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ORIGIN OF LICHEN STRUCTURES 69
In the Collemaceae, the gonidial cells of which are species of Nostoc
(Fig. 2), there appears a more developed thallus; but in general, symbiosis
in Collema has wrought the minimum of change in the habit of the alga,
hence the indecision of the earlier botanists as to the identification and
classification of Nostoc and Collema. Though in many of the species of the
genus Collema no definite tissue is formed, yet, under the influence of
symbiosis, the plants become moulded into variously shaped lobes which
are specifically constant. In some species there is an advance towards
more elaboration of form in the protective tissues of the apothecia, a layer
of thin-walled plectenchyma being occasionally formed beneath or around
the fruit as in Collema granuliferum.
In all these lichens, it is only the thallus that can be considered as
primitive: the fruit is a more or less open apothecium more rarely a perithecium
with a fully developed hymenium. Frequently it is provided with
a protective thalline margin.
b. EXOGENOUS THALLUS. In this group, composed almost exclusively
of heteromerous lichens, Zukal includes all those in which the fungus takes
the lead in thalline development. He counts as such Leptogium, a genus
closely allied to Collema but with more membranous lobes, in which the
short terminal cells of the hyphae have united to form a continuous cortex.
A higher development, therefore, becomes at once apparent, though in some
genera, as in Coenogonium, the alga still predominates, while the simplest
forms may be merely a scanty weft of filaments associated with groups of
algal cells. Such a thallus is characteristic of the Ectolechiaceae, and some
Gyalectaceae, etc., which have, indeed, been described
1
by Zahlbruckner
as homoiomerous though their gonidia belong to the non-gelatinous
Chlorophyceae.
Heteromerous lichens have been arranged by Hue 2
according to their
general structure in three great
series :
1. Stratosae. Crustaceous, squamulose and foliose lichens with a
dorsiventral thallus.
2. Radiatae. Fruticose, shrubby or filamentous lichens with a strap-
shaped or cylindrical thallus of radiate structure.
3. Stratosae-Radiatae. Primary dorsiventral thallus, either crustaceous
or squamulose, with a secondary upright thallus of radiate structure called
the podetium (Cladoniaceae).
1 Zahlbruckner 1907.
2 Hue 1899.