266 PLANT MORPHOLOGY 



In Botrychium the fertile segment is more complex than in Ophioglos- 

 sum. In nearly all species it is pinnately branched, the narrow divisions 

 bearing two rows of spherical sporangia that are not embedded but project 

 on a very short stalk (Fig. 218). In development, the sporangia arise 

 separately, each from a single initial. In Helminthostachys the fertile seg- 

 ment is spike-like and bears two rows of crowded, oval, stalked sporangia, 

 a number of which may be borne on a single stalk. The vascular anat- 

 omy of the leaf and the occurrence of occasional reversions indicate that 





Fig. 222. Cross section of the petiole of Botrychium virgiiiianum, X 10. 



the fertile segment of Ophioglossiim and Botrychium represents two united 

 basal leaflets, while in Hehninthostachys it represents a single leaflet. 



In almost all the Ophioglossales each leaf trace arises from the stele of 

 the stem as a single strand, but branches before or as it enters the leaf. A 

 cross section of the petiole of Ophioglossum shows a single row of vascular 

 bundles arranged in a circle, those on the adaxial side passing into the 

 "fertile spike" and the others going into the sterile blade. The petiole 

 of Botrychium, just below the fertile segment, usually displays two pairs 

 of vascular bundles arranged symmetrically on each side (Fig. 222). 

 From the two larger ones, which are crescent-shaped, a pair of smaller 

 bundles branch off and pass into the fertile segment, while at higher levels 

 two small bundles similarly depart to each pair of sterile leaflets. 



The sporangium wall, in the Ophioglossales, is about five layers of cells 

 thick. No annulus is formed. The tapetum is probably derived from 

 the innermost layer of wall tissue and may consist of one layer or several 

 layers of cells. In all three genera the tapetum is peculiar in that the 



