592 Thoday . — Behaviour during Drought of the Leaves of two 
Water Storage , or Absorbing Power ? 
Although folding of the walls is easily observed in the epidermal cells 
it is not confined to them. The cells of the palisade parenchyma in tightly 
closed leaves may also show fine foldings which are less easily demonstrated. 
Fig. 5 is from a microtome section of a closed leaf, fixed in absolute alcohol 
and carefully embedded in paraffin. 
Solereder 1 mentions the common occurrence of similar foldings in the 
palisade parenchyma in herbarium material of various plants, and infers 
that this tissue has water storage as a subsidiary function. To adopt such 
a suggestion in the present instance would, however, be to overlook the 
real significance of the phenomenon ; for the amount of water that can be 
stored, even in the epidermis, is small. Moreover, the contracted condition 
may of necessity persist for a considerable time. True water-storage tissue 
may act either as a reservoir of water 
on which the active tissues of the organ 
itself may draw during temporary 
shortage, so that they may remain 
actively functioning, or as a reservoir 
which may be tapped by younger parts 
of the plant, the older parts yielding up 
their water and dying. Among suc- 
culents both alternatives are repre- 
sented, often in the same plant. In 
Passerina the former alternative may 
apply to a limited extent, in so far 
as the epidermis yields water to the 
assimilating tissues. It can, however, 
only be of service in relation to tem- 
porary drought. 
As regards the assimilating tissues themselves, the folding of the walls 
is an accommodation to a greatly diminished water content which may 
persist sometimes for many weeks. As the cell sap shrinks in volume the 
cell contracts in length rather than in breadth ; it may perhaps be due in 
part to this regular and structurally determined mode of contraction that 
the cell survives. The principal fact is, however, the great diminution in 
volume, to which the cell-wall offers little resistance. There is no 
hydrostatic pressure in the contracted cell, but the full osmotic pressure of 
the sap is brought into play for the absorption and retention of water. 2 
The difficulty of detecting the foldings in the palisade cells has 
prevented extensive comparative observations of the behaviour in this 
respect of the epidermis and palisade layer. In spring, at any rate, folding 
Fig. 5. Outer epidermal and palisade 
cells from microtome sections of closed leaf, 
fixed in absolute alcohol and embedded in 
paraffin. The contents of each epidermal 
cell (shaded) form a somewhat contracted 
brown gelatinous mass. Palisade cells with 
finely-folded walls. 
1 Systematic Anatomy of Dicotyledons, vol. ii, p. 1088. 
2 Cf. Thoday, New Phyt., xvii, 1918, pp. 108-13. 
