MR. A. J. EWART ON ASSIMILATORY INHIBITION. 383 
solution show no signs of life *; and even if sprinkled repeatedly ` 
with water containing living Protococci obtained from specimens 
of the same Lichen, no regeneration and reorganization of the 
Lichen takes place as might be expected to occur if the fungal 
thyphe were living. On the other hand when, as in many cases, 
the gonidia remain living only over a part of the thallus, such 
parts remain living and normal in appearance however long they 
may be kept under observation. Also in such areas many of the 
gonidia may be killed, only a portion remaining living; and when 
this is the case after a few days to a week or more in such regions 
‘a multiplication and regeneration of the gonidia take place, the 
dead algal cells becoming less and less numerous as the living 
ones multiply. 
Hence whenever the gonidia remain living, the Fungus remains 
living also, the Algal component being apparently the first to be 
killed. The conclusion, therefore, is that the resistant power of 
the Fungal component to desiccation and dry heat is slightly 
higher than that of the Algal component, but that the difference 
is very slight, it being impossible by drying or heating to entirely 
kill the one without also killing the other. 
It is worthy of notice in this connection that the combination 
-of Alga with Fungus seems to increase the resistant power of the 
former (compare tables C & D). Whether this is (a) atemporary 
modification due to the symbiosis, or (5) a merely mechanical 
protective effect of the enclosing fungal hyphæ, or (c) a special 
permanent modification induced by natural selection, is diffieuir 
+o say. That (b) exercises some influence is certain, and that 
(a) is of some importance is rendered probable by the fact that 
the symbiotic condition does in many cases induce a change in 
the structure of the Alga, such as the breaking-up of chains of 
.cells and the thickening of the cell-wall, which latter would 
;readily account for an increased resistant power. Whether such 
temporary modification due to the environment can be rendered 
permanent by natural selection is an open question. In the case 
of the Fungal component a permanent modification, selective or 
acquired, one indication of which is the increased thickness of 
the hyphal cell-walls, fitting it for a dry habitat, has certainly 
* A. Moller (‘ Ueber die Cultur flechtenbildender Ascomyceten ohne Algen,’ 
Münster, 1887) has shown that it is possible to grow and develop in suitable 
media the isolated Fungal componente of Lichens, 
