Determining Factors in Petiolar Structure. 285 
Now a small ratio signifies that the value of * is large, or in 
other words, there is more xylem for each unit of leaf effectiveness, 
a condition we should expect to find where the rate of evaporation 
is fast. If the ratio be large as with Bignonia then less xylem 
corresponds to the same unit. 
Now of the species in question the Clematis was a large smooth 
leaved mesophytic plant, whilst the Bignonia had leaves of small area 
and possessed an effective transpiration check in the form of copious 
hairs. We thus see that there is also indicated a correspondence 
in xylem development according as the species transpires, rapidly 
or slowly. 
In fairness it should be said that attempts have been made to 
estimate the transpiration of individual leaves so that direct 
comparison with the xylem development could be made, but up to 
the present the results obtained have been worthless as the ratio 
between the transpirations of the different leaves, although the 
experiments were prolonged over several days, did not remain 
constant. If however such a definite relation does not exist, the 
results here obtained are inexplicably constant. 
With regard to the failure to obtain direct results it must be 
borne in mind that any method of ascertaining the transpiration of 
a leaf after removal from the plant is open to the same objections 
as those raised by Dixon 1 for separated branches. Further, such 
methods give us the transpiration for the conditions under which 
the experiment is conducted, which are the same for all the leaves. 
But it is doubtful whether, when the leaves are upon the plant, the 
average maximal conditions for transpiration are the same for any 
two leaves. So that it is conceivable that whilst in nature two 
leaves of similar size may transpire at very different rates, under 
experimental conditions the relation between them becomes either 
lessened or even in extreme cases inverted. 
The experiments of Dixon (Proc. Roy. Soc., Lond., B, 1907), 
Ewart (Phil. Trans. Roy. Soc., Lond., B, 1905) and others upon the 
resistance offered to the passage of water through wood, ranging 
from a head just over one, to as much as thirty-three times the 
length of the stem, have shewn how great is the influence of the 
rate of flow upon the resistance. Where the flow is normally rapid 
this increased resistance is neutralised by an increase in the size of 
the vessels. The large size of the lumina of the vessels constituting 
1 “ Transpiration and the Ascent of Sap.” Progressus rei Botanicse, 
Band 3, Heft 1, 1909, p. 59. 
