214 



THE FARMER'S MAGAZINE. 



answer our purpose ? As it is to be feared not, we will try again. 

 The lichens I have before described aa constituting a numer- 

 ous and diversified tribe, and no general definition will convey 

 au idea of what they are in the aggregate. Their simplest 

 forms are seen in the several species of Lepraria, so termed 

 from the Greek lepra, scurf or leprosy ; and the most common 

 and universally distributed of these, is the green powder, that, 

 in all parts of the country, makes its appearance upon the walls 

 of old buildings, park-paling, and the trunks and branches of 

 trees. A magnifying glass showsit toconsistofminuteglobular 

 or oval grains, closely clustered together. These are the sin- 

 gle microscopic cells of which mention has beeu already made ; 

 and the uncertain limits of the lower groups of vegetable life 

 are instanced in the fact that this Lepraria viridis has beeu 

 referred alternately, by different botanists, to the alga3 and to 

 the lichens. It is a point of little moment, so far as our pre- 

 sent purpose is concerned, to which group it really appertains, 

 as both it and its brethren of similar simple structure and habit 

 are equally pioneers of vegetation. Conspicuous on the objects 

 and in the situations just mentioned, they are no less common 

 on the ground everywhere, where their propagation proceeds 

 more rapidly on exposed and newly-turned soil than among 

 growing crops. To the student of Nature, who thus observes 

 them, it would be an absurdity to deny that their appearance 

 ia associated with a function : our common parent admits of 

 no sinecures, no superfluous offices, under her government ; 

 whatever is, muat act or cease to be ; presence is inseparable 

 from need ; and lbs living microscopic atom must work, or die 

 and become the medium of support to others. 



The powdery lichen, unseen by the cultivator, or unregarded 

 and despised as is the dust on which he treads, is truly a 

 valuable bond-slave, pursuing a slow, but unintermitting toil 

 for his benefit, and elaborating the first organic compounds of 

 hydrogen, carbon, aud oxygen, that are destined, on its decay, 

 to become the basis of a higher vegetable form. But it is not 

 within the limits of an agricultural district that we are to ex- 

 pect to see any striking manifestation of the lichen fulfilling 

 the purposes of its existence. We must aacend the mountain 

 towards the verge of perpetual snow, examine the smooth 

 worn surface of rocks recently abandoned by the sea, or walk 

 over the scarcely-cooled lavas of some volcanic region, to trace 

 the progress of primeval planf-developraent, and learu to ad- 

 mire, and estimate at their true value, the operations of these 

 its hardiest pioneers. The investigator of earlier lichen his- 

 tory must set aside all previous views of vegetable growth, 

 as associated with the necessary presence of soil aud root- 

 extension. The barest, nay, almost polished surface, to 

 which adhesion would seem scarcely possible, yields a suffi- 

 cient resting-place : the imperceptible moisture- of the 

 driest atmosphere affords the required medium of suste- 

 nance. Many of the tribe appear ia the form of a white or 

 grey powder only, and bear so little resemblance to organic 

 substance, that a person unaccustomed to the study would, ia 

 most instances, mistake them for casual stains upon the sur- 

 face of a flint or a saline efflorescence on a mass of granite ; 

 indeed it often requires no small degree of familiarity with the 

 general structure of the family, and a glass of high magnify- 

 ing power, to distinguish some of thera from the rock or earth 

 on which they grow. The decay of these miuut ? and obscure 

 kinds, however, affords a thin stratum of soil sufficient for the 

 habitation of other species of more complicated organization, 

 larger size, and more interesting appearance. Some of the 

 higher grades of lichenaceous plants are remarkable for their 

 be«uty, elcgacce of outline, and diversity, as well as brilliancv 

 of hue. •" 



The remark of an author previously quoted— that, while 

 lichens improve the soil for the benefit of other plants, they 

 derive no nutriment fiom it themselves— is apparently opposed 

 to circumstances attending their development, different 

 species being found on different substances. This fact is 

 especially instanced in those which are most conspicuous, 

 giving colour to rocks, trunks of trees, walls, and buildings ; 

