CHAPTER VIl.—PHENOMENA OF VEGETATION.—LICHENS. 415 
conical bodies like small flat apothecia of Peziza with the apex of the cone towards 
the thallus and produced by suitable branching of a tuft of hyphae; these bodies 
when they are present in large numbers unite together at the margins and form 
patches of some size; in Dictyonema and Laudatea they are smooth expanded 
layers usually resembling those of the Thelephoreae. The sporiferous structure of 
Rhipidonema is not clearly understood. Further details will be found in Mattirolo 
and Johow. 
Section CXVII. The thallus of very many Lichens forms small brood- 
buds which separate spontaneously from the thallus and under favourable conditions 
may develope into a Lichen-thallus similar to the parent structure. These have been 
known since Acharius’ time by the name of soredia. They appear to be wanting in 
some species, as in Lecidea geographica and Endocarpon pusillum ; in others they 
are very abundant, as in many species of Usnea, Bryopogon, Ramalina, Evernia, 
Imbricaria, Parmelia, Pertusaria and others. 
In their most general characters the soredia are small portions of the thallus 
which have a distinct form in each case and consist of one or more algal cells and the 
hyphae which have grown round them. As the soredium developes into a new 
thallus the two constituents behave 
in the manner which has been 
described above in the special 
types of thallus. Their structure 
at the moment of separation is 
in many cases that of the mother- 
thallus in a rudimentary form, 
as in Collema where they are 
roundish branchlets, prolifications 
of the upper surface or margin of 
the thallus appearing like granules to the naked eye, in the Graphideae also which 
contain Chroolepus, in Roccella and others. Detailed investigations in many 
species have not yet been made. 
The soredia have been carefully studied by Tulasne and especially by 
Schwendener in the typical heteromerous forms which contain Palmellaceae, especially 
Cystococcus, and there their development and structure usually exhibit the following 
peculiar features (Fig. 181). 
They are formed at spots in the algal zone beneath the rind which are distributed 
over the surface or margin of the thallus and vary according to the species. Branches 
of the hyphae twine round an adjacent algal cell or group of cells which have arisen by 
division and form a soredium ; if a group of cells is thus attacked the hyphae also 
insinuate themselves between the cells and twine round each of them. The roundish 
body thus formed consists entirely of one or more algal cells and an envelope of 
hyphae which differs in compactness and colour in different species, and in some, as 
in Bryopogon, is not perfectly closed; how it is separated from the thallus has not 
been clearly ascertained. 
By unlimited repetition of this process at one spot the soredia accumulate there 
beneath the rind, which swells up and at length bursts, the soredia emerging from 

FIG. 181. Usnea barbata. c. an isolated mature soredium, with an algal cell 
(Cystococcus) in the envelope of hyphae. @ another with several algal cells 
in optical longitudinal section. ¢, /two soredia in the act of germinating ; the 
hyphal envelope has grownout below into rhizoid-branches, and above shows 
already the structure of the apex of the thallus (see Fig. 171). After 
Schwendener. Magn. more than 500 times, 
