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i 44
MORPHOLOGY
genus Usnea which give rise to new branches. Many of the species in that
genus are plentifully sprinkled with the white powdery bodies. A short
way back from the apex of the filament the separate soredia show a tendency
to apical growth and might be regarded as groups of young plants still
attached to the parent branch. One of these developing more quickly
pushes the others aside and by continued growth fills up the soredial
opening in the cortex with a plug of tissue; finally it forms a complete
lateral branch. Schwendener calls them "soredial" branches (Fig. 82) to
distinguish
development.
them from the others formed in the course of the normal
B. SORALIA
In lichens of foliose and fruticose structure, and in a few crustaceous
forms, the soredia are massed together into the compact bodies called soralia,
and thus are confined to certain areas of the plant surface. The simpler
soralia arise from the gonidial zone below the cortex by the active division
of some of the algal cells. The hyphae, interlaced with the green cells, are
thin-walled and are, as stated by Wainio 1
, still in a meristematic condition ;
they are thus able readily to branch and to form new filaments which clasp
the continually multiplying gonidia. This growth is in an upward or outward
direction away from the medulla, and strong mechanical pressure is
exerted by the increasing tissue on the overlying cortical layers. Finally
the soredia force their way through to the surface at definite points. The
cortex is thrown back and forms a margin round the soralium, though shreds
of epidermal tissue remain for a time mixed with the powdery granules.
a. FORM AND OCCURRENCE OF SORALIA. The term " soralium " was
first applied only to the highly developed soredial structures considered by
Acharius to be secondary apothecia; it is now employed for any circum-
scribed group of soredia. The soralia vary in size and form and in position,
according to the species on which they occur; these characters are constant
enough to be of considerable diagnostic value. Within the single genus
Parmelia, they are to be found as small round dots sprinkled over the
surface of P. dubia; as elongate furrows irregularly placed on P. sulcata; as
pearly excrescences at or near the margins of P. perlata, and as swollen
tubercles at the tips of the lobes of P.physodes (Fig. 83). Their development
is strongly influenced and furthered by shade and moisture, and, given such
conditions in excess, they may coalesce and cover large patches of the thallus
with a powdery coating, though only in those species that would have borne
soredia in fairly normal conditions.
Soralia of definite form are of rather rare occurrence in crustaceous lichens,
1 Wainio 1897, p. 32.
2 Reinke 1895, p. 380.