722 G wynne- Vaughan . — Observations on the 
by reduction from a more complicated type. Vascular strands 
of exactly the same structure are also to be found in the upper 
regions of many different petioles, which lower down possess 
a more complicated vascular system, e. g. L ox soma Cunning - 
hamii 1 , Dicksonia apiifolia , Davallia ciliata and D. hirsuta. 
According to Bertrand and Cornaille these also may be 
regarded as reduced structures. 
It may be objected, however, that there is hardly any 
evidence that can be brought forward in support of a theory 
of reduction in any of these cases. There certainly does not 
seem to be any general reason why the vascular system at 
the top of the petiole should ever have been at any time 
in a more advanced condition than it is at present. It is clear 
that in any leaf the whole brunt of an increase in the leaf- 
surface must be felt in its entirety by the vascular system 
of the lower part of the petiole, and especially at or near 
its extreme base, where the water-current running directly 
upwards in the stem has to be diverted into the leaf. For 
this reason any advance towards greater efficiency in the 
water-conducting apparatus would probably make its first 
appearance in this region, and I have found, in many cases, 
that this actually does occur. On the other hand, the water- 
conducting apparatus that formerly supplied the whole leaf 
would still suffice for the amount of leaf- surface lying beyond 
it, provided that it was situated at a point sufficiently high up 
in the rachis. Apart from the disturbance due to the branch- 
ing, the vascular system would therefore experience less 
incentive to modify its form as you pass upwards towards the 
apex of the leaf, and on this account it would seem preferable 
to regard the simplicity of its structure in this region as due 
to the retention of primitive features rather than to re- 
duction. The same simple type of petiolar bundle is, more- 
over, characteristic of the earlier leaves of the young plants of 
Dicksonia apiifolia , D. antartica , and even of Cyathea excelsa , 
in which, especially in the latter, the mature petioles possess 
a very elaborate vascular system. 
1 Gwynne- Vaughan, 1 . c., p. 89, and Fig. 8. 
