Ophioglossales. 267 
(adaxial) side of the petiolar circle, and also from the large central 
strand whose origin has been described. The venation of the frond 
is dichotomous, and taken in conjunction with the repeated 
dichotomy of the leaf-trace strands, and with the partially dicho¬ 
tomous branching of the frond, suggests a primitive feature. 
Fig. 94. Hebniuthostuchys. Eight bundles in four pairs near base of 
petiole. The ventral (adaxial) side of the petiole comes between the bottom 
and left-hand pair, these pairs representing the free ends of the petiolar curve. 
The vascular structure of the genus Botrychium , which contains 
several species scattered over the world, resembles that of Helmin- 
thostachys in a quite general way. Thus the stem contains a hollow 
cylinder of xylem, with a parenchymatous pith, and with external 
phloem, pericycle and endodermis. In details there are great 
differences. The xylem is endarch; the metaxylem tracheids, 
which have highly developed bordered pits, are radially seriated, and 
these series are interspersed with medullary rays one cell thick. 
The vascular cylinder is added to by a cambium just like that of a 
dicotyledon, except that no new elements are added to the phloem. 
In the stem of the young plant there is an internal endodermis 
surrounding the pith ; this dies out when traced upwards. 
The leaf-trace again consists of a single strand which forks at 
the base of the petiole, and eventually a circle of strands is formed 
which are concentrically organised, but with a tendency, in some 
species, to collateral structure. The supply of the fertile spike 
appears to take place as in Helniinthostachys, but here no central 
bundle has been found in the petiole. The venation is like that of 
Hehninthostachys. 
