4 195 
upon as typical lichen structures, yet their morphology is of great 
importance in the consideration of lichen evolution and classifica- 
tion. Some of the changes in apothecia indicating a probable 
higher or lower stage of development have. already been referred 
to. AsI intend to consider these changes more particularly in 
a future paper on lichen classification, I shall at present omit 
further discussions. I shall now briefly consider the physiological 
characters which distinguish lichens from both fungi and algae. 
PHYSIOLOGICAL. 
In their method of growth lichens stand alone, The two 
symbionts form a microcosmos which is enabled to perform the 
necessary life functions which were originally inherent in both, 
and in addition they have acquired new characteristics during their 
phylogeny as lichens, which unmistakably stamp them as autono- 
mous structures. . As a unit they can now exist where neither sym- 
biont could exist alone. In spite-of this intimate mutualism, it 
is not at all likely that the fungal symbiont is w/ol/y dependent 
upon the algal symbiont for its food supply. For example, a 
lichen spore may develop to a considerable extent as a sapro- 
phyte upon: decaying wood, humus, and other dead organic mat- 
ter; nor is it at all likely that a.lichen can develop upon purely 
inorganic matter, as, for example, pure quartz crystals. Of course, 
the spore, with the requisite algae or a soredium, has bound up 
within itself a certain amount of extra food material, which enables» 
development to begin in the absence of all organic matter. The 
mycelial network then forms a structure for collecting within its 
meshes organic substances, carried to it by air and water currents ; 
_ this allows growth to continue. No amount of food supply will, 
however, allow the fungal symbiont to mature without its algal 
_Symbiont, excepting perhaps the lowest forms. Thus we see that 
mutualism of fungus and alga is necessary to form a lichen. 
The fungal symbiont, considered by itself, still retains its ancestral 
function as a saprophyte ; in addition it has acquired the habits Or. 
_ 4 semi-obligative parasite upon its algal symbiont. The algal 
_ Symbiont, which has the function of chlorophyll-bearing plants in 
general, that of assimilating carbon, must be looked upon asa 
| facultative parasite, since it can exist and mature independently of 
