MAXIMUM SPECIFIC ELECTRICAL CONDUCTIVITY IN CHEMISTRY. 1338 
hydrolysis, namely, the diurnal change from sugar to starch, and again from starch to 
sugar.* Foliage leaves favour evaporation and sap concentration during insolation, by 
exposing a great surface, and by opening their stomata. ‘They not less surely favour 
subsequent sap dilution by a checked evaporation, due to closure of the stomata when 
insolation ceases. Thus sunlight, evaporation, sap concentration, and conversion of 
sugar into starch, alternate with darkness, checked evaporation by closure of stomata, 
dilution with fresh sap, and consequent reversal of the reaction, and reconversion of 
starch into sugar. To increase the concentration of a premaximal solution, and to dilute 
an ultramaximal solution, is in both cases to bring about an increased conductivity. 
Bearing this in mind, we are able to correlate these two apparently opposed reactions 
as both tending towards increased conductivity. 
Local sap concentration must surely occur in the intracellular regions of the meso- 
phile where the sunlight is concentrated by the cell lenses of the epidermis on the 
light-absorbing chloroplasts congregated together close to the opened guard cells, which 
permit a free escape of water vapour. It is here that metabolism is most active and it 
is very probably here that sugar is chiefly formed. 
Applying the hypothesis, it follows that when the sap concentration increases, 
and the solution becomes ultramaximal, the sap should tend to dilute itself by con- 
verting the sugar into starch. Accordingly, it is here that the formation of starch 
granules from the sugar is first noticed. When night falls or insolation ceases, the 
stomata close, the rate of evaporation slackens greatly, and then the concentrated sap, 
bathing the starch granules accumulated during the daytime, must become diluted by 
admixture with the dilute sap which continues to circulate owing to root pressure. 
This results in the solution becoming premaximal, so that the tendency towards increased 
conductivity determines the hydrolysis of the starch and its reconversion into 
sugar. : 
To such speculation, unaccompanied by direct experimental evidence, a greater or 
less degree of plausibility can at best be conceded. More is not claimed for it at 
present. It is advanced here, in the hope of drawing the attention of botanists and 
physiological chemists to a line of investigation which, being based on a recognition of 
the importance of the actually occurring local variations in sap concentration, promises 
to be very fruitful. 
If we turn from the consideration of the metabolism in foliage leaves and other 
green parts of the higher plants to the processes belonging to the filling and ripening 
of seeds, we find analogous reversible reactions, but under conditions more amenable to 
direct experimental investigation, for here the reversals are not of diurnal, but are, as 
a rule, of annual occurrence. Anabolic polymerisations, condensations, and dehydra- 
tions characterise the filling and ripening of seeds, and these changes are certainly 
accompanied by the drying up and concentration of the sap. In presence of the 
* Brown and Morris, “The Chemistry and Physiology of Foliage Leaves,” Chem. Soc. Jowr., 1893, p. 637. 
TRANS, ROY. SOC. EDIN., VOL, XLVIII. PART I, (NO. 6). 21 
