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CONSTITUENTS OF THE LICHEN THALLUS
(N. lichenoides Kutz.) has been determined as the lichen gonidium. When
the chains are broken up it has been wrongly classified as another alga,
Poly coccus punctiformis.
Fam. SCYTONEMACEAE. Trichomes of single-cell rows, differentiated into
base and apex. Pseudo-branching arises at right angles to the main filament.
SCYTONEMA Ag. Pseudo-branches piercing the sheath and passing out
as twin filaments (Fig. 19); colour, golden-brown. This alga occurs in
genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in Petractis
a genus of Gyalectaceae, and in Dictyonema one of the Hymenolichens.
Fam. STIGONEMACEAE. Trichomes of several-cell rows with base and
apex ; colour, golden-brown.
STIGONEMA Ag. Stouter than Scytonema, with transverse and vertical
division of the cells, and generally copious branching (Fig. 20). This alga
occurs only in a few genera of Ephebaceae. S.panniforme Kirchn. (Siro-
siphon pulvinatus Breb.) has been determined as forming the gonidium.
Fam. RIVULARIACEAE. Trichomes with a heterocyst at the base and
tapering upwards, enclosed in mucilage (Fig. 21).
Fig. 20. Stigonema sp. x 200 (after
Comere).
-**-* 1
Fig. 21. Examples of Rivularia ; A, B, ,R.Biasokttiana
Menegh. ; D and E, R. minutula
Born, and Fl. A and D nat. size; B, C and E
x 48o (after West).