37 
Anatomy of the Ophioglossaceae. Ill\ 
same essential type of primary construction they present. In Helmintho - 
stachys the inner xylem is better represented than in any other existing 
medullated plant. Indications of inner xylem are found in Botrychium , 
and a mixed pith may occur as a result of traumatic disturbance. In 
Ophioglossum tracheides may be developed throughout the pith in branching 
rhizomes, while strands of tracheides occur normally in the pith of some 
specimens of Ophioglossum pendulum} 
In attempting to get a deeper insight into this distinction of inner and 
outer xylem, it must be borne in mind that the descriptive statement that 
a leaf-trace ‘ departs 5 from a stele is metaphorical. The vascular system 
has no such individuality or power of branching. In the light of develop¬ 
ment it must be regarded as laid down along certain tracts, determined 
partly by the meristematic construction, and partly by influences proceeding 
backwards from the growing points of the main axis, or of lateral leaves or 
branches. While our knowledge of the necessary facts is too imperfect to 
make any inference more than tentative, it is suggested that the central 
region of a stele may be directed from the main apex, while the peripheral 
region is largely influenced from the developing leaves. In support of such 
a conception of the vascular structure, it may be pointed out that the study 
of the Ophioglossaceae has shown clearly the reality of such backward 
influences along more or less predetermined tracts, in the case of the 
vascular connexions of the branches. This was evident in the study of 
the branches of Botrychium lunaria , and has been shown above for 
Helmintho stachys. In the latter plant we see further that such an influence 
may extend to the main stele, and modify its structure all round and for 
a considerable distance; the extensive development of accessory or secondary 
xylem in the second branching specimen was clearly due to the influence of 
a developing branch upon the mature primary structure of the stele. 
In the preceding considerations, the stem with its stele has been 
regarded as the unit. It is possible, however, to regard the rhizome of 
Helmintho stachys as exhibiting a segmental construction from three rows 
of segments, two dorso-lateral and one ventral. The distinction of the 
dorso-lateral segments is pretty clear, since each bears a leaf and its 
related vestigial bud ; the distinction of the ventral segments which do not 
bear leaves, and on which the roots are borne, is less evident. Such a con¬ 
struction differs in the existence of a ventral region not having leaves, from 
the radial construction exhibited by Botrychium , where each segment bears 
a leaf and a bud. The difference here shown between these dorsiventral 
and radial types of rhizome is of course present in Ferns at large, but this 
preliminary consideration may be confined to the Ophioglossaceae. 
The radial and strictly dorsiventral types of shoot, with the general 
appearance of a segmental construction, as seen in Botrychium and 
1 Petry : Bot. Gazette, vol. lvii, p. 183, Figs. 12, 13. 
