Determining Factors in Petiolar Structure. 287 
that the leaves of a branch from which the outer wood had been 
removed for a short distance withered no sooner than those on other 
branches. Also Darwin and Phillips (loc. cit., p. 339) found that a 
single cut beyond the centre of a stem produced no very great 
diminution in the rate of absorbtion. In experiment 5 (p. 341) a 
diminution of the area of the conducting surface by more than one 
half only produced a diminution in the absorbtion of about 6%. 
It is clear then that the principles that obtain in water mains 
hold good for the translocation of water in plants and that local 
variations where they only extend over a short distance do not 
appreciably affect the final result. 
As Dixon has pointed out the water column in a plant is 
continuous and therefore the presence of numerous walls, though 
introducing a resistance factor does not invalidate comparison with 
a single water channel. Approaching the problem from another 
standpoint leaves from two plants of Stachys sylvatica, one of which 
was a shade-form and the other a sun-form, were compared in order 
to see whether the different capabilities of transpiration were 
reflected in the xylem development, for as Wiesner 1 shewed, under 
the same conditions shade-leaves transpire more than sun-leaves* 
The leaves were taken from different nodes so that very close 
relations between the ratios are not to be expected. As the 
material was obtained from the natural habitat the areas of the 
leaves, not the peripheries, have been used in calculating the ratios. 
Sun 
Leaves. 
Shade 
Leaves. 
Area of Leaf. 
Area of Xylem. 
Ratio i 
X 
Leaf a 
54*41 sq. cms. 
•03408 sq. mm. 
15-9 
Leaf b 
48-06 ,, ,, 
•03271 „ „ 
14-6 
Leaf c 
47-72 ,, „ 
•03207 „ „ 
14-91 
Leaf d 
39-53 ,, ,, 
•0293 ,, , 
13-4 
Leaf e 
36-62 ,, ,, 
•0275 ,, ,, 
13-31 
Leaf / 
28-97 ,, ,, 
■0228 „ „ 
12-7 
Leaf g 
25-63 „ ,, 
•0226 „ „ 
11-3 
Leaf h 
22-85 „ „ 
•0222 „ „ 
10-2 
Leaf i 
(very young) 
11-32 „ ,, 
•0183 „ ,, 
61 
Comparing leaves of similar dimensions it will be seen that the 
ratios given by leaves b and c, and d and e are very close which fact 
points to the different potentialities for transpiration between sun- 
and shade-leaves being such that under their natural conditions of 
growth the differences in humidity of the habitats are almost exactly 
equalised. Comparing the sun-leaves /, g, and h, it is seen that 
1 “ Photometrische Untersuchungen auf Pflanzenphysiologischerj 
Gebiete.” Sitz.-ber. d. k.k. Akad., Wien, 1893, 