 many of them ranking among the highest forms of their 

 order: a-d the practical botanist is well aware of the prefer- 

 ences they exhibit. The various primitive, secondary, and 

 Igneous rocks yield resting-places to particular species; aud 

 so invariable IS the aUachment of certain kinds to peculiar 

 media, that the geological collector may often determine, by 

 the hue of its surface, clothed with a thin coating of primeval 

 vegetation, the nature and composition of the rock he is ap- 

 proaching. Granite, slate, sandstone, limestone, trap, &c,, are 



each likely to be thus distinguished ; and in a similar man- 

 ner the greater number of the epiphytic lichens, or those which 

 grow on the bark of trees, are not indiscriminately scattered 

 upon the stems of different species constituting the forests of 

 the same climate, but vegetate exclusively upon the surface of 

 those which afford them the necessary texture to which 

 Nature has adapted their powers of adherence. A skilful 

 landscape-painter, observing the effect thus produced, adapts 

 his colours, accordingly, to the trank of any particular kiud of 

 tree he may be desirous of representing. He sees that of the 

 oak invariably differing from that of the beech in the hue of 

 its humble dependants, that of the elm from that of the wil- 

 low, aud so on of others ; aud so perceiving, his picture be- 

 comes a transcript of reality. 



This preference of the lichenaceous plant for specific 

 hahilals is without doubt associated with difference of tex- 

 ture ; but it is no less unquestionably attributable to other 

 causes likewise. The lichen, as a cellular vegetable organism, 

 derives much of its subsistence by absorption from the atmos- 

 phere, in the same manner as the alga does from the water in 

 which it is submerged ; but the elements held in solution by 

 the former are insufiicieut to supply certain principles which 

 are invariably present in the plants now before us. They must 

 be obtained from other sources ; aud the succession in which 

 the species appear, following each other always in a corre- 

 sponding series, is a further and unmistakeable manifestation 

 of the fact. Metallic oxides (that of iron especially), earthy 

 and alkaline salts, are present in all of the higher forms; and 

 the bases of these proximate principles existing in the sup- 

 porting medium, whether earth, rock, or tree, their separation 

 from it by organic action iLight be anticipated. Again, if we 

 examine closely the attachment of many of the more minute, 

 and even of the larger, crustaceous kinds (understand by thi* 

 latter denomination, those which adhere like a crust to the 

 substance ou which they vegetate), we shall find that their 

 under- sides are not superficially adherent only, but actually 

 imbedded. This is the cas-i with some which grow upon the 

 hardest siliceous rocks and stones, and even on the glass of 

 old windows ; and it probably arises from the elaboration, 

 during their growth, of oxalic aud other vegetable acids, 

 which, however weak and powerless in thtir action on such 

 substances when mechanically applied, overcome the cohesion 

 of their atoms during the slower process of organic assimila- 

 lation. By whatever means, however, such an effect may be 

 produced, its importance is the same ; aud the ministry of Ihe 

 lichen, commencing with the modification of the mineral ele- 

 ments of the fluid- water and viewless atmosphere, is succeeded 

 by a corresponding action upon the solid crust of the earth. 



The more conspicuous aud complicated species of these 

 plants, though they may often be observed covering spaces of 

 considerable extent, to the apparent exclusion of alt others, 

 are, individually, only so many grades in the grand natural 

 scheme of fertilization. In describing one of the most beauti- 

 ful of the series, the Slereocaulon pasehah of botanists. Sir 

 James E. Smith, mentions having observed it coating the 

 lavas of Vesuvius, which looked, at a distance, in many parts, as 

 though they were covered with snow, owing to the abundance in 

 which it vegetatedthere ; but this species, chiefly,if not entirely, 

 confined to volcanic and plutonic rocks, only appears upon them 

 as the successor of many others, which have from time to time 

 vegetated and decayed to form a soil fitted for its develop- 

 ment; and the same may be affirmed of its equally or more 

 distinguished brethren. Mere microscopic specks, like 

 grains of impalpable powder, commence the mighty opera- 

 tion, solidify the air and moisture, soften and abrade the 

 rock ; broader and denser, and at length leaf-like, e.tpan- 

 sions follow in their turn, survive their time, and decaj', un- 

 til the deepening soil is prepared to receive the green moss, 

 the grass, and the herb, and eventually becomes clothed 

 with an exuberant vegetation. The course of changes under 

 which the fertilization of a world has been produced has 

 attracted the attention of an English poet, whom, as a cor- 

 rect observer of nature, a naturalist may venture to quote — 



" Seeds, to our eyes invisible, will find 



On the rude rock the bed that fits their liind. 

 There in the rugged soil they safely dwell, 

 Till showers and snows the subtle atoms swell, 

 And spread the enduring foliase ; then we trace 

 The freckled fJower upon the flinty base. ' 

 These all increase, till in unnoted years 

 The stony lower iis crey with age appears : 



