On the Primary Xylem, and the Origin of Medullation 
in the Ophioglossaceae. 
BY 
F. O. BOWER, Sc. I)., F.R.S. 
Regius Professor of Botany in the University of Glasgow, 
With Plates XLV and XLVI. 
I 'yOTR YCHIUM is the only genus of living Ophioglossaceae which 
fjk shows secondary thickening of its axial stele. The greater part of 
the vascular tissue of the axis owes its origin to the cambium, so much so 
that in some cases 4 the wood is practically all secondary, as may be learned 
from the radial arrangement of its matured elements V This prevalence of 
the secondary tissues has diverted attention from the primary tissues, 
which are apt to be ignored in the descriptions which are given of the 
structure of the axis. But for purposes of comparison with other genera 
of the family, and with other Pteridophytes, it is desirable that the primary 
tissues should be recognized, so far as they are present in Boirychium . It 
is the object of this paper to bring them more clearly forward than has 
hitherto been done. 
In the mature axis of all the genera of this family there is a central 
pith, and the xylem as seen in transverse sections forms a more or less 
interrupted ring surrounding it. The interruptions are caused by the 
departure of the leaf-traces, each consisting of a single strand, excepting in 
the more specialized species of Ophioglossum . 2 In the pith an endodermal 
structure has been observed, but not with any high degree of constancy. 
This internal endodermis was first described by Van Tieghem , 3 in young 
plants of Ophioglossum vulgatum : it was also seen in young plants of 
Botrychium Lunaria, and its origin traced by involution of the outer endo¬ 
dermis at the leaf-gap, the foliar strand being there detached with its 
special pericycle and endodermis. Van Tieghem described 4 how 4 the 
endodermis of the central cylinder, thus open, folds its two margins inwards, 
where they extend and rejoin around the pith, which is thus incorporated 
1 Jeffrey, Gametophyte of Botrychium virginianum, 1898, p. 22. 
2 Bower, Annals of Botany, vol. xviii, p. 215, and vol. xxv, p. 278. 
3 Journal de Botanique, 1890, p. 405. 1 Loc. cit. p. 407. 
[Annals of Botany, Vol. XXV. No. XCIX. July, ign.] 
