34 8 Sutherland and Eastwood. —The Physiological 
atmosphere, part of the water is given up and the leaf becomes more erect, 
making it easier for the motor-cells to roll the leaf, and thus check 
•transpiration from the upper surface. The latter danger is also lessened by 
a smaller surface being presented to the sun’s action when the leaves are 
more upright. 
Plants placed in water in the laboratory and then allowed to dry 
showed a pulvinar movement of ten to fifteen degrees. This is larger than 
occurs normally in nature. Freedom of movement at the articulation is 
facilitated by the thinning of the bundles there and by the migration of the 
stereome from a subepidermal position towards the centre, while sufficient 
mechanical support is given to the pulvinus by the thickened central sheath 
of sclerenchyma with its dorsal pad. 
Fixing Roots. The fixing roots are much thicker than the absorptive 
ones and are never branched. The epidermis and the layer beneath it 
finally decay, so that the persisting exodermis represents the original third 
layer, contrary to the usual Monocotyledon rule. The layer next to the 
latter consists of smaller cells also sclerified. Then follow five or six rows 
of compact parenchyma, the outer cortex, ultimately, showing moderate 
sclerification. Cubical crystals occur in abundance in these layers. The 
inner cortex consists of a large number of rows of extremely regular 
parenchyma with rectangular air-chinks at every corner. Even in old 
fixing-roots the regularity and embryonic appearance of these cells are 
retained, so that the sections appear like the young roots of many Mono- 
cotyledons. Lacunae, of the type more common in the absorbing roots, 
also occur, either all along except at the tip and near the insertion, or at 
irregular intervals. 
Absorbing Roots. At first these are anatomically the same as the 
young fixing roots, but are even then considerably thinner as a rule, owing 
to the smaller amount of cortex. In the adult roots all the inner cortex, 
except that at the actual growing-point, shows numerous well-marked 
radially elongated air-passages, stretching from the inner edge of the outer 
cortex to within two or three cell-rows of the endodermis. 
The lacunae, so well developed in this latter type of root and locally 
in the former, arise as follows: A certain small number of radial rows of 
cells in the inner cortex show considerable enlargement. The cells of the 
neighbouring rows, not growing in size much and sometimes even collapsing, 
meanwhile become stellate, usually four-armed, and the intercellular spaces 
at their corners thus become enlargech The radial walls of these rows of 
stellate cells become considerably thickened, and at points single cells or 
rows of two or three decay altogether, and only their radial walls plus small 
traces of their tangential walls attached to them persist round the gap. 
Each tangential wall becomes split across, and finally numerous narrow, 
complete radial rifts are formed in the inner cortex by the decay of com- 
