Fink: Classification of Lichens 
133 
adds to the ease with which some terrestrial algae may have 
assumed the hostal relation with lichens. It shows that these 
algae need not have undergone any great change in method of 
nutrition since becoming lichen hosts. Again, according to this, 
certain algae may at any time become hosts of lichens without any 
pronounced nutritional change, the modification being almost 
entirely morphological and physiological, due to the effect of the 
lichen upon form, size and functioning of the algal cells or 
filaments. 
It is plain enough that these results from physiological re- 
searches are in part conflicting, and that more investigation is 
needed. 
Breathing Pores and Other Means of Aeration for Lichens 
AND Their Hosts 
Breathing pores have been postulated for lichens, and their 
existence in crude form in some lichens is no longer to be doubted. 
Yet their general presence is not proved. Funfstiick (64) gives 
a general summary in Engler and Prantl. Rosendahl (loyi 
says that Parmclia aspidota (Ach.) shows numerous wart-like 
elevations. These are at first small and closed above ; but they 
enlarge and open later, so that there is free communication be- 
tween the interior of the thallus and the external air. The open- 
ings are formed by a loosening of the hyphal tissue of the plec- 
tenchymatous cortex, so that they are not definite canals, but 
passages through networks of hyphae. He found no such system 
in any other of fourteen species studied from the same genus and 
concluded that its presence in Parmclia aspidota (Ach.) is corre- 
lated with the unusually thick cortex of this species. Zopf (147) 
has described a new species of Ramalina which has similar struc- 
tures. Aside from such passages which are known in very few 
lichens, there are several possible ways of entrance. Air may 
enter through growing points, where the cortex is only one or 
two layers of loosely interwoven hyphae, and through the lower 
sides of thalli that have no lower cortices or very thin or incom- 
plete ones. In thalli with uneven upper surfaces, the cortex is 
often very thin over the elevations, where air may enter. Other 
