216 



PLANT MORPHOLOGY 



developed. There is no cambium. An endophytic fungus is present in 

 the outer cortical region of the rhizome. 



Sporangium. The sporangia are borne singly in the axils of the upper 

 leaves, which are bifurcated in both genera, a sporangium arising at the 

 point of forking (Fig. 170). Each sporangium is situated at the end of a 

 short stalk. In Tmesipteris the large sporangium is divided transversely 



Fig. 172. Prothallium and sex organs of Psilotum nudum. A, surface view of an entire 

 prothallium of rather small size, bearing rhizoids, antheridia, and art-hegonia, X28; B, 

 cross section of prothallium, shomng two antheridia (one discharged), two archegonia, and 

 endophytic fungus, X38; C, a nearly mature antheridium, X 145; D, several sperms, X450; 

 E, a nearly mature archegonium, X 145. {After Lawson.) 



by a sterile plate into two separate chambers, so that two sporangia seem 

 to be present. In Psilotum the sporangium is three-chambered. The 

 wall consists of several layers of cells. No tapetum is present, but among 

 the spore mother cells are numerous sterile cells that are resorbed by the 

 developing spores. Both genera of the Psilotales are homosporous. 

 Dehiscence of the sporangium occurs by means of a longitudinal slit. 



The nature of the sporangium-bearing structures of the Psilotales is not 

 clearly understood. One interpretation is that the upper, forked leaves 

 are bifid sporophylls bearing solitary adaxial sporangia that are bilocular 

 in Tmesipteris and trilocular in Psilotum. Another view is that the whole 

 structure is a sporangiophore bearing two leaves and a terminal sporan- 



