824 Yapp. — Spiraea Ulmaria, L., and its Bearing on the 
We have seen that two types of leaf (glabrous and hairy), with inter- 
mediate transitions, occur on each shoot. As was perhaps to be expected, 
the glabrous low-growing leaves were found to possess a more or less definite 
‘ shade ’ structure ; while the upper, exposed hairy leaves approximated to 
the ‘ sun * type. There is a gradual transition (often fluctuating as regards 
individual leaves) in structure between the two extremes. 
The glabrous radical leaves, which are unfolded about March, have large 
epidermal cells, with more or less sinuous lateral walls (especially those of 
the lower epidermis). The cuticle is thin. The stomata are large and 
relatively few in number. The mesophyll is composed of large, loosely 
arranged cells, with large and abundant intercellular spaces (Text-fig. 4, 
leaf 2). As more leaves are unfolded the following progressive changes 
may be observed. The epidermal cells (both upper and lower) of the succes- 
sive leaves become gradually reduced in size, and their lateral walls less 
sinuods. The cuticle of the upper epidermal cells is thicker, though in this 
plant it is always fairly thin. The stomata become progressively smaller, and 
at the same time considerably more numerous per unit area. 1 The lower 
surface of the leaves is more and more hairy. The mesophyll is more com- 
pact (especially the upper palisade layer), its cells are smaller, and the 
intercellular spaces are gradually reduced to a minimum (Text-fig. 4, leaves 
io and 18). 
The general question of relation of structure to environment will be 
discussed later. It may, however, be remarked here, that while the above 
relative differences were found in the successive leaves of each shoot examined, 
the actual structure of any given leaf depends partly on its position on the 
shoot, and partly on the general nature of the habitat. Thus the most 
lacunar leaves, with the fewest stomata, &c., are the radical leaves of plants 
grown under conditions of extreme humidity and shade. The converse also 
holds true (cf. Text-fig. 3). Again, the observed differences depend only to 
a certain extent on the actual size of the leaf lamina. Thus, a e shade ’ leaf 
has not only a larger surface area, but also larger epidermal cells, fewer 
stomata per unit area, &c., than a corresponding leaf from a plant grown 
under fully insolated conditions (see Text-fig. 5). On the other hand, if the 
leaves formed on a single shoot are considered, a curve showing the periodi- 
city in size of lamina would exhibit a very distinct rise and subsequent fall 
(both sides of this curve are steeper in shade than sun plants, cf. Text-fig. 5). 
But a curve showing the progressive changes of structure would be a con- 
tinually rising one. Thus two leaves from opposite ends of a shoot may be 
approximately equal in size, yet possess a totally different structure (Text- 
fi g- 4)- 
1 Dufour (’87), p. 407, states that stomata are more numerous on leaves grown in sun than in 
shade, while Eberhardt (’03), pp. 151-2, says that in humid air the size of epidermal cells is increased, 
and the number of stomata decreased, as compared with the effect of dry air; cf. Fig. 4 p. 81. 
