PETRONEMA fruticuiosum.
S h r u b - lik e P e tr o n e m a .
CRYPTOGAMIA Alga.
Gen. Char. P la n t s densely casspitose, erect, somewhat
regularly branched. B r a n c h e s free, with
obtuse rounded apices, and each with a connecting
cell at its base. E n d o c h r o m e annulated, increasing
in diameter towards the apices of the
filaments.
Spec. Char................... .
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JL HE present species, where it occurs, forms a frustulose,
areolated, olive-brown crust upon the wet perpendicular surfaces
of limestone rocks, to which it adheres so slightly as to
be very readily detached. The plants are densely crowded
in little hemispherical masses, which together produce the
peculiar areolated appearance of the crust.
Each separate plant consists of a single filament at the base,
which is from l-4< or even more times tri- or tetra-chotomously
branched, and thus a fruticose habit is given to thé species.
Each branch has a connecting cell at its base, and sometimes
one also at about the middle of its length. The very thick
cartilaginous sheaths are of a pale brown colour towards the
base of the plant, but darker upwards, except at the apices,
which are frequently nearly colourless. The eridochrouie is
of a dull green colour, extremely narrow and indistinctly annulated
; towards' the ends of the filaments, however, it becomes
much wider and slightly moniiiform.
The present genus is closely allied to Calothrix, but differs
in its regular mode of branching, and in the very obtuse and
smooth apices of the filaments.
Found on St. Vincent’s Rocks, near Bristol, during the
winter months.—G. H. K. T.