96 



MANUAL OF BOTANY 



Fig. 856. 



The fruticose forms have an erect thallus, which branches in 

 a somewhat shrubby fashion. A typical Uchen of this type is 

 Usnea harhata {fig. 855, a). The folia ceous ones are flattened 

 and grow closely and pressed to the substratmii. The thallus is 

 wavy and its margin lobed. The crustaceous forms are usually 

 very indistinct, their outline indefinite, and only their fructi- 

 fication distinctly seen. They become so attached to their sub- 

 stratum that they cannot be detached from it completely with- 

 out injury {fig. 856). 



In composition the thal- 

 lus consists of a fungoid 

 mycelium, which is very 

 closely matted together. 

 It includes in its substance 

 certain algae, which may 

 be distributed throughout 

 it or confined to a definite 

 layer near the upper surface, 

 constituting the hovioiome- 

 rous and heteromerouskinds 

 respectively. In the former 

 case the algae generally be- 

 long to the Cj^anophyceae, 

 and in the latter to the Chloro- 

 phycese. This distinction is, 

 however, by no means abso- 

 lute. The fungal constituents 

 are chiefly referable to the 

 sub-class Ascomjxetes, and 

 so great is their prominence 

 that the whole group has 

 by some botanists been de- 

 scribed as a family of this 

 sub-class. Other fungi, how- 

 ever, are now known to enter into the symbiotic relationship 

 with algae, especially several belonging to the Basidiomycetes. 



The proof of the true character of the lichens is based upon 

 the facts that the two organisms can be cultivated separately, 

 and that if the spores of one of the fungi be sown amongst some 

 of the algae the composite organism can be produced. 



A section of the body of one of the heteromerous foliaceous 

 lichens is shown in fig. 857, where the distribution and 

 character of the fungal mycelium are shown, g is the layer of 



Fig. 856. A, B. Graphis elegans, a crustaceous 

 lichen growing on the bark of the Holly. 

 A. Natural size. B. Slightly magnified, c. 

 PertKsaria Wulfcni, another crastaceous 

 lichen (slightly magnified). After Sachs. 



