22 BOTANICAL GAZETTE [JULY 
The attempt to measure the internal forces of seeds with semi- 
permeable coats by means of osmotic solutions was based upon 
this conception of the moisture relations, and on the assumption 
that the total osmotic pressure of the solution is transmitted through 
the membrane as force when pitted against the internal forces of 
the seed through the agency of the semipermeable coat. The 
results of this attempt have been highly satisfactory, and I can 
see no good reason for doubting that the internal forces of air-dry 
seeds are approximately equal to goo—1000 atmospheres. 
The vapor pressure method, using sulphuric acid, involves 
another assumption whose validity may be a little more question- 
able. It is generally admitted that the osmotic pressure of.a solu- 
tion can be calculated with some degree of accuracy from its vapor 
pressure. I have assumed in addition that the vapor pressure of 
the seed hydrogels measured against the vapor pressure of strong _ 
solutions can be used as a measure of the internal forces of the seed. 
The assumption seems reasonable enough, but it would be difficult 
to offer definite proof of its validity at present. 
The principal difficulty with the vapor pressure method is the 
uncertainty as to the osmotic pressure of sulphuric acid. It may 
seem quite unwarranted to some even to think of estimating the 
osmotic pressure of strong sulphuric acid by means of a formula, 
knowing as little as we do of all the factors which enter into the 
problem in this case, and knowing that these formulae have all 
been developed for the dilute “ideal” solutions. I realize fully 
the danger of basing any conclusions upon results obtained by such 
precarious methods, and offer the results merely for their suggestive 
value. The formula I have used for calculating the osmotic pres- 
sure from the vapor pressure is that given by WALKER (35), with S 
representing the density of the solution rather than that of the 
solvent. This is necessary in order to make the formula fit a 
concentrated solution with high density. This same formula has 
been used by others for the same purpose, as by DRABBLE and 
DRABBLE (12). 
Attention is called to a difference in the formulae given by 
NERNST (24) and WALKER. In WALKER’s discussion, ff repre- 
f 
