220 
Journal of Agricultural Research 
Vol. XXIII, No. 3 
caryopses than in Sudan grass caryopses, this difference being related to 
the more forward exposed position of the Sudan grass embryos. In 
other regions the coverings of Sudan grass caryopses are thicker than 
those of Johnson grass caryopses but are less compact and less darkly 
pigmented and offer less effective insulation to the embryo. 
(n) The coverings of Johnson grass caryopses are much more resist¬ 
ant to the* bleaching action of Javelle water and the corrosive action of 
chromic acid and are somewhat less readily penetrated by iodin than are 
those of Sudan grass caryopses. The lots of Johnson grass caryopses 
which are most resistant to the action of Javelle water and of chromic 
acid are most deeply dormant when fresh and most resistant to germi¬ 
nation when after-ripened. 
(12) The coverings of wheat caryopses are less resistant to the action 
of chromic acid than are those of Sudan grass caryopses. 
(13) The coverings of wheat and corn are more readily permeable to 
iodin than are those of Sudan grass caryopses. 
(14) Soaking Johnson grass caryopses in lipoid solvents kills the em¬ 
bryos and at the same time increases the rate of the subsequent pene¬ 
tration of iodin and the rate of subsequent bleaching in Javelle water. 
(15) The inner integument and the various layers of the pericarp of 
Johnson grass all contain tannin* compounds, and all are highly 
suberized, especially the inner wall of the inner integument, which con¬ 
sists of suberin impregnated with fats and to which is due the great re¬ 
sistance of the caryopses to the action of chromic acid. Probably tannin, 
suberin, and lipoids all increase the strength and diminish the extensi¬ 
bility of the coat membranes and decrease their permeability to solutes, 
and probably all are related to the inhibiting effects of the coverings of 
the caryopsis upon germination. 
(16) While this hypothesis is not subject to exact proof it seems prob¬ 
able that the character of the coverings of Johnson grass caryopses limits 
the imbibitional swelling of the embryos and thus keeps their water con¬ 
tent below the minimum required for the inception of germination at 
relatively low and constant temperatures. Removal of the distal ends, ex¬ 
posing the caryopses to increased imbibition, induces enzymic activity in 
the scutellum followed by germination, reversing the order which char¬ 
acterizes normal germination. It is possible that inhibitory substances are 
held within the coats and that these maintain the embryo in a dormant 
condition. The effective forcing agents may oxidize or precipitate these 
substances, or they may modify the permeability of the coat structures 
so they can diffuse. Breaking the coat structures would also lead to the 
exit of such materials. 
(17) Chemical treatments which increase the germination of Johnson 
grass and the wounding of the embryo may involve both a reduced re¬ 
sistance of the embryo coverings to imbibition and a direct stimulus to 
the embryo protoplasm. 
