Cape Species of Passerina, with some Notes on their Anatomy . 591 
bellows-like folding of the walls is clearly shown in a ; in b the cells have 
increased considerably in volume and the walls are nearly straightened. 
This feature is characteristic of certain kinds of water-storing tissue, a point 
which will be considered again later. In the present connexion it is a clear 
indication of cohesion tension drawing the walls together, a tension which 
must act tangentially as well as at right angles to the surface. 
As the epidermis is curved in the expanded condition, the approximation 
of the short inner to the longer outer walls during the contraction, which 
is facilitated by the folding of the lateral walls, will in itself bring about an 
increase in curvature. This effect will be more pronounced where the initial 
curvature is greater, and in fact it is in such regions that the increase of 
curvature chiefly occurs (cf. Fig. 1). Cohesion therefore acts in two ways to 
close the groove, by drawing the lateral walls together and by drawing the 
inner walls to the outer. 
Subsidiary factors. The sap of the epidermal cells contains a substance 
in solution which forms in absolute alcohol a gelatinous mass slightly 
contracted away from the cell-wall. As to the chemical nature of the 
substance I have not yet been able to obtain satisfactory evidence. So long 
as the sap is fluid or semi-fluid it can hardly assist closure, though it may 
help the cells to resist the loss of water, and may also account in part for 
the rapid expansion of sections transferred from absolute alcohol to water. 
In dead cells, such as often occur in small patches in the outer epidermis of 
old leaves, the contents are brown and the outer wall depressed inwards. 
The contraction of the lumen is, however, only partial, and it seems probable 
that complete collapse is prevented by the gelatinous contents. A corre- 
sponding phenomenon in cells with mucilaginous inner walls is described 
below (p. 599). 
Associated with the development of cohesion tension in the epidermal 
cells there will also be a contraction of the inner cellulose wall, which must 
lose some of its imbibed water in the maintenance of equilibrium between 
the imbibition and cohesion forces. This contraction would tend to bring 
about closure of the groove. In the expanded condition these walls bulge 
inwards somewhat into the palisade tissue ; in the contracted condition 
they are sometimes nearly straight. This straightening, partly due no 
doubt to the contraction of the walls themselves, partly to the cohesive pull 
of the contracting contents, involves some deformation of the palisade layer. 
Any resistance offered to this deformation would assist closure. On the 
other hand, I have seen cases in which the inner walls were straighter in the 
expanded condition. 
On the whole it seems clear that cohesive forces acting in the outer 
epidermis play the principal part in the closing mechanism. Other 
factors may also be concerned in variable degree, but their effects are 
subsidiary. 
Q q % 
