FERNS 419 



gonium is often only a small part of it. Sporangia, on the 

 other hand, are exclusively devoted to producing spores, 



FIG. 210. Adiantum, the maidenhair fern. Cross section of the stem. The fol 

 lowing tissues are evident : epidermis, a thick cortex, outer phloem, xylem 

 (thick-walled cells), inner phloem, pith. The phloem, the xylem, and the pith, 

 taken together, constitute the stele. 



and they are only a very small part of the whole sporophyte 

 generation. 



When the spores of pteridophytes germinate, they pro- 

 duce a structure called the prothallium. Since this struc- 



