CHAPTER VII. PHENOMENA OF VEGETATION. LICHENS. 417 



products of development of the hyphae. That conversely the hyphae also might proceed 

 from the Algae, was a view which was at any rate often expressed, but which could not be 

 maintained by those who looked fully into the facts on which it was supposed to rest. 

 Wallroth expressed this view most decidedly, though he could not in his day draw from 

 it the histological conclusions which have been stated above, when he named the algal 

 cells gonidia, brood-cells, and expressly understood by this name asexual organs of re- 

 production produced from the thallus and capable of developing under favourable 

 conditions into a new and perfect Lichen-thallus 1 . This view of Wallroth was in fact 

 derived from the correct observation of the origin of the thallus from soredia, and from 

 confounding small soredia of heteromerous species with the Algae of their thallus, and 

 even with other free green algal cells, a mistake quite conceivable in his day. But after 

 this confusion and the non-reproductive character of Wallroth's gonidia had long been 

 recognised, the expression was still retained in an altered sense for the Algae of the 

 Lichen-thallus, and with it the terms gonidial layer or gonimic layer (stratum gonimon), 

 hytnenial gonidia, and others of the same kind. The term chromidia proposed by 

 Stitzenberger 2 met with little favour. I have avoided the word gonidia in the foregoing 

 account, because it is a convenient expression in its original acceptation for the repro- 

 ductive organs of the Fungi, and has been so used with all possible consistency in my 

 former chapters, and to apply it in a different sense to the thallus of the Lichen-fungi 

 would necessarily have caused confusion. At the same time it is unnecessary to intro- 

 duce a new term, because the old word Alga declares the real nature of the objects 

 briefly and distinctly, and satisfies all the requirements of the terminology. 



After the discovery of the fact that the Algae of the thallus are not reproductive 

 organs but assist in the vegetation of the plant by means of their chlorophyll, a long 

 time elapsed before clear insight was obtained into their true character. They con- 

 tinued to be regarded as parts of a simple organism, the Lichen, and this view was 

 confirmed by the observations first of Bayrhoffer and after him especially of Schwen- 

 dener, which showed that they are often so placed at the extremities of branches of the. 

 hyphae in heteromerous species containing Cystococcus that they may very well be 

 supposed to be the swollen terminal cells of the hyphal branches forming chlorophyll. 

 I myself adopted this view in the case of the heteromerous Lichens in my first edition 

 in 1865. In the case of other species, especially the gelatinous Lichens containing 

 Chroococcaceae and Nostocaceae, there appeared to me to be objections to it which could 

 not be removed, and I was led to propose the following alternative : either the Lichens 

 in question are the perfectly developed and fruitful states of plants, the imperfectly 

 developed forms of which have hitherto been placed with the Algae under the names of 

 Nostocaceae and Chroococcaceae, or the Nostocaceae and Chroococcaceae are typical 

 Algae which assume the form of the Collemeae, Ephebeae, &c., when they are invaded 

 by certain parasitic Ascomycetes which spread their mycelium in the growing thallus 

 and often attach themselves to the algal cells containing phycochrome (Plectopsora, 

 Omphalarieae). Then Schwendener not only adopted the latter alternative at the close 

 of his work on the Lichen-thallus in 1868, but extended it to all Lichens and thus 

 founded the view which has been set forth in the preceding paragraphs. 



The arguments which he advanced in support of this doctrine on the subject of the 

 Lichens are drawn from \heptirely anatomical data given above. They may be briefly 

 stated in the following manner : All Lichen-gonidia, as they have been hitherto termed, 

 are so like certain Algae that every one of them can be directly placed under some well- 

 known genus or even species of Algae; the 'gonidia' have never been distinctly proved 

 to have been formed from branches of the hyphae, but all observations tend to show 

 that the Alga has been attacked by a Lichen-fungus. Schwendener did not go beyond 

 this point even in his later and more elaborate work on the subject. Bornet's investi- 



1 Wallroth, Naturgeschichte d. Flechten, I (1825), especially at p. 46. 



2 Flora, 1860, p. 216. 



[4] EC. 



