54 



CONSTITUENTS OF THE LICHEN THALLUS 



(N. lichenoides Kiitz.) has been determined as the lichen gonidium. When 

 the chains are broken up it has been wrongly classified as another alga, 

 Polycoccus 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). 



6 



Fig. 2 1 . Examples oSRivularia ; A, B, C, K. Bin- 

 solettiana Menegh. ; D and E, R. minutula 

 Bom. and Fl. A and D nat. size; B, C and E 

 x 480 (after West). 



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