ORIGIN OF LICHENS 273 



B. Algal Ancestors 



It is unnecessary to look far for the alga.e as they have persisted through 

 the ages in the same form both without and within the lichen thallus. By 

 many early lichenologists the free-living algae, similar in type to lichen algae, 

 were even supposed to be lichen gonidia in a depauperate condition and 

 were, for that reason, termed by Wallroth " unfortunate brood-cells." In the 

 condition of symbiosis they may be considerably modified, but they revert 

 to their normal form, and resume their normal life-history of spore production, 

 etc., under suitable and free culture. The different algae taking part in 

 lichen-formation have been treated in an earlier chapter \ 



C. Fungal Ancestors 



a. Hymenolichens. The problem of the fungal origin in this sub-class 

 is comparatively simple. It contains but three genera of tropical lichens which 

 are all associated with Myxophyceae, and the fungus in them, to judge from 

 the form and habit of the plants, is a member of the Thelephoraceae. It 

 may be that Hymenolichens are of comparatively recent origin and that the 

 fungi belonging to the Basidiomycetes had, in the course of time, become 

 less labile and less capable of originating a new method of existence. What- 

 ever the reason, they lag immeasurably behind Ascomycetes in the formation 

 of lichens. 



b. ASCOLICHENS. Lichens are again polyphyletic within this sub-class. 

 The main groups from which they are derived are evident. Whether there 

 has been a series of origins within the different groups or a development 

 from one starting point in each it would be difficult to determine. In any 

 case great changes have taken place after symbiosis became established. 



The main divisions within the Ascolichens are related to fungi thus: 



Series i. Pyrenocarpineae I , „ 



_ . . \ to ryrenomycetes. 



2. Coniocarpmeae J 



3. Graphidineae to Hysteriaceae. 



4. Cyclocarpineae to Discomycetes. 



II. THE REPRODUCTIVE ORGANS 



A. Theories of Descent in Ascolichens 



It has been suggested that ascomycetous fungi, from which Ascolichens 

 are directly derived, are allied to the Florideae, owing to the appearance of 

 a trichogyne in the carpogonium of both groups. That organ in the red sea- 

 weeds is a long delicate cell in direct communication with the egg-cell of 

 the carpogonium. It is a structure adapted to totally submerged conditions, 

 and fitted to attach the floating spermatia. 



* See p. 51. 

 S. L. 18 



