Determining Factors in Petiolar Structure. 2^9 
weight added to that of the receptacle. The results obtained shew 
that a considerable degree of relationship exists between area and 
breaking strain and therefore presumably resistance to bending. 
Species. 
Area of Leaf. 
Breaking 
Strain. 
Polygonum cuspidatum j 
Plant A 
180 cms. 
133 ozs. 
168 ,, 
116 ,, 
94 „ 
98 ,, 
58 ,, 
67 ,, 
Plant B 
164 ,, 
125 ,, 
144 ,, 
119 ,, 
70 ,, 
72 ,, 
66 ,, 
58 ,, 
Aucuba A 
103 ,, 
121 ,, 
70 ,, 
103 ,, 
49 ,, 
80 ,, 
Aucuba B 
79 „ 
84 ,, 
54 ,, 
68 ,, 
Heliantlius 
181 ,, 
176 ,, 
104 ,, 
83 ,, 
11 
52 ,, 
53 ,, 
Many observers have called attention to the utility of the open 
arc arrangement of the vascular bundles in maintaining the rigidity 
of the petiole in unilateral leaves, and this is still further emphasised 
by the flattening out which the arc shews in the adpressed radicle 
leaves of many species. Also in leaves such as Hydrocotyle , 
Tropceolum, Oxalis , Pterospermum acerifolium, Hernandia ovigem , 
etc., where there is either a complete ring of xylem or the bundles 
are symmetrically arranged, we see a definite relation to the almost 
equally distributed strain. A similar phenomenon is exhibited by 
many pseudo-peltate leaves, e.g„ Geranium spp., Malva rotundi- 
folia, etc., where although the leaf is morphologically unilateral, 
mechanically it is radially symmetrical. 
From these preliminary observations we can only conclude that 
the functions of the leaf have a profound influence both upon the 
amount and arrangement of the xylem. So that whether we be 
concerned with the petiolar structure of the mature organism or 
with that of the cotyledons, it is to function, rather than to phylogeny, 
that we must look for its elucidation. In so far as the vascular 
system of the stem is itself built up of leaf-traces the same 
generalisation will apply with equal force. Even in those cases 
where a special cauline vascular organisation is present the 
same regulative factors may well have determined the limits of 
development. 
