346 Sutherland and Eastwood . — The Physiological 
These latter are most numerous in the sheath, where they can be seen readily 
through the epidermis, owing to the enclosed tubes of air. 
The inner surface of the sheath is continued upwards in a short ligule 
(Fig. 5, /), terminating in a brush of fine hairs. The solid portion consists 
of moderately thin-walled, slightly elongated cells. The surface next to the 
stem, to which it is applied closely, has a smooth epiderm like the inner 
side of the sheath, while that towards the blade consists of cells, as in 
Fig. 7, with the long diameter nearly at right angles to the surface, and 
their outer walls notched, uneven, and mucilaginous. As the leaf-base is 
not always at the same angle to the stem, it seems likely that this upper 
surface of the ligule acts as a pulvinus in preventing its being torn away 
from the stem. The large, clear, colourless cells of the whole structure are 
adapted for holding water, which is retained, even under unfavourable con- 
ditions, by the mucilaginous coating. In this more or less turgid condition 
the ligule is pressed tightly against the stem, preventing the entrance of 
either water or mud, while the clefts (Fig. 7, cl) at its base permit the 
blade to sway freely without jerking the ligule suddenly away. 
Pulvinus and Articulation. Immediately above the insertion of the 
ligule there is a thickening of the blade, due to the greater development of 
the mesophyll at that point. Under this short, convex stretch of epiderm, 
which shows little cutinization or thickening, lies a mass of soft-walled cells 
with slight almost invisible pitting. In longitudinal section they present 
the outline shown in Fig. 5, 2. Beneath this cushion the cells become 
thickened, distinctly pitted, twisted, and fibrous, forming a solid sheath 
round the bundle, which is slimmer at this point, and minus the water- 
storing envelope. This investing mass of mechanical tissue is thickened 
near the middle of the pulvinar region, from which it thins away upwards 
and downwards, as in Fig. 5, 1, III and II Ik The sclerenchyma strands 
under the epidermis of the ridges above this region pass in under the soft 
tissue to meet the central supporting mass, while those under the abaxial 
epidermis stop short where the articulation bulges out on the dorsal side. 
This joint (Fig. 6) is marked by a darker, more glossy surface, where the 
epidermal cells are short, with thickened pitted outer walls and very wavy 
radial ones. Beneath this bulging portion is a pad of thick-walled cells 
with large pits ; on the inside it abuts on the central supporting sheath. 
Above and below are the ordinary mesophyll cells. 
Transversely along the middle line of the abaxial surface of the joint 
a split appears later, cutting across the strengthened pad without any 
definite order. This split forms a pseudo-articulation on which the leaf- 
blade swings more freely for some time before it falls off. 
The soft-walled tissue on the adaxial surface acts as a pulvinus. When 
the plants are submerged and contain abundant water, these cells are turgid 
and the leaf-blade is bent back, but when they are exposed to a drying 
