826 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
ing the numbers of stomata per unit area, it was at once apparent that an 
increase occurred in successive leaves. Unfortunately, while the numbers 
on the glabrous leaves are readily ascertainable, the dense pubescence of the 
majority of leaves renders accurate counting impossible. Many methods of 
surmounting this difficulty were tried, but none proved really successful. 1 
Finally, the var. denudata was used, though this is also hairy to a certain 
extent. It is from this plant that the only complete series of counts were 
obtained. A few actual examples may be given. 
SHOOT A 2 (Type form). A ‘shade’ plant, i. e. grown in ordinary 
garden soil, under currant bushes, and sheltered by a high wall on the south 
side. The shoot produced a total of 24 leaves. Of these, the first two were 
quite glabrous. A few extremely short hairs appeared on the main veins 
of the third leaf. The hairs gradually increased in number and length until 
the eighth leaf, when it was difficult to count the stomata ; above this it was 
impossible. 
Leaf No. 1234567 8 
221 224 226 240 255 302 about 348 
Shoot B (Type form). Grown under distinctly drier and sunnier condi- 
tions than A. In all, 14 leaves were produced. Very short hairs again began 
on the third leaf. The stomata could not be counted beyond the sixth leaf- 
Leaf No. 1 
No. of stomata ) 
per sq. mm. J 3 4 ^ 
283 
3 4 5 6 
273 35° 360 372 
SHOOT C (var. denudata)} Grown in a garden, fully exposed to sun. 
In all, 21 leaves were produced. A very few short, straight hairs appeared 
on the main veins of the fifth leaf. These slowly increased in number and 
length on the remaining leaves. In this variety, however, the leaves never 
become densely hairy, though in a few cases stomata counting was somewhat 
difficult. 
Leaf No. 1 
No. of stomata ) 
per sq. mm. 3 " 3 
Leaf No. 1 2 
No. of stomata ) g 
per sq. mni. 
2 
3 
4 
303 
313 
322 
13 
H 
15 
856 
945 
934 
3i3 
352 
482 
16 
i7 
18 
928 
1095 
1188 
8 9 10 11 
44° 543 678 807 
19 20 21 
1293 1083 1090 
Thus there is a gradual though fluctuating rise in the number of 
stomata through the successive leaves of the entire shoot. The numbers 
themselves are worthy of comment. Not only are the absolute numbers of 
1 Hesselman (’04), p. 419, states that he, too, was similarly baffled by this same species. 
2 The same shoots will also be referred to later as Shoots A, B, and C. 
3 The stomata counts, in this and the following cases, were taken from the terminal leaflet, 
cleared as previously described. As a rule, the numbers represent the mean of ten counts taken from 
different parts of the leaflet (lower surface only). The larger, and as far as possible even the smaller, 
veins were avoided. 
4 The leaves used for Text-fig. 4 are from this shoot. 
