434 Farmer and Freeman . — On the Structure and 
form of tracheid marking was a wide-spread one amongst 
these ancient types of Vascular Cryptogams, whereas in 
another alliance, that of the Calamites, &c., it is conspicuous 
by its absence. 
The forms of the tracheids themselves are often highly 
irregular, recalling those figured by Bucherer in the rhizome of 
Dioscorea h Some of them may branch or fork, others exhibit 
nodular outgrowths. A few of the shapes commonly to be 
met with are sketched in Fig. 17 (PL XXII). The reason for the 
remarkable outlines thus exhibited seems to be most easily 
sought in the fact that the individual tracheids continue to 
grow long after the stem has ceased to elongate, and hence 
a considerable amount of sliding growth obtains. As will 
presently be seen, the differentiation af the procambial strand 
proceeds with unusual slowness, and consequently the last- 
formed tracheids have to accommodate themselves to the 
possibilities of room as best they may. One result of all this 
is that many of them pursue a very tortuous path ; and that 
hence, in a transverse section of the rhizome it is possible to 
meet with elements which have been cut almost longitudinally, 
and this is especially the case as one examines older parts 
of the stem. Sliding growth is doubtless facilitated by the 
apparently gelatinous or pectosic character of the middle 
lamella. The tracheids are grouped together in bundles, 
each mass being separated by parenchymatous tissue, which 
in some instances can be traced as a continuous band extend- 
ing from the cortex to the axile strand of parenchyma lying 
within the xylem ring. 
Passing inwards from the xylem, it is seen that there is no 
phloem on the inner surface of the wood, and this is also 
a feature which Helm in thostachys shares in common with 
other Ophioglossaceae. Even when the continuity of the 
vascular ring is interrupted by a foliar gap, the phloem stops 
short abruptly at its margins. A remarkable feature in this 
Fern is the occurrence of an internal endodermis which often 
abuts directly on the xylem-elements, though it is sometimes 
1 Bibliotheca Botanica, vol. iii. 
