28 
POPULAR SCIENCE REVIEW. 
well ascertained that the mobile property resides in the cellular 
tissue jpar excellence, and is not primarily, at least, manifested 
by the woody or by the vascular tissues, though these may 
serve to transmit the impressions from one part to another. 
This holds good in the parts of the flower as in the leaves. 
It may be as well, however, to describe the general structure 
of the little swelling {coussinet of the French) that is so 
generally found at the bas of the leaflets, and also of the leaf 
in those cases where they are endowed with the greatest mobility, 
merely stating that the presence of the ‘’coussinet” does not 
always accompany the property of raising or depressing the 
leaf. The structure of the coussinets is readily made out, by 
an examination of transverse or longitudinal sections, from 
which it maybe seen that there is in the centre a small quantity 
of pith cells surrounded by a ring of flbro-vascular tissue ; 
longitudinal slices show that this ring is formed by the coales- 
cence of groups or bundles of vessels which in the leaf-stalks are 
separate. Outside the flbro-vascular ring are three or four 
layers of cells — the inner ones containing starch grains, the 
outer ones chlorophyll ; they are comparatively loosely aggre- 
gated, so that small spaces are left here and there between 
them. Next in order from within outwards is a much thicker 
layer of cells, so closely packed as to present no intercellular 
spaces, and which are filled with chlorophyll, and frequently 
with some powerfully refracting substance of an oleaginous 
nature ; investing this thick cellular layer is the ordinary epi- 
dermis, destitute of stomata. Similar structural arrangements 
exist in all the “ sensitive plants ” yet examined. Now, as to 
the action of these layers of tissue, Sachs* — many of whose ex- 
periments we have repeated — has shown that, if thin transverse 
slices of the coussinet be thrown into water, the thick outer 
layer of cells, in which there are no intercellular spaces, becomes 
distended and turgid ; and when a longitudinal section is made, 
as the outer cells are fixed, on the one hand to the epidermis, 
and on the other to the central tissues and to the vascular 
riug, the two ends of the section become bent, while the cen- 
tral portion, being fixed, remains comparatively unaffected. If 
the slice be bisected there will be a double curvature, the portion 
attached to the epidermis curving in one direction, that fixed to 
the central bundle bending in the opposite direction, the con- 
cavity of the curve in either case being necessarily on the same 
side as the point of attachment. It would appear that the 
turgescence of the cells in this case is due to endomose, as 
when the sections are placed in sugar and water the direction 
of the curves is reversed. 
Experiments of a like nature were made many years since by 
* Bot. Zeitung, 1867. 
