495 
discouraged ; much less a reason why old fashioned “ empiricists ” 
should gloat over these failures and make strenuous efforts to per- 
petuate the old regime. 
The knowledge gained from a purely morphological study of 
lichens, is in itself of little practical value if we can not find the 
correlated physiological interpretation. The scientific study of 
lichens is of very recent origin. Our knowledge of the physiology 
of these plants is especially deficient. Sufficient is, however, 
known in a general way, to enable us to give fairly accurate 
hypothetical physiological interpretations of most of the anatom- 
ical structures. 
It is perfectly consistent to suppose that the lichen, during 
phylogenetic history, has undergone great adaptive changes. This 
statement is axiomatic and requires no further elucidation. From 
the nature of things it is also evident that the thallus has under- 
gone the greatest adaptive changes. 
As has been known for some time, the thallus in particular per- 
forms the function of assimilation, due to the presence of the sym- 
biotic algae. Within recent years, Jumelle * has made a special 
Study of chlorophyllian assimilation in lichens. The special 
structural adaptations which we are about to discuss are due to 
the fact that the fungal symbiont, which is incapable of assimi- 
lating CO,, must make suitable concessions to the position and ar- 
rangement to the algal symbiont. This we find to be the case, 
In this paper I shall devote myself to foliaceous thalli and shall 
_ briefly consider some of the typical structures met with in such 
thalli; namely, the epidermal layer, the upper cortical layer, the 
algal (“gonidial” ) layer, the medullary layer, the lower corti- 
cal layer and the rhizoids. A typical epidermal layer is not 
Present in all foliaceous lichens. It is, however, well devel- 
Oped in Svcta and Stictina. It consists of hyphal cells, two 
or three layers in thickness. The cells are placed horizontal'y 
and are very closely crowded, thus forming a protection against 
excessive evaporation. It serves a function similar to eS ce 
dermis in higher plants. In the genus Lepéogium the epider- 
mis consists of a single layer of cells presenting a close resem- 
ia See pena : le de Bo 
*Jumalle H. Recherches Physiologiques sur les Lichens. Revue generale de Bo. 
_— tanique, 4: 49-64, 103-121, 159-175) 220-231, 259-272, 305-320. 1892 
