514 Boodle. — On the Occurrence of 
A short comparison of the structure of P silo turn and Tmesipteris is 
called for here. No secondary growth is known in Tmesipteris. It is 
possible that it may yet be found in old parts of luxuriant plants, or on 
the other hand it may have been quite lost by reduction. In Tmesipteris 
the lower part of the stem, which is covered with rhizoids, may be called 
rhizome ; the structure exhibits much the same type as the primary 
structure of the same organ in Psilotum. The xylem of the stele consists 
of a solid group of tracheides, either with no distinct protoxylem in the 
case of small steles, or exarch and diarch in larger ones k At the base 
of the ‘ aerial ’ stem of Tmesipteris the stele has a solid triangular group of 
xylem with three protoxylem-groups (see Dangeard, ’ 91 , PL XII, Fig. 10), 
and agrees with that of Psilotum (in the same region), except that the 
xylem is mesarch in Tmesipteris. 
In the aerial stem also Tmesipteris has mesarch structure. A less 
important point of difference is that the xylem takes the form of a number 
of separate strands surrounding a sclerotic pith. Such a structure might 
easily be attained in Psilotum by the suppression of the later-formed part 
of the metaxylem, and this had actually taken place by arrest in the upper 
part of the aerial branch e (Fig. 44 in the text), there being five separate 
groups of xylem, each with its protoxylem. 
The other distinction between the two genera, viz. the absence of 
mesarch structure in Psilotum , is apparently not absolute, though further 
investigation is desirable on this subject. Cases of apparent mesarch 
structure were certainly met with locally in the lower part of the aerial 
stem 1 2 , a small number of scalariform tracheides being found on the outer 
side of one or more of the protoxylem-groups. The difficulty here is to 
prove that these elements are not secondary, and the material was not 
quite sufficient for the purpose. It may be noted, however, that these 
elements were mostly just opposite the protoxylem-groups, usually in 
contact with them (i. e. without the intervention of a layer of parenchyma), 
and that none of them were found in an immature condition. It is also 
important that, internal to the peripheral scalariform element of one of 
these apparently mesarch groups, there may be one or more tracheides 
with rather dense spiral thickening before one reaches the smallest tracheide 
characterized by a loose spiral, and even a form of thickening transitional 
between scalariform and spiral may be met with between the tracheides 
with these two types of thickening. Thus if one disregards the scalariform 
tracheides (as being possibly secondary) the evidently primary elements 
outside the protoxylem indicate mesarch differentiation. 
1 Bertrand (’81), Fig. 208 on p. 482, Dangeard (’91), PI. IX, Fig. 4 and PI. XII, Fig. 7, for 
diarch structure. Dangeard (’91, p. 206) states that in the ramifications of the rhizome 4 la stele 
binaire peut perdre son caractere de determination. 5 
2 In the region where five or more xylem-rays are present. 
