830 Yapp . — Spiraea Ulmaria , Z., z'Z Bearing on the 
much more than those of the upper epidermis (cf. also the sections and sur- 
face views in Text-fig. 4). The latter cells remain relatively large in the 
upper, exposed leaves, especially at the leaf margins. 
(c) Palisade tissue . The above table shows that the ratio of the total 
depth of the palisade tissue to that of the spongy mesophyll is remarkably 
constant. The variation, however, in the ratio of the length of a palisade 
cell to its breadth is considerably greater. As one would expect, the pali- 
sade cells of ‘ shade ’ plants tend to be relatively shorter and broader than 
those of ‘ sun ’ plants. In addition to this, in an individual leaf, those pali- 
sade cells which are most remote from the larger veins are relatively longer 
and narrower than those situated near to the veins. Again, taking a lower 
and an upper leaf from each of the three shoots previously used, we get the 
following average measurements (expressed in \x ) : 
Shoot A — shade : 
Leaf 2 (a) near vein . . 
( b ) remote from vein 
Leaf 22 (a) near vein . . 
(3) remote from vein 
Shoot B — moderate sun : 
Leaf 4 ( a ) near vein . . 
( b ) remote from vein 
Leaf 10 (a) near vein . . . 
( b ) remote from vein 
Shoot C — full sun : 
Leaf 2 (a) near vein . . . 
(b) remote from vein 
Leaf 18 (a) near vein . . . 
( b ) remote from vein 
Length of 
Breadth of 
Ratio of length 
palisade cell. 
palisade cell . 
to breadth. 
30.0 ix 
15-5 V- 
i*9 : i-o 
32-0 
n.5 
2.8 : i-o 
19*0 
9*5 
2-0 : i*o 
18.5 
6-5 
2.8 : 1.0 
27-5 
9*5 
2.9 : i-o 
35*5 
8-o 
4.4: 1. 0 
24*0 
8-5 
2.8 : i-o 
31.0 
7.0 
4*4: 1*0 
30.0 
12.5 
2*4 : i-o 
36-5 
10-0 
3*6 : i-o 
20*0 
9*5 
2*1 : i-o 
24.O 
7-5 
3-2: I-O 
This table shows clearly (1) that the actual dimensions of the palisade 
cells are less in the upper than the lower leaves ; (2) that the palisade 
character is best developed at some distance from the larger veins ; 1 and 
(3) that the palisade character is least marked in shoots grown under shade 
conditions. 
§ 8. Development of the Leaves, and Time of Origin 
of the Hairs, Palisade Cells, &c. 
In order to appreciate the effect of environment on the production 
of hairs, it is necessary to follow to some extent the development of the 
leaves themselves. Comparatively little appears to have been written 
on the later stages of leaf development. 2 A brief account will therefore be 
1 Stahl (’83), p. 35, records the same fact for the sun leaves of the Beech and the Bilberry. This 
is analogous to the development of the interveinal bands of hairs in partially hairy leaflets. 
2 Goebel (’05), p. 302 et seq., and others have described the distribution of growth during 
development, &c. The former points out (p. 305) that ‘ in general, parts which have the earlier 
functions to perform appear the eajliest \ Mer (’83), p. hi, has given a very brief account, un- 
accompanied by figures, of the order of development of the tissues of the lamina itself. 
