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RADIATE THALLUS 99
those of Roccella have been found measuring 30 cm. in length ; those of
Ramalina reticulata, the largest of all the American lichens, extend to con-
siderably more.
Lichens of filamentous growth are more or less cylindrical (Fig. 58).
They are in some species upright and of moderate length^
but in a few
Fig. 58. Usnea barbata Web. (S. H., Photo.},
pendulous forms they grow to a great length : specimens of Usnea longissima
have been recorded that measured 6 to 8 metres from base to tip.
The radiate type of thallus occurs in most of the lichen groups but most
frequently in the Gymnocarpeae. In gelatinous Discolichens it is represented
in the Lichinaceae. It is rare : among Pyrenocarpeae there is one
very minute British lichen in that series, Pyrenidium actinellum, and one
from N. America, Pyrenothamnia, that are of fruticose habit.
2. INTERMEDIATE TYPES OF THALLUS
Between the foliose and the fruticose types, there are intermediate forms
that might be, and often are, classified now in one group and now in the
other. These are chiefly : Physcia (Anaptychia) dliaris, Ph. leucomelas and
the species of Evernia.
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