33° 



PART IV. — VEGETABLE TAXONOMY. 



CHAPTER X. — The Thallophyta (Continued). 



CLASS V.— THE LICHENES. 



These plants are here treated as a separate Class, more for 

 convenience than because there is anything in their structure 

 which warrants the distinction. They are, in fact, algae and 

 fungi living together as host and parasite. Each Lichen is a 

 kind of composite organism, a few of the fungi composing them 

 belong to the Basidiomycetes, but by far the larger portion to 



g h 



Fig. 534 — Portions of two Lichens, showing fungi parasitic on Algae. In A, the fila- 

 ments, g, g, belong to a species of Scytonema and the hyphae, A, k, h, are those of Stere- 

 ocaulon ramulosus. Magnification about 950 diameters. 



In B, a hyphal branch, /«, is entering the cells of a species of Nostoc, g. Magnification 

 about 650 diameters. Both after Bornet. 



the Ascomycetes, and in each case the modes of reproduction 

 are such as characterize these groups respectively. 



The algae mainly belong to the lower forms, such as the 

 Nostocs, the Palmellas, Chroococcus, Chroolepus, and, more 

 rarely, the Confervas ; and while they do not lose their power of 

 vegetative multiplication by reason of the parasitism, they do 

 lose the power to reproduce by other means, and moreover, 

 frequently undergo important structural modifications. 



The facts are analogous to those which often occur in the 

 higher plants when attacked by fungi or other parasites. The 



