862 Yapp . — Spiraea Ulmaria , Z., and its Bearing on the 
leaves. The earliest radical spring leaves are glabrous, the cauline leaves 
hairy (see PL LXXXI). (e) The non-flowering shoots of adult plants pro- 
duce only radical leaves. The earliest of these are glabrous as in (< b ). Sub- 
sequently, the successive leaves exhibit increasing hairiness up to June or July, 
after which they are decreasingly hairy, till finally glabrous leaves are once 
again produced in autumn. ( d ) The distribution of pubescence on the 
partially hairy leaves is interesting. The terminal leaflet is invariably the 
most hairy, and there is a regular decrease in hairiness from above downwards. 
Individual partly hairy leaflets generally possess a marginal band of hairs, 
with sometimes additional bands running inwards between the main veins 
(see PL LXXXII and LXXXIII). 
3. In reality this seasonal periodicity in the production of hairs is 
a widespread phenomenon. Spiraea Ulmaria was the most striking case 
observed, but many other marsh, and also certain land, plants exhibit the 
same general succession of glabrous, partly hairy, and more or less com- 
pletely hairy leaves. 
4. Similarly, there are marked seasonal differences in leaf structure. 
The lower glabrous leaves have on the whole a more or less definite ‘ shade ’ 
structure, while the upper, more exposed, hairy leaves approximate to the 
‘ sun ’ type. The former have large epidermal cells, with sinuous lateral 
walls, and few but relatively large stomata. The mesophyll is loose, with 
large intercellular spaces. On passing up the erect stem, the following 
progressive changes in leaf structure may be observed : the leaf becomes 
thinner, but the cuticle thicker ; the epidermal cells smaller, and their walls 
less sinuous. The stomata are more numerous but smaller, and the hairs 
much more abundant. The mesophyll is more compact, and the inter- 
cellular spaces are gradually reduced to a minimum (see Text-fig. 4). 
Speaking generally, the upper leaves show a distinctly greater degree 
of xeromorphy than the lower. These changes are independent of the 
actual size of the respective leaves. 
5. The variations in the number of stomata are remarkable. There is 
a gradual though fluctuating rise in the number of stomata through the 
successive leaves of the entire shoot. In one shoot, for instance, the 
numbers varied from about 300 to nearly 1,300 per sq. mm. of lower leaf 
surface. The latter number is the highest yet recorded for any species. 
At first sight it might appear that these numerous stomata of the upper 
leaves would neutralize the effect of the xeromorphy referred to above (4). 
But the stomata of the lower leaves have larger pores, and it is calculated 
that under similar conditions, and considering stomata alone , the possible 
transpiration from the upper leaves could be but little, if any more, per unit 
area of leaf surface, than that from the lower. 
6. The effect of environment on the development of certain leaf tissues, 
especially hairs and palisade cells, is considered. The results of many 
