PTEROPSIDA 139 



The prothallus in all three genera is mycorrhizal. Indeed, 

 the presence of the appropriate fungus is essential for the 

 growth of the prothallus beyond the first few cell divisions. 

 In most cases the prothallus is deeply buried in the soil and 

 lacks chlorophyll, but cases have been reported of super- 

 ficial prothalli, in which some chlorophyll was present. 

 Some have abundant rhizoids, but others are completely 

 without them. 



The prothallus of Botrychium virginianum (Fig. 20D) is a 

 flattened tuberous body, up to 2 cm long. Antheridia appear 

 first and are deeply sunken. Large numbers of antherozoids 

 are liberated from each and escape by the rupturing of a 

 single opercular cell. The archegonium has a projecting neck 

 several cells long, a neck canal cell with two nuclei, and a 

 ventral canal cell (Fig. 20E). 



The prothallus of Ophioglossum vulgatum differs in being 

 cyhndrical, and may be as much as 6 cm long (Fig. 20J). 

 Frequently, there is an enlarged bulbous base, in which the 

 bulk of the mycorrhizal fungus is located. (In both Figs. 

 20D and 20 J, the extent of the fungus is indicated by a broken 

 line.) As in Botrychium, the antheridia are sunken and pro- 

 duce very large numbers of antherozoids. Unlike Botrychium, 

 however, its archegonia are sunken too. In Fig. 20K, the 

 archegonium is illustrated at a stage just before maturity, 

 when there are visible two nuclei in the neck canal cell, but 

 just before the basal cell has divided. Indeed, a ventral canal 

 cell has rarely been seen, presumably because it disintegrates 

 almost as soon as it is formed. 



As in the Marattiales, the first division of the zygote is in a 

 plane at right angles to the archegonial axis. In Helmintho- 

 stachys, the outer (epibasal) hemisphere undergoes a second 

 division, so as to produce a suspensor of two cells, while 

 the hypobasal hemisphere gives rise to a foot, a root and, 

 later, the stem apex. The embryo is thus endoscopic, but 

 during its further development its axis becomes bent round 

 through two right angles, so as to allow the stem to grow 



