286 
E. J. Salisbury. 
the spring as compared with the autumn wood is an accompaniment 
of the periodic change in the rapidity of the transpiration current, 
and a comparison of the vessels of mesophytic and xerophytic 
plants shews the same relation. Similarly in the development of 
the leaf itself the xylem of the immature condition is composed of 
tracheae which have a larger average lumen than those of the adult. 
A temporary excess over the average maximal transpiration 
provided for will of course be met by a corresponding closure of the 
stomatal mechanism, but if this condition continues as a permanent 
change the decreased rate of flow in the transpiration current 
(reacting perhaps upon the food-supply, Jost, loc. cit.) probably acts 
as a stimulus to the production, by the developing leaves, of an 
augmented conducting system. 
Sinnott 1 observes that “ it is a noteworthy fact that in very 
many ferns the leaf becomes slender at its attachment and that in 
almost all of them the vascular supply, besides being much less 
complicated at this point than higher up the petiole, is also 
composed of a much smaller number of cells.” He concludes 
therefore that it is “ extremely doubtful if the size of the transpi¬ 
ration current has had much influence on the development of the 
vascular supply.” 
The writer has confirmed the above generalisation for several 
species of ferns and it was found that in a Polypodium sp. the total 
cross-sectional area of the xylem in the middle region and in fact 
throughout the greater part of the petiole was about 0-026 sq. mm. 
At the base, however, the area was only 0-011 sq. mm. or a reduction 
of nearly 60%. In Clematis aristata a similar reduction was found 
at the extreme base of the petiole, the area throughout the greater 
part was -0385 sq. mm. and at the base 0-364 sq. mm. Also this 
divergence is to some extent increased by the larger size of the 
vessels in the upper part (-000447 sq. mm.) as compared with the 
lower (-000327 sq. mm.) 
In every case examined, however, the marked contraction is 
confined to a short distance at the base of the petiole. Mr. Ormsby, 
who has worked on the effect of contraction in water mains, has, 
however, shewn that where the length of contraction is small as 
compared with the total distance traversed the effect is negligible 
(see Middleton, “ Water Supply,” pp. 152-153). Hales (“Vegetable 
Staticks,” Ed. 2, 1731, p. 138, Exp. XLV) records the observation 
1 The Evolution of the Filicinean Leaf-Trace.” Ann. Bot.. Vol. 25, 1911, 
p. 187. 
