LICHENS. 



[ 463 ] 



LICHENS. 



Cyclostomatous Polyzoa (Tubulipora, pt., 

 Johnst.). Four species ; on marine zoo- 

 phytes, stones, and shells. (Hincks, Polyzoa, 

 471.) 



LICHENS. A class of Thallophytes or 

 cellular plants standing between the Algae 



Fig. 395. 



Sphscrophoron coralloides. 



Thallus with apothecia . 



Nat. size. 



and the Fungi, exhibiting in the various 

 genera relations sometimes approaching very 



Fig. 396. 



Fig. 397. 



Fig. 396. Borrera ciliaris. Thallus with apothecia. 



Nat. size. 

 Fig. 397. Opegrapha atra. Thallus with lirellse. Nat. 



size. 

 Fig. 393. Section of thalamium. Magn. 150 diams. 



closely to the one, sometimes to the other of 



these two classes. The parasitic Lichens, 

 such as ABROTHALLTJS, being destitute of 

 a free thallus and of green gonidia, are un- 

 distinguishable from Fungi by any definite 

 character. 



The Lichens are almost universally either 

 dry incrusting bodies, growing upon bark 

 of trees, stones, earth, &c., as a pulverulent, 

 or rough and horny, or laminated and mostly 

 wrinkled and curled crust (fig. 397) ; or as 

 horny or leathery, foliaceous or shrubby, 

 ragged or bristling patches (figs. 39o & 396), 

 seldom rising much from the surface which 

 they overgrow ; of grey, greyish green, 

 brown, yellowish, or even reddish colour, 

 and with a dead, pulverulent, and opaque 

 surface. Some, however, are parasitic on 

 other Lichens, as Abrothallus, or upon living 

 leaves, as Strigula. They derive their 

 nourishment from the air ; and are usually 

 recognized by their dry crustaceous habit, 

 and the presence of gonidia. 



In the simplest kind of Lichens, the 

 thallus consists of microscopic branched 

 filaments (hyphee), penetrating among the 

 superficial layers of the cells of the bark or 

 epidermis upon which the plants grow. 

 These filaments usually present globular 

 cells here and there arising from them, 

 filled with green matter, which are capable 

 of reproducing the plant when detached ; 

 they are called gonidia, and are regarded as 

 analogous to the buds of the Fowering 

 plants and the cellular gemmce of the higher 

 Cryptogams. The gonidia consist either of 

 simple cells containing green granular 

 matter (PI. 37. fig. 6^), or of clusters or 

 strings of green granules, when they are 

 called granula gonima (PI. 35. fig. 13). 

 Sometimes these granules form spherical 

 concretions upon the thallus, and are called 

 cephalodia. The gonidia have been stated 

 to produce zoospores ; while these again are 

 declared to be granules in molecular motion. 

 In the simple forms here alluded to, the 

 gonidia are not sufficiently numerous to give 

 a coloured tinge to the structure as seen by 

 the naked eye j in some even the filaments 

 make no show, while in others they form 

 whitish patches (Opegrapha, Verrucaria). 

 In the forms rather more developed we find 

 a layer of globular epidermal cells, with 

 whitish contents, closely coherent together, 

 constituting a " cortical layer " covering the 

 upper surface, to which the filamentous 

 structure or liypotTiallus then forms the 

 " medullary layer." The crustaceous kinds 

 overgrowing stones have this filamentous 



