PTERIDOPHYTES 151 



bordered pits, elements characteristic of gymnosperms (fig. 547), but 

 not of pteridophytes. 



Fertile spike. In Ophioglossum the fertile spike begins to appear 

 very early in the history of the leaf. As it begins to project from the 

 adaxial face of the young leaf, a superficial band of cells becomes differ- 



FIG. 354. General habit of Helminthostachys. After HOOKER, 



entiated on each side, from near the apex downwards. As the spike 

 elongates, these two bands elongate and deepen, eventually giving rise 

 to two continuous bands of sporogenous tissue (figs. 355, 356, 359). 

 Later, sterile plates appear across the band, and the individual sporangia 

 become outlined, the single large sporogenous mass being broken up into 

 a great number of sporangia by sterilization (figs. 357, 358). 



The situation has suggested the idea that the fertile spike of Ophioglossales is 

 a sporangiophore extremely developed. If this interpretation is true, sporangio- 



