PTERIDOPH YTA 215 



Sporophyte. The sporophyte consists of a rhizome bearing rhizoids 

 and giving rise to slender, green, aerial stems (Fig. 170^, E). A peculiar 

 feature is the absence of roots. The aerial stems of Psilotum branch 

 dichotomously and generally grow erect upon the ground, but are some- 

 times epiphytic and drooping. They reach a length of 20 to 100 cm. 

 The leaves are few, scale-like, and without veins. Tmesiptcris may grow 

 as an erect terrestrial plant but is generally epiphytic and pendulous, 



Fig. 171. Cross section of the central portion of the aerial stem of Psilotum nudum, X 100. 

 The protoxylem lies at the tips of the xylem rays, the phloem between them. The center 

 of the stele is occupied by a group of fibers. 



especially on tree ferns. The stem is unbranched, rarely showing a single 

 dichotomy. It reaches a length of 5 to 25 cm. The leaves are narrow, 

 12 to 18 mm. long, and have a single vein. In both genera the leaves are 

 more or less spirally arranged. The rhizome and aerial stem grow by 

 means of a tetrahedral apical cell (having the form of a triangular 

 pyramid). 



Vascular Anatomy. In both genera the rhizome is a protostele. In 

 the aerial stem the xylem forms a star-shaped mass enclosing a pith that, 

 in Psilotum, is occupied by a group of fibers (Fig. 171). The rhizome is 

 exarch in both genera, the first-formed xylem (protoxylem) lying outside 

 the later-formed xylem (metaxylem) . The aerial stem is exarch in Psilo- 

 tum but mesarch in Tmesipteris, in the latter the metaxylem surrounding 

 the protoxylem. The phloem, which lies outside the xylem, is poorly 



