2l8 
Journal of Agricultural Research 
Vol. XXIII, No. $ 
attributed the result to effects upon oxygen or water absorption. To be 
sure, Kiessling (22) reported that coating the wounded endosperms 
with substances which he claims prevented increased imbitition as a 
result of the wounding did not prevent the stimulating effect upon 
germination and used this fact in support of his “stimulus” hypothesis. 
But it is difficult to accept this hypothesis, because the only avenue for 
the transmission of such a stimulus to the embryo through the medium 
of living cells appears to be along the aleurone layer, and even this 
path is interrupted along the adjoining faces of the scutellum and 
endosperm by a mass of compressed nonliving endospermic cell walls. 
Furthermore, in our experiments with Johnson grass caryopses the 
first evidences of vital and enzymic action were observed, not where the 
aleurone approaches the scutellum—that is, at the extreme distal end 
of the scutellum, but in the area nearest to the cut surface—that is, 
several epithelial cells from the extreme distal end. 
There is one strong indication that the coat structures are more or 
less directly responsible for the dormancy and germination physiology 
of Johnson grass caryopses independently of any effect upon the embryo 
protoplasm of the operation of removing or weakening these structures, 
namely, the correlation which has been shown in previous sections 
between the resistance of these structures to certain reagents, on the 
one hand, and the germination of the fresh caryopses, on the other. 
That differences in these coat structure occur as between different kinds 
of seed with different germination requirements might be wholly inci¬ 
dental and irrelevant. But when in addition similar differences occurring 
in different lots of the same kind of seed parallel in a logical manner 
differences in germination under identical external conditions, the 
parallelism can hardly be without significance. These considerations 
stJ g£ e st that the inhibitory effect of the unusually tough and compact 
coverings of the embryo may be related primarily to a purely physical 
restriction of the imbibitional swelling of the embryo, especially its 
axial organs, a restriction which is enhanced by the location of these 
organs almost wholly enveloped by the massive wings of the scutellum 
and is increased also by the pressure of the tight-fitting scales within 
which the caryopses are inclosed. That such a physical restriction is 
paralleled by a decrease in permeability of the coat structures and an 
increase in their resistance to chemical attack is entirely natural, since 
the tensile strength, elasticity, and extensibility of the coat structures is 
determined by the degree of drawing together of the individual elements 
and the degree of their impregnation with substances which are insoluble 
in water and which confer relative impermeability. It should be stated, 
however, that actual proof of this hypothesis, which relates the tardy 
germination of Johnson grass seeds to restricted imbibition by their 
embryo, is, in fact, wanting. As already pointed out, certain facts 
indicate that other factors, perhaps involving direct stimulation of the 
embryo protoplasm, enter into the forcing of their germination by 
chemical reagents. A sharp distinction should be maintained between 
mechanical and chemical forcing of germination even when, as here, the 
same kind of seeds is used in both cases. The fundamental explanation 
may be quite different in the two cases. 
