418 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



hemispheres. The uses of the species have abeady been amply 

 noticed. There is a trace of Lichens in Amber. 



458. No vegetable productions are more liable to variation 

 than Lichens, a circumstance which makes their study very 

 difScult"; and without a knowledge of the fruit it is almost im- 

 possible to distinguish species accurately. Not only is the crust 

 liable to put on various forms, by the over-production of some- 

 of its constituent parts, but even where these are in a nonnal 

 condition, the degree of division of its lobes, the difference of 

 colour, the obliteration of the margin of the apothecia, the 

 exposure of those which have a true excipulum partially 

 covered by the crust, the greater or less crowding of the fruit, 

 the reduction of compound forms to simple, and many other 

 circumstances, induce variations which can only be appreciated 

 by the practised student. The tropical VerrucaricB, for in- 

 stance, assume forms so different, that without a comparison 

 of the fruit it is almost impossible to come to any correct 

 judgment, and in these the Lichenoid character is sometimes 

 completely obliterated by the non-development or evanescence 

 of the crust. 



459. A'general hypertrophy of the crust is also a source of 

 much embarrassment. A very peculiar form of Parmelia 

 saxatilis was lately figured by myself in the Gardeners' Chro- 

 nicle, 1856, each plant lying free upon the ground and forming 

 a dense round ball consisting of narrow lobes, in which their 

 peculiar sculpture was almost obliterated. There is also 

 some reason to believe that other free Lichens, as Lecanora 

 affi,nis and esculenta, may be due to a similar hypertrophy ; 

 but this at present is mere conjecture. Many other forms 

 are assumed by the crusts of Lichens ; the gTanulated arising 

 from the development of chlorophyl, bursting through the 

 cortical stratum, or from "the external cells of that stratum 

 itself, the squamulose from an analogous hypertrophy, the 

 soredioid from the protrusion of groups of gonidia, which, 

 when excessive, gives rise to such productions as Variolar ia; 

 the proliferous, which is an exaggeration of the squamulose; 

 the isidioid, in which the thallus is broken up into short erect 

 cylindrical projections. These must all be kept in view by 



