FILICES 



65 



divisions. The prothallium is most commonly moncecious, though the 

 sexual organs may not appear at the same time, and is strictly bilateral 

 or dorsiventral, the result, according to Leitgeb (Sitzber. x\kad. Wiss. 

 Wien, Ixxx., 1880, p. 201), of the action of hght. The archegones 



Fig. 42. — Germination of prothallium of fern, with exospore still attached, a, b, Dicksonia 

 antarctica Lab. (x 240) ; c, d, Aspidiiun Jilix-mas Sw. (x 120). (After Luerssen.) 



are found exclusively (except in Alarattiaceae) on the under side of the 

 cushion. Rhizoids are produced in large numbers on the under side 

 of another part of the cushion ; the antherids also on the under side 

 among the rhizoids, or less often on the margin. In Gymnogramme 



F 



