182 



THE AMEKICAN MONTHLY 



J 



ly, has a close connection with the de- 

 velopment of the spores ; hence, it 

 concentrates and lodges in large quan- 

 tity in the hymenium of the apothe- 

 cia, which, when fresh, often presents 

 the appearance of a small clot of jelly. 



The presence of lichenine constitutes 

 a point of difference between the me- 

 dulla and paraphyses of Lichens, and 

 the hyphoid parts of fungi. The hy- 

 pha and paraphyses of the latter, are 

 generally brittle, and non-elastic ; and 



Fig. 32. 



they dissolve away in a solution of 

 hydrate of potash. But those of 

 Lichens are thicker, and more flexi- 

 ble, and do not readily dissolve in the 

 said solution like the filaments of 

 fungi. On the ground of this differ- 

 ence. Dr. Nylander says : " No fungus 

 is present in the formation of Lichens. 



Fig. 33. 



This is demonstrated from their very 

 first beginnings ; for the spores and 

 primary filaments of germination at 

 once show themselves to be of a 

 lichenose nature." * Beside the three 

 layers of cells already described as 

 constituting the thallus, there is, par- 

 ticularly in crustaceous Lichens, a 

 hypothallus. This is the first vegeta- 

 tion of the germinating spore, which 



* Vide Grevillea, Vol. VI, p. 44. 



underlies the thallus : and, eventually, 

 from the increasing growth above it, 

 becomes a dead mass. It is generally 

 of a black or brown color, though 

 sometimes it remains white. It will 

 be often seen bordering the thallus, 

 or protruding through it ; or, if scat- 

 tered, filling up the intervening spaces. 

 The "biological action" of the 

 Lichen-thallus is mostly superficial. It 

 does not increase in thickness beyond 

 a certain point ; but it grows upward, 

 or outward from a common centre, ac- 

 cording to the form it assumes. The 

 outer portions of the thallus are con- 

 sequently always the younger. It is 

 not an uncommon thing in foliaceous 

 species, to see the centre of the thal- 

 lus — the oldest part, entirely decayed 

 and gone, while the periphery still 

 adheres to the rock or tree, and grows 

 on. The centre of vegetative action 

 in the thallus, is undoubtedly in the 

 gonidial cells. They are the chief 

 agents in the growth of the plant. 

 These green cells, says Nylander, 

 " originate in the lower portion of the 

 cortical stratum, and are enclosed in 

 the cells of that stratum, and subse- 

 quently, as the development of the 

 same stratum progresses, the gonidia 

 are observed in a free condition." * 

 In their activity, the gonidia frequent- 



* Vide Grevillea, Vol. VI, p. 44. 



