jan. 20,1923 Structure of the Pericarp of Johnson Grass Seed 
217 
are not completely impermeable to any solute. These substances, even 
the highly toxic salts of mercury, may therefore reach the embryo in 
exceedingly small, subtoxic or only slightly toxic doses—much weaker 
than the solutions in which the caryopses are soaked—and stimulate it 
into growth. Moreover, treatment of freshly harvested Johnson grass 
caryopses with tenth molar to hundred thousandth molar solutions of 
hydrochloric, acetic, oxalic, citric, and tartaric acids and of sodium 
hydrate—all of which are colloid hydrators—did not increase their ger¬ 
mination as we should expect if decreasing the coat resistance by increas¬ 
ing the hydration of coat colloids were the only factor involved. 
Apparently with chromic acid the treatments which stimulate ger¬ 
mination are such as just fall short of serious injury, exactly as Kidd and 
West ( 21) reported for dormant white mustard seed with a number of 
stimulating agents, which are not classifiable under any other head. 
Also some at least of the other substances which increase germination 
are toxic to the intact, dormant caryopses if employed in too great con¬ 
centration. In the case of the wounding of the scutellum with a needle 
there is always definite, concrete evidence of a reaction of the living pro¬ 
toplasm in the prompt darkening (probably from suberization) of the cell 
walls along the wound surfaces. This reaction takes place only slowly if 
at all in dead embryos when these are scratched. 
The argument set forth in the preceding paragraph might lead to the 
hypothesis that the favoring effect of various treatments upon germina¬ 
tion is entirely the result of stimulation of the embryo protoplasm. This 
hypothesis, however, leaves unexplained one important earlier obser¬ 
vation (15). If the distal ends of the dormant caryopses are cut off 
just back of the ends of the embryos and the embryo portions are 
put to germinate, the following reactions ensue. First, a slight disten¬ 
sion of the starchy endosperm beyond the edges of the cut surfaces of 
the coat structures; second, after a day or two the cells of the epithelial 
layer of the scutellum begin to elongate in the region of the cut surface 
and the digestion of the starch begins in the endosperm cells underlying 
such areas of elongation exactly as in the early stages of normal germina¬ 
tion of inmutilated caryopses; third, a few days later, normal germination. 
The most probable explanation of this set of phenomena is the effect of 
an increased swelling capacity of the embryo due to the reduction of the 
mechanical pressure upon it, this time working from behind the embryo 
and effecting it more slowly than when the coverings were removed from 
the embryo itself. To be sure, the radicle and epicotyl break through 
their coverings when germination occurs exactly as in the germination of 
unmutilated caryopses, but as the result of growth forces which are greater 
than the imbibitional force of the partially imbibed embryo, and which 
could not be initiated so long as the swelling of the embryo was greatly 
restricted. And it has been shown in a previous section (see p. 205) that 
the embryos of dormant imbibed caryopses are not completely satisfied 
with water. Theoretically increased oxygen supply to the embryo or 
the removal of an inhibitor to germination may play a part also, but the 
theory of increased imbibition seems much more plausible. 
Cereals and other grasses which were not after-ripened have been in¬ 
duced to germinate by this method of cutting the grain in two just back 
of the embryo. Others (2, 18) also have induced germination of cereals 
which were not after-ripened by wounding the endosperm, and have 
