existence of a third element. Behind and before the manifesta- 

 tion of the hyphse, which are to play so great a part in the 

 lichen-world, is a dimly-seen, primordial tissue a web or net- 

 work of exceedingly delicate filaments (Hypliema, Minks) which 

 gradually pass into the hyphae proper (Gono-hyphema, Minks) 

 as these accomplish their highest result in generating the goni- 

 mous cells (Gonidema, Minks ubi supra, p. 39). 



As regards external form, lichens differ according as they 

 ascend vertically from the substrate, or are spread out horizon- 

 tally upon it. In the first case the development is, for the most 

 part, into branched or shrub-like (fruticulose) types, becoming 

 often finally pendulous ; of which Usnea barbata offers familiar 

 examples. But this is evidently an extreme of lichenous evolu- 

 tion ; and we find, much more commonly, the horizontally ex- 

 panded thallus, which is either foliaceous or crustaceous. Of 

 the foliaceous thallus (exhibited in Parmelia) the frondose (of 

 Peltigera, etc.) is a more entire expression; and the squamulose 

 often (in Pannaria, etc.) a reduced one. Cladonia is remarka- 

 ble as uniting in itself a horizontal and a vertical thallus, and 

 has, on this account, been sometimes taken for the highest exhi- 

 bition of lichenose vegetation. Foliaceous lichens are attached 

 generally to the substrate by variously modified, and more or 

 less conspicuous, fibrillose processes (fibrils ; hypothallus). The 

 crustaceous thallus ascends now into lobed, and even fruticu- 

 lose expressions (as in the highest types of Placodium and Leca- 

 nora) not always readily reducible to their real rank ; and in its 

 squamulose types it approaches yet closer to the foliaceous ; it 

 is however, as respects the great majority of species, well char- 

 acterized by its uniform (neither lobed nor branched) habit, and 

 the peculiar intimacy of its relation to the substrate. In the 

 lowest of all forms of the crustaceous thallus, we have only a 

 web of hyphse, with some few clusters of gonimous cells nestling 

 beneath the outermost layers of cells of the bark upon which 

 these humble plants grow. 



The lichen-fruit is called Apotheclum. Apothecia are vari- 



