76 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



a less degree. In most species of Lichen, the outer walls are 

 densely compacted and cartilaginous, so as to present some diffi- 

 culty of penetration in that direction, except from accidental 

 fissures or normal ruptures, while the spongy base more readily 



Fig. 19. 



a. Cladonia retipora, Florke, nat. size. 



h. Portion of stem magnified to shew the porous structure. From an 

 Australian specimen given to me by Sir W. J. Hooker. 



absorbs moisture. Though even in the largest AlgcB there is 

 no wood, properly speaking, still the stems sometimes acquire 

 considerable dimensions, and even shew concentric zones 

 (Fig. 15), which arise, probably, from distinct periods of growth 

 and rest, as in Dasydadus clavceformis, and in those Poly- 

 iwri whose flesh exhibits similar markings. 



59. A second mark of distinction is, that they have no true 

 leaves. As was before stated, they have foliaceous expansions 

 of various kinds and forms, resembling, in some cases, true 

 leaves very closely, and in some degree, perhaps, performing 

 their functions, at least, so far as exposing a larger surface 

 to the light, and the medium in which they grow, and from 

 which they derive their nutriment. The processes of absorp- 

 tion, nutriment and aeration are frequently, however, in these 

 plants, altogether confused, the seeming roots being little 

 more than grappling threads, intended to fasten them in their 

 proper place, and enable them to withstand the force of 

 wind and waves. In some cases, indeed, amongst Algge and 

 Lichens, and very generally among Fungi, the rootlets may 



