4 36 Farmer and Freeman. — On the Structure and 
In the stems of the youngest plants which we were able 
to obtain it was seen that the axile strand of parenchyma 
which we may term the pith, is but scantily developed, xylem- 
elements extending into the centre. But nevertheless it is 
obvious (see Fig. 23, PI. XXIII) that a pith is beginning to form 
by the increased differentiation of parenchyma at the expense of 
the tracheidal tissue. It can hardly be doubted that in still 
younger plants the xylem would form a more or less solid 
axile core in the stele just as is the case with other Ferns. 
As regards the question of the existence of secondary 
thickening in the stem of ' Helminthostachys, we have come to 
the conclusion that it is quite absent. But owing to the slow 
differentiation of the procambial strand, already alluded to, 
it is easy to mistake the late differentiation of tissues which 
are really primary for a secondary formation. In spite of 
careful search we did not find any satisfactory evidence for 
anything like a secondary cambial division and subsequent 
new formation of fresh tissues, such as goes on in the stem of 
species of Botrychium , in which secondary tissue-production 
is a characteristic feature. There appears to be a slight 
cambial activity in Ophioglossum J , but it would seem to be 
quite wanting, at least normally, in Helminthostachys. 
Following the tissues of the rhizome to the apex, the 
vascular strand is seen to terminate immediately behind the 
somewhat sharply marked meristem. The actual apex is sunk 
in a depression formed by the over-arching of the youngest 
leaves on the dorsal and lateral edges, whilst on the ventral 
edge the tissues of the stem itself contribute to the same 
end. The cells which occupy the apical depression are all 
elongated at right angles to the surface, and it is extremely 
difficult to identify any one cell as the parent of the whole. 
But we succeeded in more than one instance in recognizing 
one cell as the probable common ancestor of the rest, though 
even in the most favourable cases it is rather difficult to be 
quite certain. The character of the protoplasm and nucleus 
affords a valuable clue, whilst the argument drawn from the 
1 Boodle, Anatomy of Ophioglosseae, p. 386 of the present number. 
