134 



THE BIOLOGY OF A PLANT. 



for the growth and enlargement of the archesporium. The latter now 

 divides first into 2, then into 4, 8, and finally 16 cells, the mother-cells 

 of the spores. These remain for a time closely united, but eventually 

 separate and again subdivide, each into 4 daughter-cells (Fig. 65, Z). The 

 64 cells thus formed are the asexual spores. In their mature state they 

 have a tetrahedral form and certain external markings, indicated in Figs. 

 63, 66. Each spore acquires a double membrane, viz., an inner, endo- 

 sporium, delicate and white, and an outer, exosporium, yellowish brown, 

 hard, and sculptured over the surface with very close and fine, but 

 irregular, warty excrescences. 



Germination of the Spores. Development 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 Fi^. 63. Germination of the 



o 



spores normally occurs only after a considera- 

 ble period (perhaps not before the following 

 spring) ; it begins by a rupture of the exospo- 



~FiG. 66. (After FIG. 67. (After Suminski. V- Germinat- FIG. 68. (After Sumin- 



Suminski.) 

 Single spore of 

 Pteris serrula- 

 ta. 



ing spores of Pteris serrulata. A, in an 

 early stage ; B, after the appearance 

 of one transverse partition ; s, spore ; 

 p, protonema ; r, rhizoid. 



ski.) Very young pro- 

 thallium of Ptcris, 

 showing the spore (s), 

 two rhizoids (r), and 

 the enlarging extrem- 

 ity. 



rium which is probably 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 endo- 

 sporium 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 (Fig. 67). It develops very quickly 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 



