PDF - CES (IISc)

282 PHYLOGENY
crustaceous: at first an accretion of separate granules which may finally be
united into a continuous crust with a protective covering of thick-walled
amorphous hyphae forming a " decomposed " cortex. The extension of
a granule by growth in one direction upwards and outwards gives detachment
from the substratum, and originates (2) the squamule which is, how-
ever, often of primitive structure and attached to the support, like the granule,
by the medullary hyphae. Further growth of the squamule results in (3)
the foliose thallus with all the adaptations of structure peculiar to that form.
In all of these, the principal area of growth is round the free edges of the
thallus. A greater change takes place in the advance to (4) the fruticose
type in which the more active growing tissue is restricted to the apex, and
in which the frond or filament adheres at one point only to the support, a
new series of strengthening and other structures being evolved at the same
time.
The lichen fungi associate, as has been already stated, with two different
types of algae: those combined with the Myxophyceae have been designated
Phycolichenes, those with Chlorophyceae as Archilichenes. The latter predominate,
not only in the number of lichens, but also in the more varied
advance of the thallus, although, in many instances, genera and species of
both series may be closely related.
B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS
One of the first questions of inheritance concerns the comparative an-
tiquity of the two gonidial series: with which kind of alga did the fungus
first form the symbiotic relationship ? No assistance in solving the problem
is afforded by the type of fructification. The fungus in Archilichens is
frequently one of the more primitive Pyrenomycetes, though more often a
Discomycete, while in Phycolichens Pyrenomycetes are very rare. There
is, as already stated, no corelation of advance between the fruit and the
thallus, as the most highly evolved apothecia with well-formed thalline
margins are constantly combined with thalli of low type.
Forssell 1
gave considerable attention to the question of antiquity in his
study of gelatinous crustaceous lichens in the family Pyrenopsidaceae, termed
by him Gloeolichens, and he came to the conclusion that Archilichens
represented the older combination, Phycolichens being comparatively.young.
His view is based on a study of the development of certain lichen fungi
that seem able to adapt themselves to either kind of algal symbiont. He
found 1 in Euopsis (Pyrenopsis) granatina, one of the Pyrenopsidaceae, that
certain portions of the thallus contained blue-green algae, while others con-
tained Palmella, and that these latter, though retrograde in development,
1 Forssell 1885.