828 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
with 1,188 stomata (cf. Text-fig. 4). Many of the stomata on the upper 
leaves had even smaller pores than C. Moreover, abortive stomata, or what 
appear to be such (Text-fig. 6, d), with no actual pore at all, are very 
frequently found. These are much more common on the leaves with 
numerous stomata than on the lower leaves, and have been included in the 
countings given above. 
The question at once arises, so far as the stomata alone are concerned, 
will the transpiration from the numerous small stomata of the upper leaves 
be greater or less, per unit area of leaf surface, than that from the fewer but 
larger stomata of the lower leaves ? If very much greater, it would seem 
possible that any economy of water effected by the production of hairs and 
other xerophytic devices in the upper leaves might be rendered nugatory by 
the numerical increase of stomata. Appended below are rough calculations, 1 
1 Unfortunately, the only measurements available at the time of writing this paper are taken 
from the stomata of leaves fixed and cleared in the manner described above. As, however, the several 
leaves were treated in exactly the same way, it is probable that the respective sizes of the stomatal 
pores bear much the same relations to each other as when fully open in nature. Still, the measure- 
ments, &c., given must be taken as approximations only. 
Regarding the stomatal pores as ellipses, the areas of the pores shown in Text-fig. 6, B and c 
(i. e. from the second and eighteenth leaves), are 0.0000136 sq. mm. and 0*0000036 sq. mm. respec- 
tively. As the second leaf has 303 stomata per sq. mm. of leaf surface, the total area of the stomatal 
openings amounts to about 0*00412 sq. mm., i. e. 0*41 % of the whole lower surface of the leaf. In 
the eighteenth leaf, however, which has 1,188 stomata per sq. mm., a considerable number of the 
stomata are abortive. Moreover, the pores of many of the remainder are smaller than the one figured 
(see Text-fig. 6, C and d). It has been estimated (roughly) that the number of effective stomata on 
this leaf may amount to about 80 % of the whole. On this assumption, the total area of stomatal 
openings would be about 0.00342 sq. mm., or 0.34 % of the lower leaf surface ; that is, actually less 
than in leaf 2. 
For calculating the amount of water vapour which could be transpired through the stomata of 
the two leaves respectively, the formula given by Brown and Escombe (’ 00 , p. 276) for the absorption 
of C 0 2 may be used. It is assumed that the air in the intercellular spaces of the leaves is saturated 
with water vapour, and the free air outside the leaves two-thirds saturated. The measurements are 
calculated at 18 0 C. and 760 mm. The formula in question is : 
o = k p • A ' y ‘ 3 > 6o ° 
^ / + x 
where Q = the amount of water vapour (in c.c.) transpired per hour per sq. cm. of lower leaf- 
surface, and 
(1) in the case of leaf 2 : 
k — diffusivity of H 2 0 vapour at 18 0 C. in C.G.S. units = 0.248. 
p = difference of pressure of H 2 0 vapour inside and outside leaf, in atmospheres = 0-0067. 
A = area of pore of stoma = 1*36 x io -7 sq. cm. 
y — number of stomata per sq. cm. = 30,300. 
I — length of stomatic tube = o-ooio cm. 
x = f 7r x diameter of circle of same area as elliptical pore of stoma = 0-00016 cm. 
Hence, Q — 21.2 c.c. of water-vapour transpired per hour per sq. cm. of lower leaf surface. 
(2) For leaf 1 8, the differences will be : 
A — 3.6 x io -8 sq. cm. 
y (reckoning 80 % of the total number as effective) = 95,000. 
I = 0.000714 cm. 
x = 0-000084 cm. 
In this case, Q = 25.4 c.c. of water vapour transpired per hour per sq. cm. of leaf surface. 
It will be seen that there are several possible sources of error in such a calculation as the above. 
