140 THE MORPHOLOGY OF PTERIDOPHYTES 



vertically Upwards. The embryo of some species ofBotrychium 

 is likewise endoscopic and has a small suspensor (Fig. 20F), 

 but in others including B. lunaria, there is no suspensor and 

 the embryo is exoscopic; and this is true of all species of 

 Ophioglossum (Fig. 20L). In all cases there is considerable 

 delay in the formation of the stem apex, and in some species 

 it may be several years before the first leaf appears above 

 the ground, by which time many roots may have been 

 formed. These long delays suggest that the mycorrhizal 

 association is an important factor in relation to the nutrition, 

 not only of the prothallus, but also of the young sporophyte. 



Chromosome counts show a surprising range within the 

 group, for Botrychium has a haploid number n = 45, Helmin- 

 thostachys n = 46 or 47, while in Ophioglossum vulgatum 

 n = 250-260 and in Ophioglossum reticulatum n = 63i + io 

 fragments. 



Despite these divergent chromosome numbers, there can 

 be Httle doubt that the three genera of the Ophioglossales 

 are fairly closely related, nor that they represent an ancient 

 and primitive group of ferns despite the lack of fossil 

 representatives. The reticulate venation of Ophioglossum, 

 its consohdated fertile spike and its complete lack of a 

 suspensor together suggest that it has reached a more 

 advanced stage of evolution than either of the other two 

 genera. As in the Marattiales, it seems that the upright 

 stem is the basic condition, since even in Helminthostachys 

 the young plant has an erect axis. 



Regarding the relationships between the Ophioglossales 

 and the Marattiales, it is not easy to decide which characters 

 are significant. Of the many characters common to the two 

 groups, most indicate merely that they have reached roughly 

 the same stage of evolution, rather than that they are closely 

 related. These may be briefly listed as i. basically erect axis, 

 2. stipules at the base of the petiole, 3. absence of scleren- 

 chyma, 4. sporadic endodermis, 5. massive sporangium wall, 

 with stomata, the sporangia showing a tendency to fusion, 



