829 
Problem of Xeromorphy in Marsh Plants . 
from which it seems probable that the difference is not great. Thus, 
considering the stomata alone , and under certain specified conditions (the 
same for the two cases), it is calculated that for Shoot C transpiration from 
the second leaf (Text-figs. 4 and 6, b) might amount to 21 -2 c.c. per hour 
per sq. cm. of leaf surface. From the eighteenth leaf (Text-figs. 4 and 6 , c) 
the corresponding amount would be 25-4 c.c. per hour per sq. cm. The 
difference is not very pronounced ; so as the upper leaves possess a dense 
covering of hairs, more compact mesophyll, &c., it may safely be asserted 
that, in spite of the numerous stomata, they are, on the whole, distinctly 
more xerophytic than the lower leaves (see § 13). The argument here is 
from leaf structure alone ; the ultimate test is of course the measurement of 
actual transpiration (see § 1 2). 
(b) Thickness of leave s, &c. The leaves of Spiraea U Unarm are fairly 
thin, varying in total thickness (excluding the hairs) from about 78 to 140 [x. 
Curiously enough, the thickest leaves on a shoot are invariably the glabrous 
spring leaves. In these the individual cells are large and loosely arranged. 
On passing up the stem, the successive leaves show a gradual shrinkage in 
the size of these cells, 1 which at the same time become more tightly packed 
together (see Text-fig. 4). The leaves thus exhibit a progressive reduction 
in thickness. The upper leaves, in fact, may almost be said to be shrunken 
miniatures of the lower. This of course applies to structure, not to surface 
area of the lamina. It is chiefly in this variation in size of the individual 
cells that the plasticity of the leaves of Spiraea consists, rather than in any 
marked change in the number of layers comprising the leaf, or the propor- 
tions assumed by the various tissues. There are, however, some changes in 
these proportions, which may now be instanced, and a few average measure- 
ments given (in f). 
Total thick- 
ness of leaf , 
between veins. 
Depth of 
cells of 
upper epi- 
dermis. 
Depth of 
cells of 
lower epi- 
dermis. 
Depth of 
palisade. 
Depth of 
spongy 
mesophyll. 
Ratio of 
palisade 
to spongy 
mesophyll. 
Shoot A (referred to above) — grown in shade : 
Leaf No. 
2 
138*0 [A 
22-0 /A 
x 5-5 /* 
54-5/* 
46-0 jA 
1*2 : 
1*0 
n 
18 
78*0 
15-5 
8.0 
31.0 
23-5 
i-3 = 
1-0 
Shoot B 
—grown 
in light of moderate intensity : 
Leaf No. 
2 
140.0 
1 9*5 
I 5‘5 
6o-o 
45-° 
i-3 = 
1*0 
12 
104*0 
18.0 
8*o 
47.0 
31.0 
i'5 = 
1*0 
Shoot C 
— fully insolated : 
Leaf No. 
2 
i35-o 
19-5 
x 5-5 
57 -o 
43-o 
i-3 : 
: i-o 
18 
80.5 
17.0 
8.0 
31.0 
24-5 
i-3 : 
: 1.0 
These figures illustrate the general shrinkage referred to above. 
Further, the lower epidermal cells decrease in depth in the upper leaves, 
But it is probable that the difference in transpiration could not be very much greater, and might even 
be considerably less than that calculated here. 
I am indebted to Professor G. A. Schott for assistance in making this calculation. 
1 This has already been noted for the epidermal cells and stomata. 
