420 SPHENOPHYLLALES. B. PSILOTACEAE 
coherent ring surrounding a central pith, but with the protoxylem 
mesarch. 
The chief anatomical difference between the two genera appears thus 
to be in the position of the protoxylem. But Boodle points out that 
locally a mesarch position may be found in Psz/oftum also, and he con- 
cludes that both genera might be referred to a common parent form, 
in which the aerial stem had a rayed mesarch xylem-mass, the suppression 
of leaf-traces having caused the loss of centrifugal wood in the one genus, 
and the influence of the leaf-traces in the other genus having broken up 
e Fic. 234. 
Tmesipteris tannensis, Transverse section of the sterile region, high up. The proto- 
xylem (fr. xy.) is mesarch. The xylem of the stele is fading out, and being replaced 
by parenchyma; three of the tracheides (z. ¢7.) show incomplete development; there 
is no longer a complete ring, and the leaf-trace bundles (7. ¢.) enter the gaps which result, 
in much the same way as in a phyllosiphonic type. There is no definite endodermis. 
X150. 
the xylem into distinct bundles! He further recalls the fact that in 
Cheirostrobus there are indications of a mesarch structure, while paren- 
chyma is present among the tracheides towards the centre of its stele: 
such cells in response to mechanical requirements might readily be 
converted into mechanical tissue, as in Psc/otum. It thus appears that 
the Sphenophylleae and Psilotaceae show uniformity in the general type 
of their vascular construction, though the details are subject to consider- 
able fluctuation. This result adds point to the comparisons already 
based upon the external characters and the spore-producing parts. At 
the same time, it is to be remembered that a structure resembling that 
of Psilotum and Cheirostrobus is seen in certain of the Lycopodiales ; in 
© Etsy pe 51S: 
