i8 



Fu.yci 



Lichen- FORMING Fungi. 



These are strictly parasitic fungi which, without the aid of algal 

 hosts, do not develop beyond the earliest stage of germination. Their 

 nutritive inter-relations with their hosts, however, mark them off from 

 other parasites. The hyphse of the lichen-fungus embrace the algal 

 cells, and the two elements together compose a thallus of definite form. 

 The algal cells form by means of their chlorophyll-contents organic 

 carbon-compounds by which the fungal cells benefit ; Ijut here the 



resemblance to true parasitism 

 ceases. The host exhibits no 

 sign of exhaustion, since a reci- 

 procal accommodation exists be- 

 tween the two elements. The 

 rhizoid filaments of the fungus 

 draw from the substratum mineral 

 substances, the raw material of 

 food. The hyphal cells are fed 

 by the exosmose of starch and 

 the like from the algal cells, and 

 the inference is justifiable that 

 the algal cells receive in exchange 

 by endosmose the waste products 

 of the fungal protoplasm. There 

 thus exists a lasting consortism or 

 symbiosis between the elements, 

 and the result is a thallus which 

 may be treated from the point 

 of view of the systematist as an 

 autonomous oro-anism. It must 

 never be forgotten, however, that 

 it is fundamentally two organisms, one of which, the fungal, cannot live 

 without the other, while the latter can and does exist separately and 

 independently in nature. It is a question not definitely decided whether 

 certain algal forms thus living in consortism can or cannot live separately, 

 and it is also doubtful whether the fungal portion of such lichens as live 

 on the bark of trees or substrata rich in humus, does not live partially 

 as a saprophyte. Evidence certainly points in this direction. The sym- 

 biotic relations existing in lichens are comparable with those described 

 by Geddes, Brandt, and others, as in operation in Radiolarians and 

 other animals, the ' yellow-cells ' of which are actively vegetating algae. 



Fig. 279. — Algal cells of Lichens. A, spore of 

 Physcia parietina Nyl. germinating on Protococ- 

 cics viridis Ag. B, Synalissa symphorea Nyl. 

 with Glococapsa. C, Cladonia furcata Hofim. 

 with P7-otococcHs. D, Stereocaulon ramu:osuiii 

 Ach. with Scytonema. (A, B, and C, x 950, 

 D X 650.) (After Bornet.) 



