468 Ewart . — The Effects of Tropical Insolation. 
in it the adaptive modifications are most markedly developed. 
The entire under surface is markedly red and hairy. The 
outer wall and cuticle of the epidermal layer, all over the 
under surface of the leaflet, are thicker than on the upper 
surface. Stomata are present all over the under surface, but 
only one-third to one-fifth, or less, of the number present on the 
upper surface, and the guard-cells have markedly thickened 
and cuticularized walls. On the upper surface stomata are 
abundant all over the leaflet, being less abundant at the 
base and especially abundant at the apex. 
When therefore the leaflets of Mimosa are folded together, 
only the red and hairy parts of the under surfaces of the 
overlapping leaflets are exposed, whilst the fully exposed 
under surfaces of the basal pair, and the exposed edges of the 
other leaflets, are markedly red and hairy and contain as 
compared with the upper surfaces but few or no stomata. 
The red dye protects the exposed portions of the more 
sensitive under surface from any excessive photo-chemical 
action caused by the unnatural exposure, whilst at the same 
time transpiration is reduced to a minimum. 
The leaf of Mimosa is very thin, consisting besides the 
upper and under epidermis, of a single layer of palisade- 
parenchyma with large and numerous chlorophyll-grains, and 
of two or three layers of spongy mesophyll. The plant is 
very sensitive to drought and is readily injured by it, the 
least scarcity of water causing the leaflets to fold and to 
remain folded for some time after the plant has been fully 
supplied with water again ; whilst if the drought be at all 
prolonged the leaflets may never recover or re-expand. It 
is hence obviously an advantage that, whenever a wind of 
sufficient velocity to agitate the leaflets is blowing, any 
excessive and injurious transpiration which it might cause 
should be guarded against by the mechanical stimulation 
causing the leaflets to fold together. At night-time the folded 
position of the leaflets and the hairiness of the exposed 
portions act as a protection against the loss of heat by both 
transpiration and radiation. 
