DEVELOPMENT OF THE PROTHALLIUM. 



93 



in Figs. 51, 54. Each spore acquires a double membrane, viz., an 

 inner, endosporium, delicate and white, and an outer, " exo- 

 apwium? yellowish-brown, hard, and sculptured over the sur- 

 face with very close and fine, but irregular, warty excrescences. 



Germination of the Spores. Origin of the 

 Prothallium. In the brake the spores ripen 

 in July or August and are set free by rupture 

 of the sporangium under the strain exerted 

 by the elastic annulus, as indicated in Fig. 51. 

 Germination of the spores normally occurs 



FIG. 54. (After 

 Suminski.) Sin- 

 gle spore of Pte- 

 ris serruluta. 



FIG. 55. (After Suminski.) Germinating 

 spores of Pteris serrulnta. A, in an 

 early stage; B, after the appearance of 

 one transverse partition; s, spore; p, 

 protonema; r, rhizoid. 



FIG. 56. (After Suminski.) 

 Very ycr.mg prothalli- 

 um of Pteris, showing 

 the spore (s), two rhi- 

 zoids ( c), and the enlarg- 

 ing extremity. 



only after a considerable period (perhaps not before the following 

 spring) ; it begins by a rupture of the exosporium which is prob- 

 ably immediately due to an imbibition of water. The spore 

 bursts irregularly along the borders of the pyramidal surfaces, and 

 from the opening thus formed the endosporium protrudes as a 

 papilla filled with protoplasm in which numerous chlorophyll- 

 bodies soon appear. 



This papilla is known as the protonema, or first portion of 

 the prothallium. It develops very quickly (Fig. 55) into a stout 

 cylindrical protrusion divided into cells joined end to end. 

 Close to the spore one or more rhizoids are put down from the 

 growing protonema to serve as anchors and roots. At the oppo- 

 site or distal end longitudinal partitions soon appear (Fig. 56), 

 which speedily convert this portion into a broad flat plate at first 

 only one cell thick, but eventually several cells thick along the 

 median line. This thickening is the so-called " cushion" (see Fig. 

 58). The whole prothallium is now somewhat spatulate (Fig. 57), 



