382 INTRODUCTION TO CRTPTOGAMIC BOTANY. 



which may be sprinkled with a few heaps of gonidia, while in 

 shady crevices the frond itself will not come to perfection, but 

 will assume the form of a Lepraria. It is very necessary to 

 attend to these circumstances in the determination of species, 

 and more especially in ascertaining their value. 



423. Lichens occur in various situations. A large portion 

 of them grow on exposed rocks, not excepting even the softest 

 chalk, the barest flint, or the hardest quartz ; the top of the 

 mountain and sea-shore are equally productive ; for the salt 

 spray does not impede the growth of many species. Trunks 

 of trees, again, or fabricated wood, support a host of Lichens, 

 which are not unfreqtiently similar to those which grow on 

 rocks. Dry, exposed soil, whether peaty or not, sustains its 

 own species ; while damp mossy woods or open heaths contri- 

 bute many kinds, sometimes of such a shrubby habit, and in 

 such numbers, as to hide the soil. The same species may, 

 however, occur indifferently on rocks, trees, or soil ; while it 

 may be capable of enduring considerable climatic differences 

 without much variety of form or habit. In some localities 

 the species on trees and rocks are so exuberant as to con- 

 ceal them scarcely less effectually than the terrestrial species 

 do the soil. 



424. Many tropical and one or two European species occur 

 on leaves. A few of these are simply superficial, in which case 

 the leaf serves merely as a base of growth, in the way of any 

 other indifferent substance. It frequently, however, happens 

 that the connection between the leaf and parasite is far more 

 intimate ; the Lichen originating after the fashion of many 

 Fungi, beneath the true cuticle. This is the case with most, 

 if not every, species of Strigula. To ascertain this fact, how- 

 ever, they must be examined in an early stage of growth. 



425. The uses to which Lichens are put are of various kinds, 

 and sometimes of considerable importance. Cetraria islandica 

 affords a nutritious mucilage, in consequence of its containing a 

 considerable proportion of amylaceous matter ; though not in 

 the granular form in which it commonly exists in vegetables, but 

 diffused through the whole mass. It acts at once as an aliment 

 and tonic, but is not adapted for forming a large portion of food. 



