Botryopteris antiqua , Kidston. 
1051 
toxylem groups, in which case B. antiqua may be said to have either 
a monarch or diarch, while B. ramosa and B. hirsuta have a diarch or 
triarch trace. 
Since this is the case the trend of evolution in the genus Botryopteris 
would seem to be towards a multiplication of the protoxylems rather than 
towards a reduction. If we accept the monarch type of leaf-trace as the 
older, and regard the diarch type as derived from it, we may explain the 
diarch type as due to arrest of branching at a primary phase. Such a view 
could be extended to the derivation of the later triarch forms, which may 
well bear the same relation to the diarch forms as the latter to the monarch. 
The series would then be continuous, and is represented diagrammatically 
in Text-fig. 2. In this figure the gradual protrusion of the protoxylem is 
simultaneously indicated. 
Text-fig. 2, a, b , c, d } e. A series of diagrams of transverse sections of petiole bundles or 
leaf-traces of Botryopteris (vide text), a = the mesarch form of the monarch trace which is found 
while the trace is still only partially separated from the stem stele (cf. Fig. 2) in Botryopteris 
antiqua. b = the very common monarch petiole bundle of B. antiqua. c = the diarch trace of 
B. antiqua. d = the branching diarch petiole bundle of B. antiqua (cf. Fig. 24). e — the 
unbranched petiole bundle of B. ramosa. The position of the protoxylem is indicated by hatching, 
and is shown to be mesarch at one end of the series and endarch at the other. 
In support of this view we may add the facts (a) that diarch petioles 
are known fairly commonly among the later triarch forms, and ( b ) that with 
the exception of the slightly mesarch position of the protoxylems (Fig. 24), 
it would not be easy to distinguish, as already stated, a branching diarch 
petiole of B. antiqua from a non-branching triarch petiole of B. ramosa. 
VII. The Root-bearing Zones of the Stem Stele. 
Sections of stems bearing numerous roots occur both in transverse and 
longitudinal planes. This part of the stem is covered with delicate 
