MR. A. J. EWART ON ASSIMILATORY INHIBITION. 881 
leaves by the outer, a power of active assimilation may be present 
in the inner leaves and be readily detected by the Bacterium 
method, though none can be detected by the gas-analytical method. 
It is impossible by the latter method to make any allowance for 
the fact that commonly the different parts of the same plant may 
‘be differently affected by the same exposure. Thus in the young 
leaves, after a given exposure, a power of assimilation may still be 
present, but in older leaves absent, whilst in the oldest leaves 
many or all of the cells may be killed. As seen above, such a 
plant by the gas-analytical method appears to have no perceptible 
power of assimilation. 
In the case of Lichens the error is still more increased by the 
fact that here the assimilation has to act against the respiration 
of both Fungus and Alga, and that the assimilating cells are 
normally more or less shaded from light by the fungal hyphe 
with which they are surrounded or covered. The substance of 
a Lichen is always rather opaque, and even where the gonidia are 
on the outside of the thallus, the inner gonidia are largely shielded 
from light which falls on the outside of the thallus. Jumelle *, 
as the result of his experiments upon the assimilation of Lichens, 
concludes that there is no maximum or optimum intensity of 
ilumination for assimilation in Lichens, but that sunlight is 
always better than diffuse daylight. The explanation of this, 
however, may be that even when exposed to direct sunlight the 
Algal cells in an intact Lichen are for the most part exposed to 
an intensity of illumination not greater than that of bright 
diffuse daylight. With crustaceous Lichens, Jumelle finds that 
it is only when exposed to direct sunlight that they give off more 
oxygen than CO,, the result being probably partly due to the 
opacity of the Lichen, and by no means indicating that the gonidia 
only assimilate actively when exposed to intense light. 
When examined in the form of thin sections, it is found that 
with increasing intensity of illumination beyond bright diffuse 
daylight no further increase in the rapidity of the evolution of 
oxygen takes place. The assimilation of the algal cells, whether 
free or combined, in a Lichen remains the same, provided the 
other external conditions are alike. The amount of oxygeu 
given off by any Lichen when assimilating is dependent upon 
* Jumelle, *L'Assimilation chez les Lichens," in Comptes Rendus, Paris, 
£. exii. 1890. 
2Eg 2 
