THALLOPHYTA -LICHENES 



97 



Fig 



algse which are symbiotic with tlie fungus. In the fruticose 

 forms the centre of the branches is occupied by a dense strand of 

 hyphee running longitudinally. This is surrounded by a looser 

 layer which is in turn enclosed by a compact cortex. The algse 

 are found in the loose tissae. 



The reproductive organs of the lichen are always produced 

 by the fungal constituent. The fructification is usually an 

 apothoecium produced after the fusion of a spermatium with the 

 trichogyne of an archicarp, as described in the case of CoUema 

 (page 82). 



The hymenial layer of 

 the fructification of the form 

 into whicli a basidiomycete 

 enters has basidia and para- 

 physes like those of the 

 ordinary Basidiomycetes. 



The mode of growth of 

 the lichen is in most forms 

 determined by the fungus 

 constituent, the algae divid- 

 ing and multiplying as the 

 thallus extends. In some 

 cases the algs are more 

 prominent, the fungus-hy- 

 phse being very fine and 

 delicate. The algse occupy 

 nearly the whole substance 

 of th§ branches, and are 

 covered by a gelatinous en- 

 velope derived from their 

 cell-walls. In this the deli- 

 cate fungus-hyphae ramify. 



The lichens reproduce 

 themselves vegetatively by 

 forming peculiar gemmae 



termed soredia. These are composed of a small bundle of 

 fungal filaments investing a few algal cells. They are cast off 

 from the surface of the thaUus and speedily reproduce it. 



The Algae which take part in the composition of lichens are 

 usually very simple unicellular or filamentous forms, such as 

 Nostoc, Chroococcus, Scytonema, Protococcus, &c. Earely 

 higher forms are found, such as members of the Coleo- 

 chstaceae. 



VOL. II. u 



Fk/. 857. Transverse sectiou of the thallus of 

 Sticta fuliginom (x300). o. Cortical layer 

 of the upper side ; u, of the under side, r, r. 

 Attaching fibres, m. Medullary layer, con- 

 sisting of fungal hyphas. g. The algal cells. 



