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'. 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 gonddial layer or gonimic layer (stratum gonimon), 
hymenial gonidia, and others of the same kind. The term chromidia proposed by 
Stitzenberger? 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 the purely 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- 

} Wallroth, Naturgeschichte d. Flechten, I (1825), especially at p. 46. 
? Flora, 1860, p. 216. 
[4] Ee 
