178 STRUCTURE AND LIFE HISTORIES 



ized, sometimes being entirely so; while in other cases 

 sporangia occur on the foliage-leaf. As in the replace- 

 ment of sporophylls by sterile leaves in the ostrich, fern, 

 Onoclea struthiopteris (paragraph 148), these abnormalities 

 indicate the close relationship between leaves and spore- 

 bearing organs, and clearly show that the latter may be 

 completely transformed, by sterilization, into foliage- 

 leaves. 



In Ophioglossum the foliage-leaf and spore-bearing spike 

 are both unbranched, the latter suggesting an adder's 

 tongue, whence the name, Ophioglossum. In both Opkio- 

 glossum and Botrychium the sporangia originate horn a group 

 of epidermal and sub-epidermal cells, and are consequently 

 imbedded in the surrounding tissue. Their walls are 

 more than one cell in thickness, the annulus is lacking, 

 and they open by a slit. Ferns of this type are called 

 eusporangiate. Their prothallia are usually fleshy and 

 subterranean, bear the antheridia and archegonia on the 

 dorsal instead of on the ventral surface, and are perennial, 

 often living on after the sporophyte has died. In general 

 the sporophyte possesses less sterile tissue in proportion 

 to the fertile tissue than is the case with the leptosporan- 

 giate forms. These characters mark the group as more 

 primitive than the leptosporangiate ferns, and they are 

 much less numerous, only about 100 species being known 

 from the entire world, while of the leptosporangiate ferns 

 between 3,000 and 4,000 species have been described. 



