed in the laboratory in a nutrient solution): ‘The alge, on the other hand, are involuntary partners; they belong to species which normally live independently by themselves. Often several different kinds of lichen-making fungus make use of the same species of alga ; there is one kind which uses two different species.

Perhaps the relations between fungus and alga will be most clearly understood if we consider the methods of reproduction of lichens. Of these there are three. The first is simple fragmentation ; if any part of a lichen be broken off and if it blows or tumbles or rolls into a suitable situation, the fungal threads will sprout and grow and the alge will multiply, so that the fragment gives rise to a new lichen plant. The second method, slightly more complicated, is by means of special and characteristic buds called soredia. A soredium is a tiny spherical particle, consisting of a number of fungusthreads surrounding and holding one or more of the alge; it is like a party of settlers moving to a new region, the fungi being the active members of the expedition and the alge corresponding to such livestock as they take with them to ensure a food-supply when they arrive and settle. Soredia are liberated in thousands from the lichen as a powdery mass, and are distributed by wind. In the third method the fungal part of the lichen forms spores after the usual manner of fungi, without the participation of its algal associates. Such spores, when they settle and germinate, are faced with the immediate necessity of finding and entangling appropriate alge; unless they alight in a place where such alge may be had in plenty they perish of starvation. Reproduction by soredia is evidently the

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safer method, since the fungal emigrants take a stock of alge with them ; on the other hand, soredia are large and heavy compared with fungus spores—although to our eyes they are minute powdery granules—so that the spores can be carried for much greater distances by the wind, and spread the species through a wider range.

We have noted that the association of these two very different kinds of plants enables its participants to live under conditions which would be impossible to either by itself. This fact is clearly shown by the geographical distribution of lichens, which are found almost everywhere on the face of the earth. The largest and most elaborate lichenous plants are found in the tropics ; from thence they range to the otherwise bare rocks of Arctic and Antarctic coasts and to mountain heights uninhabitable by other plants. They are even found on rocks far above the snowline. In Arctic regions lichens are more plentiful than any other form of plant, and such forms as the “reindeer moss,’ ‘‘ Iceland moss,’ and “tripe de roche” are important articles of food, both for reindeer and for hard-put men. In the barren steppes stretching from Algeria through Palestine to Tartary the manna-lichen, growing on the ground in little, pale greyish lumps, is eaten by men and beasts—indeed, by saving the Israelites from starvation during their wanderings this species may claim to have had more influence on the present political and economic situation of the world than any other vegetable organism. Before the extensive use of aniline dyes the lichens were used as sources of pigment; litmus, the acid-alkali indicator of chemists, is an example.

The lichens are the classical example of the biological phenomenon known as symbiosis. This term (which is Greek for “living together ”) is applied to cases when two organisms of different kinds live in intimate union, and to the benefit of both. In the lichen, as we have seen, both the alga and the fungus derive some benefit from their union, although the fungus is the active member in bringing it about and certainly profits most. It is interesting to find that lichens can be divided into two groups, each derived from a different fungus type. The two groups must have taken to oo cating green algz quite independently of each other. There are other cases where single-celled alge are captured and domesticated in this manner by plants, and even by animals. A characteristic feature of the

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