FILICES 65 



divisions. The prothallium is most commonly monoecious, though the 

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

 or dorsiventral, the result, according to Leitgeb (Sitzber. Akad. Wiss. 

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



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

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



are found exclusively (except in Marattiaceae) 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 



