LICHENS. 163 



LICHENS. 



T T was observed many years since, by the celebrated Robert 

 Brown, that two thirds of the lichens found in New Holland 

 are identical with European species ; and more recent authors 

 have proved that many of the common forms (such as Usnea bar- 

 bata, Cladonia pyxidata, Lecanora subfusca, &c.), are truly cosmo- 

 politan. In treating of the mosses that occur in this neighbour- 

 hood, I have already had occasion to notice that as we descend 

 in the vegetable kingdom, there is a remarkable similiarity in the 

 species which are found in different parts of the world, and that 

 under certain conditions the same plants invariably spring up. 

 How this happens, learned men are unable to explain ; but per- 

 haps it may assist our comprehension of the subject, if we reflect 

 on the amazing fecundity of well-known cryptogamous plants, 

 and the probability that their spores are almost indefinitely dif- 

 fused, and ever ready to appear when conditions favourable to 

 their development occur. In the common English fern " hart's 

 tongue" (Scolopendrium) , it has been calculated that in a good- 

 sized frond, there are at least fifty sori or patches of spore cases, 

 that each sorus contains from four to six thousand thecse or cases, 

 and that these thecae themselves inclose about fifty spores. Now 

 these facts, which the miscroscope has revealed to us, lead to the 

 wonderful conclusion, that a single leaf of the plant may give rise to 

 no less than ten millions of young ferns ! The sporules of some 

 species of moss (such for instance as Dicranum bryoides, Funaria 

 Jiygrometrica, Tor tula muralis, Didymodon purpurewn, &c.,) are 

 likewise exceedingly numerous, and probably almost indefinitely 

 diffused. Nor, indeed, must it be overlooked, that innumerable as 

 the sporules may be, they do not appear in every case necessary 

 for the propagation of these minute plants, for of the many species 

 which clothe the rocks of Spitzbergen, few are ever seen in a state of 

 fructification. It may be mentioned, therefore, on the authority 

 of Dr. Hooker, that what are considered the seeds of mosses, are 

 not requisite for the increase of these plants. On this subject, 



