5i5 
Secondary Xylem in P silo turn. 
Hencej though examination of young shoots showing a suitable stage 
of differentiation at this level is required, one may provisionally regard 
local mesarch structure as established for Psilotum , and the most natural 
conclusion is that the structure of its aerial stem has been reduced from 
the mesarch to the exarch type in connexion with the disappearance of the 
leaf-traces. Psilotum and Tmesipteris might then 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 the xylem into distinct bundles. If we make a com- 
parison once more with Cheirostrobus , it is interesting that in that plant 
there are indications of mesarch structure 1 (see Scott, ’97, p. 9, footnote, 
and PI. IV, Fig. 1), and that towards the centre of the stele a considerable 
amount of parenchyma is found separating the tracheides (Scott, 5 97, PL I, 
Phot. 3). This parenchyma might easily become converted into sclerotic 
tissue in response to mechanical requirements, the intervening tracheides 
becoming replaced by similar elements 2 * * . 
The material of Psilotum triquetrum , which was chiefly used for this 
investigation, was a plant which had lain in spirit for several years, and 
was probably grown at Kew, though its origin had not been recorded. 
For comparison fresh specimens of one or two old aerial shoots with 
adjacent subterranean parts were obtained from a plant grown at Kew, 
and these showed the same structural characters. 
Summary. 
The solid mass of tracheides described and figured by Bertrand in the 
subterranean parts of Psilotum triquetrum does not represent the whole 
of the xylem present in parts of the specimens examined, but additional 
tracheides are found outside it but internal to the ring of sieve-tubes. 
These tracheides are formed considerably later than those of the central 
mass, and show successive and somewhat irregular development. Certain 
of them are to be found still incompletely differentiated in quite old parts 
1 And that secondary xylem is present in the peduncle of the cone (Scott, ’97, p. 15, and 
PI. VI, Fig. 21). 
2 It may be mentioned here that in a large aerial branch of Psilotum , preparation for the first 
dichotomy by the appearance of one tracheide in the middle of the central sclerotic tissue of the stele 
occurred at four inches below the fork. At two and a half inches below the latter there were eight 
tracheides in this position, and the group then increased in size to form a bridge across the 
sclerenchyma, afterwards splitting for the dichotomy of the stele. This differs from Bertrand’s 
description (’81, p. 405 et seq. and Fig. 179) and may be exceptional. In Tmesipteris 
Bertrand (’81, p. 494 and Fig. 215) mentions that occasionally certain of the xylem-strands are 
interior with regard to the rest, and figures a small strand of five tracheides placed roughly at the 
centre of the pith, which is surrounded by a ring of six xylem-groups. It is possible that this central 
group may have some such connexion with the branching of the stem, as in the case just referred to 
in Psilotum. 
N n 
