CHAPTER VII.—PHENOMENA OF VEGETATION.—LICHENS. 399 
us the germ-tube from the spore puts out other branches, which penetrate into the 
nutrient substratum and evidently obtain the necessary mineral matter from it. If 
there is no substratum of this kind, as when the plants are grown on glass plates, the 
above processes do not go beyond the very first stages. 
The two observers just named carried the cultivation of their plants, with the 
precautions suggested naturally by the circumstances stated above, from germination 
to the production of the fully grown thallus, which in some cases also produced 
sporocarps. The same results were obtained in a series of other cases by the 
investigations of Bornet and Treub at least as regards the earliest stages, and these 
investigations and a comparison of 
fully developed thallus-forms show 
distinctly that the first stages of 
the development have a close re- 
semblance to one another in all 
or almost all Lichens, though ac- 
companied with certain specific 
modifications which were naturally 
to be expected throughout. Even 
the peculiarities observed by Frank 
in the development of the thallus 
of certain Graphideae, as Arthonia 
vulgaris and Graphis scripta, which 
grow on the bark of trees, are 
really only specific modifications. 
In this case the hyphae of the 
thallus, which can only have come 
from the germ-tubes of the spores, 
though the observations on this 
point are still imperfect, make their 
way into the outer layers of the 
periderm in smooth stems of oaks 
and ashes and there grow as FiG. 170. Ea pound pluricellular pore of Exdocarpon pusti- 
¢um, cultivated on a microscopic slide and putting out numerous germ- 
saprophytes independently, that is tubes. T a bicellular compound spore of Thetidium minutulum also 





ger ona micr pcells of Pleurococcus ejected with the 
* tf spores from the h i of Endocarpon and v ing on the slide; 
without Algae, into a thallus formed px a group of several cells of Pleurococcus formed at the spot by the 
of an abundance of slender hyphae ersmihofsngt cls. a ses of Pewroocnsatacked and partly grown 
which spread through the cells of eee es those which are vegetating in freedom. After Stahl. Magn. 
the periderm. Then its proper 
Alga, Chroolepus umbrinum (Fig. 169), finds its way from without through the 
cell-walls of the peridermis into the previously formed hyphal thallus and is seized 
by it. The cells of the Chroolepus are in rows forming filaments with apical 
growth, and it is by means of this growth that they penetrate into the thallus, in 
the same way as mycelial hyphae pierce through membranes. The Alga is a frequent 
inhabitant of the bark of trees and makes its way into the periderm for its own 
purposes. Its penetration into the thallus of the Fungus can scarcely be supposed 
to be caused by the Fungus, but is merely an adaptation which favours the formation 
of a Lichen. 
