XVill INTRODUCTION 
The form of the sterigmata (or sporophores) varies, and the 
differences have been made use of in classification. Nylander 
divided the spermogones into two groups: those with simple 
““sterigmata ” and those with “arthrosterigmata.” In the former, 
the more usual type, the sterigmata are more or less upright, 
variously branched, and sometimes anastomosing; they are 
usually sparingly septate, and the spermatia are acrogenous on 
the tips of a secondary branch. The arthrosterigmata are divided 
up into short cells, each cell directly giving rise to a pleurogenous 
spermatium (or spore). This type of sterigma is found in Physcia 
and Sticta. In Endocarpon the arthrosterigmata form a tissue 
lining the wall. 
The spermatia also show great variation in form. Usually 
they are extremely minute, or if elongate, extremely narrow and 
thread-like. Some undoubted pycnidia contain larger stouter 
spores which do not differ from the others except in size, though 
the term “‘ pyenides” was wholly reserved for these by Lindsay. 
PHYSIOLOGY. 
Nutrition.—In the higher plants there are green cells containing 
chlorophyll and colourless cells forming tissues with different 
functions. We find the same distinctions between the tissues in 
lichens, but with this great difference, that whereas, in the higher 
plants, all the cells have the same initial starting point, in lichens, 
the two kinds of cells have a widely separate origin. In lichens, 
as in the higher plants, the green cells—the lichen gonidia—do 
the work of assimilation and by photosynthesis prepare carbo- 
hydrates for the whole plant. The colourless cells or hyphe 
are the organs of absorption, and, in return for the algal carbo- 
hydrates. in the form of sugars or glucoses, they supply water, 
nitrogenous substances and salts, which they have absorbed from 
the substratum or from the environment and converted into a 
state in which they can be used by the protoplasm of the 
green cell, 
Symbiosis.—The relationship between the two organisms was 
at first regarded as that of algal host and fungal parasite, but 
further consideration showed that this view was not entirely 
satisfactory, and Reinke pointed out that each member of the 
joint thallus might be regarded as the “consort” of the other. 
This view was further elaborated by De Bary, who brought 
forward a similar theory of “ symbiosis,” or conjoint life with more 
