1913] SHULL—SEMIPERMEABILITY OF SEED COATS 189 
difference between the two figures is very large, the former being to 
the latter as 3:1, inspection of the curves in Putir’s little book 
shows that at a concentration of 700 grams of sucrose per liter of 
solution, the observed pressure would be to the theoretical pressure 
calculated on the basis of solution volumes as 3:1. Moreover, the 
figures of RAcrBorskI for saturated LiCl solutions (80.7 per cent at 
20° C.) were obtained by the same methods which Diererict used 
for NaCl; his results give to this solution an gs 
osmotic pressure of 965.3 atmospheres. This = 
does not seem too high if NaCl has a value of os 
375 atmospheres; for NaCl at saturation at 18° C. 
is a 5.42 M solution, and LiCl a 13.3 M solution 
(Smiru’s General chemistry). + te 
These pressures are of interest on account of 
their magnitude. If this measure is correct for 
the osmotic pressure of saturated lithium chloride, ro 
then the internal forces causing entrance of water 
into air-dry Xanthium seeds must be at the initial 
moment in the neighborhood of 965 atmospheres. com, 
It is as difficult as it is interesting to think of a 
saturated aqueous salt solu- 
tion being as “dry” as an 
air-dry seed. 



. 60% 30% 20% oz ‘ of 
From saturated solutions «« ha «2A ARBRE BE 
of NaCl these air-dry seeds 54, Curve of imbibition in M-NaOH, 
will imbibe 7 per cent of showing the development of osmotically active 
their air-dry weight. It igs substances within the semipermeable coat 
: : during the second day; the pressure thus 
“plabesee rt then, if the developed bursts the outer layers, but leaves 
pressures given for the two the inner one intact. 
solutions are correct, that 
the imbibition force in a seed which contains 8-9 per cent of hygro- 
scopic moisture (air-dry) is 965 atmospheres, and that the addition 
of 7 per cent more water reduces the capillarity and imbibition 
force from 965 atmospheres to 375 atmospheres, a loss of 590 at- 
mospheres. 
The rapid increase of the internal forces with the decreasing 
water content of the seeds is shown graphically in fig. 8, which is 
