1913] SHULL—SEMIPERMEABILITY OF SEED COATS 193 
that the forces which extract water from the soil particles and move 
it through the plant may not be entirely osmotic whenever the force 
needed runs high, but may involve capillarity and imbibition as . 
well. A study of surface tension forces in soils should throw some 
light on this subject. 
Many theories have been advanced to explain the cause of semi- 
permeability. Chief among these are (1) the filter or sieve theory, 
(2) the solubility theory, (3) the “Haftdruck ” theory of TRAUBE, 
and (4) the hydrone theory of ARMSTRONG. Each theory has a 
certain amount of supporting evidence, and any of them could be 
applied to the problems of physical or physiological semipermea- 
bility. 
Since ARMSTRONG (3) used the hydrone theory to explain the 
semipermeability of the coat of Hordeum vulgare as described by 
Brown, I have given it more attention than the others, not that it 
seemed to offer the best explanation, but because of its weakness. 
ARMSTRONG’S conception of water as a mixture of hydrone, 
hydrol, and hydronol was developed several years ago (1, 2, 4). 
The hydrone compounds are composed of =OH,, which is related to 
the hydrones much as =CH, is to the carbon compounds. The 
more complex molecules, as penthydrone, are believed to be inert, 
resembling the polymethylenes in this respect; whereas the simple 
hydrones are much more active. 
The size of the hydrone molecule is believed to be influenced 
by temperature. As the temperature of water rises, the complex 
hydrones are supposed to split up into simpler molecules, which 
renders the water much more active in all its relations.’ This 
conception has received some support recently from the work of 
Brown and Wortey (12), who studied the relation of temperature 
to the rate of water imbibition by seeds of Hordeum vulgare. 
Their results indicate that the velocity of water intake may 
be a logarithmic function of the temperature. The temperature 
coefficient for the imbibition rate follows approximately the VAN’T 
Horr law, indicating that chemical processes are involved in water 
intake. They suggest that the chemical change thus indicated is 
the simplification of the complex hydrone compounds into smaller, 
more active molecules. 
