Ewart . — The Effects of Tropical Insolation. 469 
On the upper surface of the pulvinus the epidermis bears 
some resemblance to a columnar epithelium, the outer ends 
of the cells being rounded or conical, whilst the cell-cavity 
is filled with a red sap. This layer probably acts as a pro- 
tection to the sensitive pulvinar tissues, preventing them from 
being too readily affected by light falling from above. As 
has been already shown, the pulvinus is, probably owing to 
this fact and to the transparency of the pulvinar tissues, almost 
or quite as readily affected by illumination from beneath as 
from above ; the only condition necessary being that the 
light when it reaches the sensitive pulvinar tissues shall be 
of sufficient intensity to act as a stimulus. 
From the foregoing it is clear that the red colouration found 
on the parts of the under surfaces of the leaflets of Mimosa 
which are exposed when folded, comes under the same 
category as in the plants previously mentioned, being for 
the purpose of shielding the transparent spongy mesophyll, 
in which the higher processes of synthetic metabolism, as 
distinguished from the simpler primary assimilatory pro- 
cesses, are probably especially active, from the injurious 
effects of the exposure to which it might wholly or in part 
be liable when the leaflets are folded together. 
In some cases, however, such as in certain Begonias, in 
which the leaf is horizontally expanded and the red dye is 
present only on the under surface in the adult leaf, Stahl’s 
interpretation of the functional importance of the red coloura- 
tion may be the correct one ; for even excluding all such 
cases occurring in cultivated garden-plants in which the 
peculiarity might be due to artificial selection, still cases 
remain for which this explanation does not suffice. Granting 
that the red pigment must always be formed for a useful 
and definite purpose, its well-defined and apparently purpose- 
ful distribution precluding the idea of its being an accidentally 
coloured useless waste product of metabolism, it is difficult 
to see in such cases, even though they are but few in number, 
what other useful function it could possibly have. 
As regards the young hanging-foliage of certain tropical 
