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 

 saprophytes independently, that is 

 without Algae, into a thallus formed 

 of an abundance of slender hyphae 

 which spread through the cells of 

 the periderm. Then its proper 



Alga, Chroolepus umbrinum (Fig. 169), finds its way from without through the 

 <^ cell-walls of the periderm-is- 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. 



FIG. 170. E a compound pluricellular ascospore of E>idocarpon pitsil- 

 turn, cultivated on a microscopic slide and putting out numerous germ- 

 tubes. T a bicellular compound spore of Thelidium tnututulum also 

 germinating on a microscopic slide, p cells oi PUiirococcus ejected with the 

 spores from the hymenium of Endocarpoti and vegetating on the slide ; 

 p * a group of several cells of Pleitrococciis formed at the spot by the 

 growth of single cells, a cells of Pleurococcus attacked and partly grown 

 round by branches of the germ-tubes, and consequently considerably 

 larger than those which are vegetating in freedom. After Stahl. Magn. 

 320 times. 



