INTRODUCTION xiii 
VEGETATIVE STRUCTURES PECULIAR TO THE 
LICHEN-THALLUS. 
Soredia.—These are small powdery bodies that consist of one 
or several algal cells closely surrounded by lichen-hyphe. They 
occur as grayish powder on trees, or on the ground, representing 
the first stage of thallus formation ; as white or greyish granules 
on the squamules and podetia of many Cladoniz, or they are 
massed on the surface of the thallus into well-defined pustules 
called soralia. The soralium is formed by an increased growth 
of soredial gonidia and hyphe at certain points within the 
thallus, the overlying tissue finally bursting and allowing the 
escape of the soredia. Soralia are generally constant in form 
and in position on the thallus. Each soredium can give rise to 
a new lichen-plant. It is a method of vegetative reproduction 
that secures a ready and wide dispersal of species. 
Isidia.—The isidium is a small outgrowth from the surface 
of the thallus which contains both the algal and fungal con- 
stituents, but differs from the soralium in being covered by a 
cortex. Usually the isidia are like minute cones on a narrow 
stalk ; occasionally they are branched. 
Cephalodia.—On the thallus of various lichens there are 
developed abnormal, and generally darker-coloured swellings, 
ee 0, \ 
Si oes ee ORES Git 
. i (CREB oN ES]. IN 
a = }9; SRN SOS 
Q <\\> 2 
> > \ 5 
ANS oe iS 
Ke BS 
o SESE 
mR 
\ 
2 Cie 
Be 
NODORROROF 
: < BRaOS G5 —C 
De eae? 
ae 
se See <ee 
st — Serle 
Sg soe Se—m 4 
iS aN 
SA SR PALS 
S SS 
FN 
we eN 
Oe 
meal oa 
Fra. 5.—Vertical section of thallus and cephalodium of Peltigera 
aphthosa. A, Cephalodium; B, Thallus ; ¢, cortex ; g, gonidia ; 
m, medulla. (Greatly magnified.) 
tubercles or warts, composed of the normal lichen hyphz, but con- 
taining a different algal constituent, which is almost invariably 
one of the Myxophycex (Nostoc, Stigonema, etc.). Cephalodia 
are constant in form and occur as flat orbicular light-coloured 
expansions (Lecanora gelida), groups of dark branchlets (Lobaria 
