338 Sutherland and Eastwood . — The Physiological 
short ones, singly or in pairs. Their radial walls may be straight, but 
more frequently they present an undulating margin, most pronounced in 
positions of great strain like the pulvinar region, as in Fig. i, /. The 
inner surface of the sheath, protected by being pressed firmly against the 
stem, has thin, straight walls, while the outer epidermis (Fig. i, 3 and/) 
is coated with a thick development of cuticle. The adaxial surface of 
the blade is cutinized slightly, whereas the abaxial side has a strong, 
resistant layer which not only protects the mesophyll, but adds materially 
to the rigidity of the leaf. The poor development of this coating on 
the upper surface is compensated by the protection afforded by the fur- 
rows, and by the curling of the leaves when the water-supply is limited 
or transpiration excessive. 
Long cells . These occur alone only in protected regions like the 
inner epidermis of the sheath and along the sides of the leaf-grooves. 
In most other parts they alternate with short cells, and possess strongly 
thickened outer and radial walls with numerous round or elongated pits 
(Fig. i, 2, y and /). Over the articulation between sheath and blade 
the thickening is most pronounced, forming strong radial flanges or girders 
between elongated large pits (Fig. 6). Freedom of movement at this 
point is facilitated by the shortened cells, whose folded fan-like walls 
(Fig. i, /) are capable of lengthening with a kind of bellows-action. 
The most interesting of these long cells are the motor-cells (Fig. 4,2, m.c .), 
which were first described for grasses by Duval-Jouve, who regarded them 
mainly as silica-containing cells, to which he gave the name ‘ cellides 
bidliformes \ In Spartina Townsendii they form belts three cells wide 
running along the bottom of the furrows of the blade. They are clear and 
colourless, containing little solid matter but abundant water easily given up. 
While their inner and radial walls are very thin and collenchymatous, their 
outer walls differ only slightly from the adjoining epidermal cells. They are 
well developed towards the middle of the blade, being much deeper than the 
other cells along the furrows, but towards the margin little difference is seen. 
This accounts for the leaves rolling up completely only when excessive 
drying takes place. 
Short cells . These show greater variation and are of two distinct 
types, one containing no silica or only traces, the other with a relatively 
massive, definitely shaped silica body. The former, which occur singly 
or in pairs between long cells, may be regular, but frequently they 
become saddle-shaped, cruciform, biscuit-shaped, or even dumb-bell-like 
(Fig. 1, sa). They invariably show less thickening than the adjacent long 
cells, and no pits are present on their outer walls. More interesting, 
however, is the second type containing a distinct silica mass. Each is 
accompanied by a short cork-cell placed always on the side towards the 
base of the plant, and forming a kind of saddle, in the hollow of which 
