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STRATOSE-RADIATE THALLUS 113
between the older cortical filaments and so provide for the expansion as
well as for the renewal of the cortical tissue.
b. GONIDIAL TISSUE. The gonidia consisting of Protococcaceous algae
form a layer immediately below the cortex. Isolated green cells are not
unfrequently carried up by the growing hyphae into the cortical region, but
they do not long survive in this compact non-aerated tissue. Their empty
membranes can however be picked out by the blue stain they take with
iodine and sulphuric acid.
Krabbe 1 has described the phases of development in the growing region :
he finds that differentiation into pith, gonidial zone and cortex takes place
some little way back from the edge. At the extreme apex the hyphae lie
fairly parallel to each other; further back, they branch upwards to form the
cortex, and to separate the masses of multiplying gonidia, by pushing
between them and so spreading them through the whole apical tissue. The
gonidia immediately below the upper cortex, where they are well-lighted,
continue to increase and gradually form into the gonidial zone; those that
lie deeper among the medullary hyphae remain quiescent, and before long
disappear altogether.
Where the squamules assume the upright position (as in Cladonia cei~vi-
corms), there is a tendency for the gonidia to pass round to the lower
surface, and soredia are occasionally formed.
c. MEDULLARY TISSUE. The hyphae of the medulla are described by
Wainio as having long cells with narrow lumen, and as being encrusted
with granulations that may coalesce into more or less detachable granules;
in colour they are mostly white, but pale-yellow in Cl.foliacea and blood-red
in Cl. miniata, a subtropical species. They are connected at the base of the
squamules with a filamentous hypothallus which penetrates the substratum
and attaches the plant. In a few species rhizinae are formed, while in others
the hyphae of the podetium grow downwards, towards and into the sub-
stratum as a short stout rhizoid.
d. SOREDIA. Though frequent on the podetia, soredia are rare on the
squamules, and, according
to Wainio 2
, always originate at the growing
region, from which they spread over the under surface rather sparsely in
Cl. cariosa, Cl. squamosa, etc., but abundantly in Cl. digitata and a few others.
In some instances, they develop further into small corticate areolae on the
under surface (Cl. cocci/era, Cl. pyxidata and Cl. squamosd).
1 Krabbe 1891.
2 Wainio 1897.