882 MR. A. J. EWART ON ASSIMILATORY INHIBITION. 
(1) the relative number and nature of the gonidia and the intensity 
of light to which they are exposed ; (2) the proportionate amount 
and respiratory activity of the fungal component; (3) the respira- 
tory activity of the algal cells themselves. In testing a Lichen 
for its assimilating powers by the Bacterium method, (2) is 
eliminated, and it is possible to be always certain that the algal 
cells are optimally illuminated. 
It is not easy to understand why with equally vigorous plants 
the resistance of the gonidia of Cladonia to dry heat should be 
so much more pronounced than those of Hvernia. The algal 
cells in both are extremely alike, being a similar Protococcus 
form. In Cladonia the gonidia are scattered or in clumps, inter- 
woven with loose hyphæ on the outside of the thallus, the thick 
clear hyaline layer of closely packed hyphe being internal. In 
Evernia the latter layer is external and the gonidia form a dense 
layer immediately beneath it and are also scattered through 
the central loose hyphal network. The gonidia in the latter 
case are therefore better protected from desiccation than they 
are in the former, but are nevertheless less resistant. The 
explanation may be that the more exposed position of the 
apparently similar gonidia of Cladonia has caused them to 
become by adaptive modification more resistant to desiccation, 
and this to such an extent as to more than counterbalance their 
greater exposure. 
To determine the resistance of the Fungal component is not 
at all easy. It is extremely difficult to say on immediate ex- 
amination whether the Fungal hyph: are dead or living. To 
plasmolytic methods they do not answer, and microscopical 
examination commonly fails at first to detect any perceptible 
difference between dead and living hyphæ, even where these are 
loosely woven as in the medulla. After being kept for several 
days in a damp chamber, the hyphx of a dead plant begin to 
show various disintegratory changes, saphrophytic fungi begin to 
appear, the gonidia bleach, and the plants become much less 
water-absorbent, even in a damp atmosphere being more or less 
shrivelled. It is, however, always possible that in such cases the 
Fungal component was originally living, its death following only 
as a result of the death of the Algal component. 
An exposure which kills all the gonidia seems also sufficient to 
destroy the vitality of the Fungus, for such plants if fed with sugar 
