[Illustration: FIG. 134.--_Nostoc lichenoides_, which is attached by a germinating thread (_h_) of _Collema glaucescens_.] [Illustration: FIG. 135.--_A_ Germinating spore of _Physcia parietina_ with _Protococcus viridis_. _B Synalissa symphorea_ with _Glæocapsa_. _C Cladonia furcata_ with _Protococcus_.] The thallus of the Lichen appears mainly under three forms:-- 1. The CRUSTACEOUS, which adheres firmly to the substratum (bark, stone) throughout its entire surface, without being raised into any free patches or lobes. It has, in many instances, no definite outline, and hyphal-branches from it often penetrate deeply into the substratum. It grows at the circumference and sometimes dies away in the centre (Figs. 138, 139, 140). 2. The FOLIACEOUS. This also lies flat upon the substratum, but is not firmly attached to and has a definite outline. It grows at the margin, and raises itself a little by free outgrowths and lobes (Fig. 141). The rhizoid-strands spring out from its whitish under surface (Fig. 131, _r_). [Illustration: FIG. 136.--Portion of a hymenium: _d_ a thin stratum on which the asci (_s_) are situated.] [Illustration: FIG. 137.--Spores of, _a Cladonia_, _Lecanora_ and _Pertusaria_; _b Bæomyces_; _c Sphinctrina_; _d_, _e_, _f_ various species of _Parmelia_; _g_, _h Verrucaria_ in its younger and older condition; _i_, _k_ species of _Leptogium_.] 3. The FRUTICOSE, which is attached to its substratum at a small point from which it projects freely, either erect or pendulous. It is more or less tufted, in the form of a bush (Figs. 142, 143). These three thallus-forms gradually pass over by many intermediate forms into one another. [Illustration: FIG. 138.--_Lecanora subfusca_: _a_ the bark on which it is situated; _l_ the thallus; _s_ the ascocarp; _s’_ an ascocarp.] [Illustration: FIG. 139.--_Graphis_ (two species).] [Illustration: FIG. 140.--_Pertusaria communis._] The Lichens, like other Ascomycetes, have very variously constructed ascospores (Fig. 137), which are enclosed in asci (Fig. 136), usually surrounded by paraphyses attached together. Furthermore they possess pycnidia (Fig. 141) containing numerous microconidia. These were formerly considered as organs of fructification, and were termed “spermatia,” and the pycnidia, “spermogonia.” Alfr. Möller proved, in 1887, that the microconidia are able to germinate and produce a mycelium with new conidia, just as in other Ascomycetes. VEGETATIVE REPRODUCTION takes place by _soredia_, which to the naked eye appear as whitish powder on the surface of the thallus. They are small round bodies, formed by one or a group of gonidia, which are surrounded by a mass of felted hyphæ. After the rupture of the cortex they are set free, and readily carried by the wind to other places, where under favourable circumstances they establish a new thallus.