VEGE TA TIVE RETROD UC TION. 



233 



those bearing male organs only. A similar separation of 

 sexes in the gametophytes frequently occurs when the spores 

 are equal in size, as in Marchantia and horsetails, but it 



FIG. 238. 



FIG. 237. 



FIG. 239. 



FIG. 237. A, the "flower" of a seed plant (/.ainia tnuricata). It is composed of 

 crowded sporophylls, of which one is represented in />' as seen from the side. It has 

 a stalk capped by a hexagonal top, s, with numerous sporangia, x, on the under side. 

 A, natural size. A, magnified about 6 diam. After Karsten. 



FIG. 238. A bit of a section of the wall of a sporangium of a horsetail. The cells of 

 the outer layer thicken their walls along spiral lines. The two inner layers of cells, /, 

 become disorganized at maturity of the sporangium. Magnified 250 diam. After 

 Campbell. 



FIG. 239. Two spores of a horsetail (Eguisetuin arvenseY, one showing the elaters 

 open, as when dry, the other with them coiled, as when moist. Magnified 25 diam. 

 After Kerner. 



