Anatomy of the Ophioglossaceae . /. 219 
as Phots. 1, 2, 16, 17, while Phot. 3 is less highly magnified. It will be 
evident that the diameter of the stele has greatly increased, and also that 
the zone of xylem is much thicker. Next the pith comes a continuous zone 
of primary xylem, which is best seen in Phot. 4, and outside this the 
secondary xylem. The two facts that the secondary tracheides are in 
regular radial rows, and that the medullary rays are confined to the 
secondary wood, and therefore do not extend to the pith, enable the 
distinction between primary and secondary xylem to be readily made 
in B. Lunar ia. The secondary tracheides are wider and rather shorter than 
those of the primary xylem. This is seen in PL XXI, Phot. 22, which is, 
however, selected to show another feature of the stele. 
While primary and secondary xylem can thus be distinguished in 
B. Lunar ia some qualification is necessary as to the sense in which the term 
secondary xylem is used. The succession indicated by the steles in Phots. 1, 
2, and 3 is that found in the extended ontogeny of the plant. At the level 
of each section the stele was complete, and under normal conditions of 
growth no additions would be made to the xylem. Even in the case of the 
large stele of the adult region, with its well-marked zone of secondary 
xylem, the secondary thickening is not progressive. The tracheides of the 
primary xylem are the first to be lignified behind the apex, but the tangen- 
tial divisions in the procambial cells outside the primary xylem are in main 
part completed close to the apex. What happens in the region somewhat 
further back is the maturing and lignification of the elements thus arranged 
in radial rows and forming the secondary xylem. The secondary thicken- 
ing in this case thus appears to be an arrangement to increase the amount 
of xylem in the primary construction of the stem, and does not proceed 
indefinitely. The justification of the comparison of the secondary thicken- 
ing in B. Lunar ia with that of the higher plants is, however, clearly shown 
by the power of the meristem to resume the active addition of new elements 
to the xylem under special circumstances. A good example of this will be 
described below in one of the branching plants (Text-fig. 14). 
The fact that in a considerable region of the rhizome all the xylem 
actually formed may be primary explains the divergent statements found in 
the literature regarding Botrychium Lunaria , since, according to*the region of 
the rhizome examined, there may appear to be marked secondary thicken- 
ing or none. On account of the importance of the memoir in which 
it appears, it seems advisable to specifically correct the recent statement by 
Campbell 1 , that ‘ in Eubotrychium the cambium is absent, and apparently 
there is no secondary increase in diameter 5 . 
The further question as to whether, in addition to the centrifugal 
primary xylem distinguished above, any representative of the well-marked 
centrifugal xylem found in HelmintJiostachys can be recognized in B.L2inaria , 
1 The Eusporangiatae, p. 102. 
