144 
AIycologia 
counted wholly for dead cells, the living must here exceed the dead 
cells in number. So it is reasonable to suppose that the large pro- 
portion of dead cells in this region must be due mainly to paras- 
itism or saprophytism. Elenkin thinks more probably the latter, 
the enzyme killing the algal cells, which are afterward absorbed. 
The dead cells are not wholly absorbed in the haustorial zone; 
but they are largely pushed into the cortex by growth, and there 
their consumption is completed by the lichen hyphae. 
This absorption of dead walls, which Elenkin has observed in 
many lichens, it seems, must be carried on saprophytically, though 
the contents of the cells are probably consumed parasitically. 
Of course Elenkin’s hypothesis, if true, makes mutualism a little 
more untenable, if possible, than does pure parasitism of the 
lichen upon the alga. He studied a wide range of lichen types and 
various kinds of algal hosts, and concluded that the hyphae, aided 
by the postulated enzyme, eat into the walls of the algal cells, not 
so much to obtain the protoplast .within as to obtain food from 
the wall saprophytically. This makes the lichen, at least mainly, 
a saprophyte, on both the external, organic substratum and the en- 
closed algal host. Elenkin’s hypothesis of endosaprophytism is 
far from proved, but the lichen is probably partly parasitic and 
partly saprophytic upon the algal host. Whether the hypothesis is 
valid or not, its author has placed before us a series of facts re- 
garding the relation of the lichen to its algal host which must 
provoke much careful investigation and has added materially to 
a solution of the problems involved. 
Danilov (46) found that a hypha enters the algal protoplast, 
and branches into a delicate network of slender hyphae, which 
penetrate through the protoplast in various directions. The walls 
of the hyphae of this network are very thin and can be seen only 
with great difficulty. One also finds in the algal cells hyphae like 
those of other portions of the lichen. These occur alone, or with 
the slender hyphae, one or both kinds filling the entire space within 
the cell. Danilov thinks that the slender hyphae pass into the 
ordinary kind, after absorbing the contents of the cell, grow out of 
the protoplast and attack other algal cells in similar fashion. 
Through the effects of the slender branched hyphae, the algal cells 
are deformed and finallv killed. He often found the protoplast 
