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RELATION BETWEEN THE ALGA AND FUNGUS OF A LICHEN. 
The Relation between the Alga and Fungus of a Lichen, By ROBERT 
Pautsoy, F.L.S., and Somervitte Hasrines, M.S., F.R.C.5. 
(PLATES 21 & 22.) 
{ Read 6th June, 1918.] 
Recenr writers on the subject of the relation that exists between the alga 
and fungus of a lichen have supported the view that hyphe, functioning as 
haustoria, penetrate the living algal cells, and gradually absorb their 
contents until only empty colourless cell-walls, or walls enclosing a network 
of hyphal threads, remain. Upon this conclusion is based, to a great extent, 
the theory of parasitism on the part of the fungus, the algal cell being 
regarded as a helot that gains little, or nothing, by reason of the close 
association of the two organisms. 
The literature of the subject shows a complete lack of agreement as to 
what takes place after the algal cell has been penetrated, or as to whether 
the cell is pierced at all in the living state. Hedlund (6) 1892 was of 
opinion that the protoplasm retreated, forming a concavity before the 
invading hyphal thread. Schneider (8) 1897 says, “ The haustorium, after 
entering the cell, develops a much branched network which encloses but 
does not penetrate the cell-plasm.” Peirce (7) 1899, affirms “ That certainly 
the haustoria do penetrate the protoplasm of the gonidial cells of Ramalina 
reticulata.” Elenkin (4) 1905, when developing his theory of endosapro- 
phytism, concluded that ** The parasitic action of the fungus haustoria on the 
algal cells has been more or less proved in comparatively few cases” ; while 
Danilov (3). a translation of whose paper appears in the current June 
number of the ‘Journal of Botany,’ holding views similar to those of Peirce, 
claims to have shown, by drawings and photo-micrographs, that branches of 
excessively fine hyphal filaments, possibly without cell-wall, or with a wall 
so attenuated that it cannot be demonstrated, pierce the cell-plasm in every 
direction. He gives no definite information as to the frequency of pene- 
tration by hyphe in the ease of the material he used, but leaves one to 
infer that the condition is to be regarded as quite normal, and therefore 
frequent, for he bases his theory of the relation between the two organisms 
upon his statement respecting the hyphæ entering algal cells. On the other 
hand, Elfving (6) 1913 says, respecting Hvernia furfuracea, * The formation 
of haustoria upon the hyphae which grow into the lumen of gonidial cells, as 
represented in popular textbooks and described for other species of lichens, 
was extraordinarily rare; in my material only on a single occasion have I 
seen such a haustorium ; while the extremely fine haustoria described by 
Danilov, as specially occurring in the present species, I have never seen.” 
"The present investigation was undertaken for the purpose of obtaining 
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