Ewart . — The Effects of Tropical Insolation . 465 
tion, and are at the same time protected from excessive 
transpiration by the presence of numerous stalked mucous 
glands of epidermal origin. The red colouration is most 
marked when the leaf is nearly fully expanded and about 
two-thirds its full adult size, being less marked in the young 
inrolled leaflets than in the still young but just expanded 
ones. As the leaf becomes adult, the red colour rapidly 
diminishes from the base upwards, and the mucous glands 
wither away and disappear. The red dye is formed in the 
mesophyll-layers only, and chiefly and most markedly in 
the cells of the palisade-parenchyma. As the dye disappears 
the special protection which the mucous glands afford against 
excessive transpiration is no longer needed, and the glands 
dry up. 
As regards the cases in which the red dye is present only 
in the under epidermis or in the spongy mesophyll, it must 
be remembered that in many such cases the young leaves 
expose their ventral surfaces to the light. Thus in Musa , the 
young leaves are vertical and rolled up with the ventral 
(under) surface outermost ; and in one variety of Musa a red 
dye develops on the under exposed surface, disappearing as 
the leaf unrolls and expands horizontally. The adult leaves 
of Musa (see above, p. 440) are quite resistant to even prolonged 
insolation, but the exposed portions of the young leaves are 
more sensitive, and may have their power of assimilation 
weakened or temporarily inhibited by a full day’s insolation. 
In Uncaria sclerophylla the young leaves have a red dye in 
the under epidermis only, which is also covered by adpressed 
sclerotic hairs. The young leaves are at first folded longi- 
tudinally, with the under (ventral) surfaces outwards ; and 
commonly even as they expand and before the fixed light 
position is taken up, the ventral surface is more exposed than 
the dorsal. The red dye acts as a protection against sun- 
light, whilst the adpressed hairs guard against excessive 
transpiration. The dye begins to disappear when the leaf 
is from one-third to one-half its full adult size, and is almost 
entirely gone when two -thirds grown. Young expanded 
