7 8 Sykes . — The Anatomy and Morphology of Tmesipteris . 
leaf-traces can take their origin from the centripetal xylem in the stem, and 
do not require any especial provision for their formation. In Psilotum 
there are no leaf-traces, and in Isoetes and the Selaginellas the stelar 
structure is essentially aberrant. But in Tmesipteris we have a plant of 
which the leaves are supplied by traces arising from a stele comprising 
both centripetal and centrifugal wood, and the latter alone, in addition to 
the protoxylem, is concerned in the formation of the leaf-traces. The 
presence of mesarch xylem in the large-leaved cone of Cheirostrobus } 
which belongs probably to a group in which exarch structure is prevalent, 
also suggests the correlation of mesarch protoxylem with the formation of 
large leaves. 
V. Comparison with Psilotum and Sphenophyllum : General 
Morphology. 
Tmesipteris and Psilotum are the only two living members of the 
Psilotaceae. The two plants are in many respects similar in structure, but 
Psilotum 2 is simpler than Tmesipteris . Their rhizomes agree both in 
appearance and in detailed anatomy, but no bulbils such as those 
described in Psilotum 3 appear to be present in Tmesipteris. The 
difference in the structure of the stems can be easily explained on the 
theory of reduction in Psilotum. The leaves of the latter plant are 
extremely small and have no vascular supply, so that the greater part of 
the function of assimilation is performed by the stem. The stele of the 
stem has a ring of xylem with three to five projecting groups of exarch 
protoxylem. Inside the xylem is a lignified pith, and outside is a ring of 
phloem. An endodermis is present in both rhizome and aerial stem, and 
in the aerial stem the phloem is slightly lignified. A rudimentary kind of 
secondary thickening has been described in the transition region of 
Psilotum l and one of the chief reasons for the investigation of Tmesipteris 
was the desire to see if anything of a corresponding nature was present in 
this plant. Nothing of the kind was, however, found in any of the specimens 
examined. 
Boodle 4 supposes that the common parent of the Psilotaceae had an 
aerial stem with a rayed mesarch xylem mass, and possessed also the 
power of secondary thickening. The anatomical resemblance between 
such a plant as this hypothetical ancestor and Sphenophyllum is obvious. 
He suggests that the suppression of the leaf-traces has caused the loss of 
centrifugal xylem in the one genus, while the influence of the large leaf- 
traces in the other has broken up the xylem into masses. 
It appears also possible that the secondary absence of large leaf-traces 
in Psilotum has caused the fusion of originally separate masses of xylem 
1 Scott, 1897. 2 Ford, 1904. 
3 Solms-Laubach, 1884. 4 Boodle, 1904. 
