I 3 6 



GENERAL BIOLOGY 



under the rolled margin of the leaf, in small clusters (called 

 sori). Each sporangium is borne on a slender pedicel; its 

 walls are composed of thin epidermal cells ; one line of these 

 cells, encircling the top of the sporangium is differentiated 

 into a ring (the annulus) of thick-walled hygroscopic cells, 

 which at maturity burst the capsule by their elasticity, 

 scattering the spores. The spores which fall in suitable place 

 germinate after the manner shown in fig. 85, and grow 

 into thin, flat heart-shaped prothallia. 



FIG. 85. Development of the fern gametophyte from the 

 spore; figs. 1 to 6 show successive stages; figs. 3 and 4 

 show the establishment of an apical cell and growing 

 point ; fig. 6 shows the second rhizoid. 



Comparing fern and liverwort, we see great similarity in 

 reproductive organs and methods, considerable similarity in 

 the gametophyte phase, and great divergence in form, size, 

 structure and manner of life of the sporophyte. 



Other pteridophytes. The common horsetail (Equisetnm) 

 will serve to illustrate the kind of differences presented by 

 another group of pteridophytes. The sporophyte phase of 

 the horsetail is leafless, but bears green naked branches 

 which arise from an underground rhizome. The chlorophyl 



