The Osmotic Strength of Cell Sap. 19! 
all these cases the danger of excessive loss of water was small. 
Geranium Robertianum growing near a salt marsh was, however, in 
a different physical environment, and the presence of salt is usually 
associated with a marked xerophytic tendency. Here the cell sap 
was found to be of considerably greater strength than in the 
other plants mentioned above. The conditions on the rocks above 
Cwm Idwal again are such that plants growing there are subjected 
to strong insolation and high winds. Here again a high strength 
of cell sap was found in the three plants investigated—•23 and -28 
gram molecules. 
At first sight it would appear that in the greater strength of 
\ 
the cell sap might be found a valuable xerophytic character, the 
presence of salts in solution being known to retard evaporation. 
But the concentration of the cell sap is so slight that but little 
importance can be attached to this as a means of checking loss of 
water. If, however, the effect on the absorption of water be con¬ 
sidered, it at once becomes evident that a very important physio¬ 
logical benefit may accrue to plants with increased strength of 
cell sap. Many plants growing in high latitudes and exposed to 
wind and insolation are well provided with a hairy coating to the 
epidermis. This is usually regarded as a means of checking tran¬ 
spiration, and doubtless it does play an important part in this respect. 
At the same time it must be remembered that many plants with 
hairy epidermis are capable of absorbing water through the general 
leaf surface. Plants growing in mountain regions and in other 
situations conducive to a xerophytic habit are frequently exposed to 
periods of drought, during which only slight and transient showers 
fall—quite insufficient to penetrate the earth and reach the roots. 
The leaves, however, are wetted, and in those plants which have a 
relatively high concentration of the cell sap the water will be rapidly 
absorbed. It is conceivable that in this way a high osmotic strength 
of sap may prove to be a valuable physiological character in con¬ 
nection with water-absorption. The strength of sap as determined 
by the sodium chloride solution is being worked out in atmospheres, 
allowance being made for ionization of the salt. Further experi¬ 
ments on plants from various localities are being conducted, and 
the relation of the strength of sap in the guard cells to that of the 
rest of the epidermis is also being investigated. 
