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STRATOSE-RADIATE THALLUS
thus distinguishing it from the purely hyphal stalks of the apothecia in
Caliciaceae. Even in the genus Baeomyces,
while the podetia of some of the species
are without gonidia, neighbouring species
are provided with green cells on the upright
stalks clearly showing their true
affinity with the Cladoniae. In one British
species of Cladonia {Cl. caespiticia) the
short podetium consists only of the fibrous
chondroid cylinder, and thus resembles the
apothecial stalk of Baeomyces rufus, but
in that species also there are occasional
surface gonidia that may give
rise to
squamules.
Krabbe 1 concluded from his observa-
tions that the podetial gonidia of Cladonia
arrived from the open, conveyed by wind,
water or insects from the loose sored ia that
are generally so plentiful in any Cladonia
colony. They alighted, he held, on the
growing stalks and, being secured by the
free-growing ends of the exterior hyphae,
they increased and became an integral part of the podetium. In more
recent times Baur 2 has recalled and supported Krabbe's view, but Wainio 3
stage of central tube and of podetial
squamulesx 100 (after Krabbe).
on the contrary, claims to have proved that in the earliest stages of the
podetium the gonidia were already present, having been carried up from
the gonidial zone of the primary thallus by the primordial hyphae. Increase
of these green cells follows normally by cell-division or sporulation.
Algal cells have been found to be common to different lichens, but in
Cladoniae Chodat 4 claims to have proved by cultures that each species
tested has a special gonidium, determined by him as a species of Cystococcus,
which would render colonization by algae from the open much less probable.
In addition, the fungal hyphae are specific, and any soredia (with their
combined symbionts) that alighted on the podetium could only be utilized
if they originated from the same species; or, if they were incorporated, the
hyphae belonging to any other species would of necessity die off and be
replaced by those of the podetium.
c. CORTICAL TISSUE. In some species a cortex of the decomposed type
of thick-walled conglutinate hyphae is present, either continuous over the
whole surface of the podetium, as in Cl. gracilis (Fig. 68), or in interrupted
1 Krabbe t!
2 Baur 1904.
3 Wainio 1880. Chodat 1913.
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