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 Psilotum 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 



FIG. 234. 



Tinesipteris tannensis. Transverse section of the sterile region, high up. The proto- 

 xylem {pr. -xy>) is mesarch. The xylem of the stele is fading out, and being replaced 

 by parenchyma ; three of the tracheides (/. tr.~) show incomplete development ; there 

 is no longer a complete ring, and the leaf-trace bundles (/. t.~) enter the gaps which result, 

 in much the same way as in a phyllosiphonic type. There is no definite endodermis. 

 Xi 50. 



the xylem into distinct bundles. 1 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 Psilotum. 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 



1 L.c., p. 515. 



