Origin of Medullation in the Ophioglossaceae . 545 
the plant was found to be normal, and the stock of moderate size. PL XLVI, 
Fig. 17, shows its condition, with a large pith, slightly compressed. But 
quite suddenly, owing no doubt to the damage of which signs are seen pene¬ 
trating deeply from the outside, the stele contracted to a much smaller size 
(Fig. 18), while the pith, which hitherto had been free from isolated tracheides, 
now showed many of them interspersed throughout the parenchyma. The 
central region of the stele is shown on a larger scale in Fig. 19, from which 
it is seen that tracheides lie at the very centre of the pith. Presently, as the 
damaged region was left behind, the stele again expanded, the pith enlarged 
and gradually became clear of tracheides. In fact a normal structure was 
resumed (Fig. 21), though of smaller size than that of the original stock. 
The interest of this case lies in the light it sheds on the relation of the 
primary wood of Botrychiam to the pith. If by traumatic conditions the 
plant, as Professor Jeffrey holds, is liable to reversionary changes, then, since 
in this special case the parenchymatous pith has many of its cells replaced 
by tracheides right through to its centre, that would appear to indicate that 
the pith, right through to its centre, was originally of xylem origin. 
The condition shown by the pith in this injured stock of Botrychinm 
ternatum may be compared with that of the centre of the stele in a small 
rhizome of Helminthostachys , which was described by Farmer and Freeman. 1 
The structure which is there shown follows naturally on that of the still 
younger stems represented by Lang. 2 A photograph of one of Professor 
Farmer’s sections is shown in Fig. 22. In this, as in the injured stock of 
B. ternatum , the centre of the stele is occupied by a tissue composed of 
tracheides and parenchymatous cells. It has in fact the character of what 
is known as a ‘ mixed pith ’. This condition is represented in greater or less 
degree in the young plants observed from the other genera of the Ophio¬ 
glossaceae. It is shown in the young protostelic plant of O . reticulatum in 
PL XLV, Fig. 2, and for Botrychinm Lunaria in Fig. 6. In each of these cases 
there is a tract of tissue where parenchyma and tracheides are intimately 
associated, though it extends only a short distance along the axis. But in 
Helminthostachys that condition may be continued for a long distance. 
Whether it be short or long, such a mixed condition exists as the progres¬ 
sion takes place from the solid protostele to the medullated state. There¬ 
after, by gradual increase in the proportion of the parenchyma cells, and 
a decrease leading to complete absence of the tracheides, a substantial part, 
or it may be the whole, of the central medulla is established. It appears 
that owing to injury Botrychinm ternatum has, in the case quoted, returned 
structurally to the phase of mixed pith commonly passed over rapidly in 
the individual life, but represented more effectively in young plants of 
Helminthostachys than in other members of the family. This interesting 
example of the principle of traumatic reversion, suggested by Professor 
1 Ann. of Bot., vol. xiii, p. 421, PI. XXIII, Fig. 23. 2 Ibid., vol. xvi, PI. Ill, Figs. 65-70. 
