2l6 
Journal of Agricultural Research voi. xxm, no. 3 
their permeability to substances in aqueous solution. We have shown 
that this is true so far as the effect of lipoid substances upon the penetra¬ 
tion of I 2 KI solution is concerned. If on account of restricted permeabil¬ 
ity metabolically developed inhibitors to germination were prevented 
from escaping from the embryo as suggested by Kidd ( 20 ) and by Maz6 
(. 23 , 24 ), or if the concentration of oxygen within the embryo were thus 
maintained below minimum required for germination, dormancy would 
result. 
It has been shown (r6) that carbon dioxid in a wide range of concen¬ 
tration forces the germination of dormant Johnson grass caryopses, 
showing that Kidd’s earlier hypothesis which indeed he and West ( 21) 
modified in a latter paper, does not apply in this case. We have found, 
furthermore, that increased partial pressures of oxygen in the atmosphere 
or even very high oxygen pressures are not effective in forcing the 
germination of Johnson grass. Limited oxygen supply, therefore, does 
not seem to play the rdle here that has been shown for dormant wild 
oats (j) Xanthium (10 , 29) and other seeds; the forcing action of hydro¬ 
gen peroxid on the germination of Johnson grass must apparently have 
some other explanation than increasing the oxygen supply of the embryo. 
As for Maze’s hypothesis of acetic aldehyd as an inhibitor to germina¬ 
tion Brown ( 4 ), Schroeder (28), Collins (9), and Brown and Tinker (5, 6) 
have shown that acetic aldehyd and smilar compounds pass through the 
selective permeable membranes of wheat and barley rather readily. In 
our own work ether, acetone, chloroform, 95 per cent alcohol, and Xylol 
all entered Johnson grass caryopses in 24 hours at room temperature in 
sufficient quantity to kill the embryos. It is highly improbable, therefore, 
that acetic aldehyd would be kept in by the coats in sufficient concentra¬ 
tion to hold the caryopses in a dormant condition for months or years under 
good conditions of moisture and aeration, as we have found to be the case 
with Johnson grass caryopses. The force of this argument is increased 
when we consider that if acetic aldehyd is present in the caryopses it is 
there as a product of respiratory activities, and that respiration is on a 
very low level in the dormant caryopses. If there were other possible 
water-soluble inhibitors present in the embryos, the same logic would 
apply to their removal. 
If we turn now to the possible explanation of coat effects as related to 
the swelling of the embryo, we find the following situation: Sulphuric 
acid removes the coat structures; chromic acid weakens them; mercury 
salts probably tend to coagulate the coat colloids and may thus weaken 
the coat structures; lipoid solvents dissolve a part at least of the lipoid 
substances with which they are impregnated, thus increasing their per¬ 
meability and probably also weakening their physical resistence; and 
salts other than those of mercury which increase germination (chlorates, 
sulphates, nitrates, sulphyocyanates) tend to increase the hydration of 
colloids and may thus weaken the coat structures. All these substances 
might be supposed to produce their beneficial effects upon germination, 
in part at least, by altering coat colloids. This can hardly be the case, 
however, with carbon dioxide, which probably passes through the peri¬ 
carp and integument in solution and acts upon the embryo itself. Un¬ 
doubtedly also all of the other substances which have been shown to 
stimulate germination pass through the covering membranes of the cary¬ 
opses at least in very limited amounts. These membranes apparently 
