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- 
