The Scottish Naturalist. yi 



scarlet. In form they vary much; from conspicuous tufts or 

 branched leaf-like masses, or cups on soil, or crusts clinging closely 

 to stones or bark, down to forms so minute as to be scarcely 

 visible. The nature of these plants has been hotly contested for 

 several years between those who regard them as distinct and true 

 species, and the followers of Schwendener, who believe them to be 

 each an association of a fungus with an Alga, each aiding and 

 nourishing the other. But it would be out of place to discuss this 

 vexed question here ; and Lichens may still fairly enough be re- 

 garded as distinct from true Fungi and true Algae, in aspect, in 

 habitats, and also in mode of nutrition. One interesting peculiarity 

 of the group is, that they are an excellent test of the purity of the 

 air, and of its freedom from injurious gases, since they die out 

 where the atmosphere becomes^ contaminated with such im- 

 purities. 



In many Lichens starchy and gummy substances occur, in some 

 along with small quantities of sugary matter. The sugar is usually 

 mannite, already mentioned as obtained from certain seaweeds. 

 The presence of these various substances renders some Lichens 

 nutritious, and a few have been used as food by northern races. 

 The best known of these are the Iceland Moss (Cetraria Islandica), 

 a large species common on our higher hills and mountains ; the 

 "Tripe des Roches" (Umbilicaria sps.) of the Artie Regions, 

 familiar, by name, at least, to all who have read the accounts of 

 the heroic explorations of Franklin, and of so many others, in the 

 frozen North ; and the " Manna " (Leca?iora esadejita), of the sterile 

 plains of Central Asia. Many other Lichens might be used freely 

 as food were it not for the trouble of removing an unpleasant bitter 

 substance which they contain, and which for its removal requires 

 plants to be steeped for a considerable time in water. No Lichen 

 is known to be poisonous. "Reindeer Moss" (Ctadonia rangi- 

 ferind) forms the chief support in winter of the reindeer, so in- 

 valuable to the Laplanders. This Lichen is plentiful on our 

 own hills, but is of no economic value to the domestic animals of 

 Scotland. 



In former times Lichens were held in considerable estimation 

 on account of their supposed medicinal properties as tonics, pur- 

 gatives, vermifuges, and astringents ; and the very common " Dog 

 Lichen " (PelUgera canina) was considered almost a certain cure 

 for hydrophobia. Unfortunately closer investigation has not 



