Ewart . — The Effects of Tropical Insolation . 467 
exposure, when young, of the ventral surface to illumination, 
was peculiarly necessary. In various Zingibers, Marantas, &c. ? 
in which the under surface of the leaf is red, the leaves when 
adult are horizontal with the ventral surface downwards, and 
the red pigment disappears in the post-adult stages. In some 
cases the leaves remain partly erect, in which case a slight 
red colouration may persist on the under surface. 
When exposed to strong illumination, Mimosa padica 
acquires a distinct reddish colouration on the upper exposed 
surfaces of the petioles and younger branches, which is slight 
or absent in the young still folded leaflets ; in the adult leaves 
the red colouration is especially marked, but is restricted to 
the under surfaces of the leaflets. In shaded plants the 
red colouration is but slight or absent, and this is commonly 
the case in plants grown in European hot-houses. The red 
dye is formed in the epidermal cells at the edges of the 
leaflets and in those parts of the under surface which are 
exposed when the overlapping leaflets are all folded together. 
At the apex of the leaflet the under epidermis has a thicker 
outer wall and cuticle than the upper epidermis, but in the 
basal part of the leaflet this difference does not exist ; if 
anything, the upper epidermis and cuticle are here slightly 
thicker than the under. On the basal pair of leaflets the 
under epidermis and cuticle, which are outermost and entirely 
exposed when the leaflets are folded, are thicker than the 
upper all over the leaflet. On the upper surfaces of the leaf- 
lets, stomata are abundant and increase in number towards 
the apex, being especially abundant at the extreme apex. 
The average number of stomata on the under surface is 
slightly greater than on the upper, but not quite so abundant 
as at the apex on the upper surface. On the under surface at 
the apical one-eighth to one-twelfth of the length of the leaf, 
i.e. over the portion which, being exposed when the leaflets 
fold together, has a protective red pigment, no stomata at all 
are present. Of the basal pair of leaflets belonging to one of 
the outer petioles, the outer one is generally larger and better 
developed besides being more exposed than the inner, and 
