THALLOPHYTA--LICHENES 



97 



Fig. 857. 



algae which are symbiotic with the fungus. In the fruticose 

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

 hyphse running longitudinally. This is surrounded b}' a looser 

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

 are found in the loose tissue. 



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 which a basidiomycete 

 enters has basidia and para- 

 phj-ses 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 algae are more 

 prominent, the fungus-hy- 

 phae being very fine and 

 delicate. The algae occupy 

 nearly the whole substance 

 of the 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 

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

 from the surface of the thallus 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. Karely 

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

 chaetaceae. 



VOL. II. H 



Fig. 857. Transverse section of the thallus of 

 Hdcta fuli'jinosa (x300). o. Cortical layer 

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

 Attaching fibres, m. Medullary layer, con- 

 sisting of fungal hypha3. g. The algal cells. 



