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94
MORPHOLOGY
or less characteristic of all the species of Parmelia with the exception of
those belonging to the subgenus Hypogymnia in which they are of very rare
occurrence, arising, according
1
to Bitter , only in response to some external
friction. They are invariably dark-coloured, rather short, about one to a
few millimetres in length, and are simple or branched. The branches may
go off at any angle and are sometimes curved back at the ends in anchorlike
fashion. The Parmeliae grow on firm substances, trees, rocks, etc., and
the irregularities of their attaching structures are conditioned by the obstacles
encountered on the substratum. Not unfrequently the lobes are attached
by the rhizinae to underlying portions of the thallus.
In the genus Gyrophora, the rhizinae are simple strands of hyphae
(G. polyrhizd) or they are corticate structures (G. murina, G. spodochroa
and G. vellea\ They are also present in species of Solorina, Ricasolia,
Sticta and Physcia and very sparingly in Cetraria (Platysma).
c. HAPTERA. Sernander 2 has grouped all the more distinctively aerial
organs of attachment, apart from rhizinae, under the term "hapteron" and he
has described a number of instances in which cilia and even the growing
points of the thallus may become transformed to haptera or sucker-like
sheaths.
The long cilia of Physcia ciliaris occasionally form haptera at their tips
where the hyphae are loose and in active growing condition. Contact with
some substance induces branching by which a spreading sheath arises; a
plug-like process may also be developed which pierces the substance encountered
not unfrequently another lobe of its own thallus. The long
flaccid fronds of Evernia furfuracea are frequently connected together by
bridge-like haptera which rise at any angle of the thallus or from any part
of the surface.
The spinous hairs that border the thalline margins in Cetraria may also,
in contact with some body often another frond of the lichen form a
hapteron, either while the spermogonium, which occupies the tip of the
spine, is still in a rudimentary stage, or after it has discharged its spermatia.
The small sucker sheath may in that case arise either from the apex of the
cilium, from the wall of the spermogonium or from its base. By means of
these haptera, not only different individuals become united together, but
instances are given by Sernander in which Cetraria islandica, normally a
ground lichen, had become epiphytic by attaching itself in
trunk of a tree (Pinus sylvestris}.
this way to the
In Alectoria, haptera are formed at the tip of the thallus filament as an
apical cone-like growth from which hyphae may branch out and penetrate
any convenient object. A species of this genus was thus found clinging to
1 Bitter 1901.
2 Sernander 1901.