590 Thoday — Behaviour during Drought of the Leaves of two 
mounted on a cover-slip over a cell containing chloroform rapidly closed, 
but opened again when remounted in fresh water practically as widely as 
before. Careful measurement showed that the surface of the cuticle 
increased in length over chloroform vapour. In the particular section 
measured the increase was 3*3 per cent. Comparison of tracings from 
a section of a closed leaf, (a) in absolute alcohol and (b) after opening in 
water, showed no appreciable change in circumference. With chloroform, 
then, an increase in circumference is responsible for closure ; whereas 
normally closure is due to the contraction of the tissues within the 
cuticle. 
These experiments indicate that the normal position of the cuticle and 
water-imbibed cell-walls corresponds to the open groove, and that but little 
further opening results 
from turgor. Any in- 
creased opening in short 
spells of moist weather 
due to osmotic expansion 
of the living cells can be 
of little account. On the 
other hand, in sections 
killed in alcohol the 
grooves open in water. 
Closure must therefore 
be due to imbibition or 
cohesion forces or a com- 
bination of both. 
In considering the 
exact location of these 
forces the thin inner epi- 
dermis and loose spongy 
mesophyll may at once 
be dismissed ; so, too, may 
the bundles (but see below as regards the fibres in P. filiformis, p. 597). The 
palisade tissue and outer epidermis both contract radially. It is, however, 
a tangential force which is required, and this, by its structure, the palisade 
parenchyma is hardly adapted to develop. Moreover, in extreme cases the 
palisade cells have the appearance of being pressed closely together. The 
epidermis alone has the tangential continuity necessary to bend the resistant 
cuticle by its contraction. 
Cohesion. That cohesion forces are developed in the outer epidermis 
of the closing leaf is shown by the lateral folding of the walls. Figs. 4 a 
and b are from camera lucida drawings of cells from corresponding parts of 
adjacent sections cut dry, one mounted in oil, the other in water. The 
Fig. 4. Epidermis of corresponding parts of two adjacent 
sections ; («) in oil, showing bellows-like foldings ; (3) expanded 
in water. 
