Cape Species of Pass erina, with some Notes on their Anatomy. 593 
appears to begin much sooner in the epidermis. In leaves of P. cf. falcifolia , 
after partial drying in the laboratory, the epidermis showed beautifully 
regular foldings, while no folding could be detected in the palisade cells. 
On the other hand, Fig. 5 illustrates a case in which the folding was more 
marked in the palisade cells ; this was summer material. Differences of 
age may perhaps be concerned here, e. g. the walls may be more flexible 
and extensible in young palisade cells. 
However this may be, a considerable change of volume is apparent 
also in the palisade tissue, even when the epidermis alone shows well-marked 
folding. 
The most reliable data are those obtained with sections mounted in 
absolute alcohol and afterwards transferred to water, as thinner sections 
can be cut with a razor moistened with absolute alcohol and the optical 
conditions are more favourable. In the case of a widely opening leaf of 
P.cf. falcifolia the following were the averages of numerous radial measure- 
ments of a section treated in this way : 
In Water. 
In Absolute 
Alcohol. 
Difference. 
ix. 
IX. 
%• 
Epidermis, outer wall 
2 3 
23 
— 
Epidermis, lumen and inner wall 
72 
46 
36 
Palisade layer 
5 2 
36 
3 2 
Spongy tissue 
J 97 
107 
46 
Here, of course, the expansion" is that of the tissue killed in absolute 
alcohol, and does not necessarily correspond with what would have occurred 
had they been still living. Rough measurement of sections measured dry 
and in water, and comparison of adjacent sections in oil and water, 
supported these data in a general way ; but in some cases the expansion 
of the palisade tissue of dry sections was proportionately greater than that 
of the epidermis. 
In view of these observations, I am inclined to regard water storage 
as a function of subsidiary importance even in the epidermis. The 
properties of the tissues will allow of some expansion during the night, 
when transpiration is minimal. But it is the increased power of holding 
and absorbing water which is of prime importance ; for cut shoots, even in 
the laboratory in shade, when deprived of any water-supply, become nearly 
air-dry in two or three days in the dry season. 
Significance of the large-celled Outer Epidermis. 
If water storage is only of subsidiary importance, the question of the 
significance of the thickness of the outer epidermis still remains. Relative 
to the size of the leaf the cells are very large. They are also large 
compared with those of many larger leaves. The following table gives the 
height of the epidermal cells in a few instances, and in some cases the 
thickness of the outer wall : 
