544 
Bower.—On the Primary Xylem, and the 
It becomes thus a question of interest to inquire what evidence there 
is of the existence of such vestigial remains of primary xylem in the genus, 
and what is its distribution in the different species. Frequently no such 
isolated tracheides exist in the pith of B. Lunaria. But where they do 
occur, they may be found to occupy positions which would be difficult to 
harmonize with a theory of intrusive origin of the whole pith. That is par¬ 
ticularly the case in the section represented in PI. XLV, Fig. 12. This shows 
in a weak plant the separation of a leaf-trace from the xylem-ring. The 
gap has opened, and parenchymatous continuity is established between the 
central pith and the outer conjunctive tissues. If the connecting tract were 
intrusive tissue of cortical origin, it should contain no tracheides. But here 
a tracheide is found occupying a central position in the connecting tract. 
A similar difficulty arises in the case of Fig. 10, where a great part of the 
medulla at the level of the fourth leaf takes the character of a mixed pith 
with scattered tracheides, though lower down it was purely parenchymatous. 
Passing to B. virginianum , which has already been quoted as having 
virtually no primary xylem in the seedling, this condition is usually 
maintained in the older axis. But occasionally isolated tracheides occur. 
One of these is shown by Professor Jeffrey in his PI. IV, Fig. 6 2, while 
Atkinson has also delineated two isolated tracheides in the pith of the 
same species (‘Biology of Ferns,’ Fig. 152), which can only be held as 
representing vestigial remains of a primary xylem. In my own sections, 
which were all from young plants, such a state is only occasionally met 
with in B. virginianum, but an example is shown in Fig. 13. 
Very interesting conditions are seen, however, in B. ternatum. I have 
not had the opportunity of examining young sporelings of this species, but 
by cutting the base of the stock of fairly advanced plants collected in 
Jamaica, I was able to obtain results which show that the species retains 
the vestigial xylem more firmly than B. virginianum. Examples are seen 
in Figs. 14-16 of isolated tracheides, or groups of tracheides, lying in the 
pith: these, as they cannot be referred to a cambial origin, can only 
represent the vestigial primary wood. But since B. ternatum is thus seen 
to be more prone to such vestigial characters in its xylem, it was a matter 
of special interest to observe in one specimen of that species the con¬ 
sequences of an injury to the stock, brought about probably by the attack 
of some insect, or it may have been by some mechanical injury. Professor 
Jeffrey has lately brought forward evidence in support of the view that 
traumatic changes are apt to be reversionary, the tissues affected by the 
injury showing ancestral characters. This gives additional interest to the 
study of this case in B. ternatum 4 
The stock was cut into serial sections. The structure at the base of 
1 Wound Reactions of Brachypliyllum , Ann. of Bot., vol. xx, p. 383 ; Traumatic Ray-Tracheids 
Cnnninghamia sinensis, Ann. of Bot., vol. xxii, p. 593. 
