PTERIDOPHYTA 217 



gium ; that the sporangiophore is not a lateral branch but a reduced mem- 

 ber of a dichotomous branch of the main stem. The existing evidence 

 favors this second interpretation. It is also not clear whether the sporan- 

 gium should be regarded as a single sporangium or a synangium, which is a 

 group of united sporangia. In early developmental stages each mass of 

 sporogenous tissue has an independent origin, but it is luicertain whether 

 or not the partitions represent sterile sporogenous tissue. 



Gametophyte. The prothallia of both Psilotum and Tmesipferis are 

 tuberous, subterranean, saprophytic bodies without any chlorophyll 

 (Fig. 172 A, B). They are light brown, cylindrical, simple at first but 

 later branched, and up to 20 mm. in length. Long rhizoids uniformly 

 cover the surface. An endophytic fungus is present. In Psilotum, but 

 not in Tmesipteris, the largest prothallia sometimes possess a strand of 

 conducting tissue consisting of a few tracheids. Both kinds of sex organs 

 occur in abundance on the same prothallium. The antheridia are glob- 

 ular and projecting, each producing many multiciliate sperms (Fig. 172C, 

 D). The antheridium initial, which is superficial, undergoes a periclinal 

 division, the outer cell giving rise to the sterile jacket and the inner cell to 

 the spermatogenous tissue. The archegonia are sunken in the prothal- 

 lium, but the neck protrudes (Fig. 172E). Apparently two neck canal 

 cells are present. 



Embryo. The embryos of Psilotum and Tmesipteris are very similar. 

 They are peculiar in that no suspensor, root, or leaf is present. The fer- 

 tilized egg divides transversely, the outer cell giving rise to the stem and 

 the inner cell to the foot. In Tmesipteris a second stem tip often appears 

 near the base of the first one; later both may grow erect and produce 

 leaves. 



Summary. The stem of Psilotum is elongated and branched, the 

 leaves scale-like and relatively few. The stem of Tmesipteris is also elon- 

 gated but generally unbranched; the leaves are small and numerous. 

 Roots are absent in both genera. The stem is differentiated into an 

 underground and an aerial portion. The rhizome is an exarch protostele. 

 The aerial stem, which is medullated, is exarch in Psilotum and mesarch in 

 Tmesipteris. The arrangement of xylem and phloem is amphicribral. 

 There is no cambial activity. The sporangia are borne singly in the axils 

 of the upper leaves, each at the end of a short sporangiophore. There is 

 no definitely organized strobilus. The sporangium is either bilocular 

 (Tmesipteris) or triloeular (Psilotum), homosporous, and longitudinally 

 dehiscent. A tapetum is not organized. The prothallia are subterra- 

 nean and tuberous. The antheridia produce many multiciliate sperms. 

 The archegonia have two neck canal cells. The embryo is without a sus- 

 pensor. The Psilotales form an isolated order not closely related to other 

 living pteridophytes. 



