AN INTRODUCTION TO THE STUDY OF LICHENS. 103 
chief agents in the growth of the plant. These green cells, says 
Nylander, ‘originate in the lower portion of the cortical stratum, 
and are enclosed in the cells of that stratum, and subsequently, as 
the development of the same stratum progresses, the gonidia are 
observed in a free condition.”* In their activity, the gonidia 
frequently burst through the cortex, and appear upon the surface 
of the thallus in clusters, called soredéa; or scattered over it 
like green dust, when the plant is said to be in a pulverulent 
condition. 
The colour of Lichens is very varied and changing. This is 
greatly dependent upon the place of growth, and surrounding 
influences. ‘There are many shades of colour, but few direct and 
primary colours,—browns and greys predominate. Green ranges 
from dark olive to a pale greyish or bluish-green. Yellow, from 
orange-red and lemon to cream colour and white. Brown changes 
from pale into chestnut and black ; and grey ranges between white 
and black. These colours are due to the gonidia, and the contents 
of the cells of the cortex or outer stratum of the thallus. 
Having, though but, in a very limited way, touched upon the 
chief features of the Lichen-thallus, we must now endeavour to 
convey an idea of its reproductive system. Lichens are said to 
have many modes of reproduction. Korber enumerates six. Two 
by spores and four by gonidia. But without accepting that state- 
ment, the Lichen undoubtedly has a secondary or indirect method 
of reproducing itself, by its green cells or gonidia. Still, whether 
the gonidial cell alone can produce a perfect plant, that is a plant 
which will bear fruit and develope spores, we have not yet seen 
authenticated. ‘The proper and normal way of fructification in the 
Lichen, is by sfortdia. ‘These are developed in a special organ 
adapted for their formation, protection, maturity, and dispersion, 
when ripe. This organ is denominated the afothecium. (Gr. 
apotheke, a storehouse or repository.) It is always found upon 
the surface, or attached to the margin of the thallus. It may be 
sesstle—resting” upon the surface; zzvzate—sunk in the thallus; 
stipitate—on little stalks; or surmounting the top of fodetia, 
These latter are cylindrical and vertical prolongations of the thallus, 
crowned with a cuplike cavity, on the toothed margins of which 
grow the apothecia, as in Cladonia. In some cases the cup is sub- 
stituted by globose fruit, singly or conglomerate. The apothecium 
assumes many different shapes on different plants. It is typically 
round and flat, or slightly concave—when it is termed scufellate. 
Sometimes it rises up from the margin of the thallus like a target, 
as in Peltigera; then it is pe/fafe. At other times, it is oblong 
and furrowed ; when it is called /re//ate. It also appears like a 
* Vide Grevillea, Vol. VIL, p. 44. 
