FERNS 419 



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

 other hand, are exclusively devoted to producing spores, 



Fig. 210. — Adianlum, 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- 



