jan. 20,1923 Structure of the Pericarp of Johnson Grass Seed 
215 
COAT CHARACTERS IN RELATION TO GERMINATION 
It can be seen from the preceding section that many solutes of various 
chemical nature pass through the membranes covering Johnson grass 
embryos. It has also been shown that certain chemical treatments 
greatly favor germination. These treatments include removal of the 
coverings over the embryo by means of concentrated sulphuric acid, 
treatment with chromic acid for a long enough time to weaken these 
coverings without killing the embryo, treatment with mercury salts, 
subjection to atmospheres with high concentration of carbon dioxide, 
and etherization (15, 16). Furthermore, certain of the salts mentioned 
in the preceeding section which were toxic in molar solutions slightly 
stimulated germination when used in tenth to hundred thousandth molar 
solutions and hydrogen peroxid in proper concentrations is also an effi¬ 
cient forcing agent. 
The question arises: Do the beneficial effects of these chemical treat¬ 
ments result from stimulation of the embryo protoplasm or from the 
removal or lessening of coat restrictions ? If the latter they may produce 
the effects observed either by increasing the permeability of the coat 
structures to solutes, thus admitting oxygen or releasing inhibitors which 
are held by them, or by decreasing the mechanical resistance of the coat 
structures to the expansion of the embryo. 
Denny (13) has shown that tannins, lipoids, and pectic substances 
greatly decrease the permeability to water of seed coats which are im¬ 
pregnated with them, while suberized membranes were not significant 
in the seeds which he studied except as these membranes became impreg¬ 
nated with fatty substances, which he showed did decrease their per¬ 
meability. 
The pectic substances and hemicellulose in the coat structures of Sudan 
grass caryopses including the inner integument indicate that these mem¬ 
branes would probably take up water slowly, but in larger total amount 
than those of Johnson grass, thereby becoming more distended, with a 
greater weakening of their mechanical resistance. The greater abun¬ 
dance of tannin and of suberin and its associated fatty substances in the 
coat structures of Johnson grass, on the other hand, would tend to limit 
the total amount of water which they are able to absorb and their per¬ 
meability to water and to substances in aqueous solution below the level 
obtaining for Sudan grass but should not effect the total amount of water 
which might in course of time pass through them if the embryo and 
endosperm were able to absorb it. These limitations would reach their 
maximum in the inner walls of the inner integument and in the closing 
tissue of the hilar orifice. 
Johnson grass caryopses, either in the scales or with the scales removed, 
take up water so rapidly that limited permeability of their coverings to 
water can not be considered as a possible cause of their dormancy and 
germination physiology. In fact the freshly harvested, dormant, naked 
caryopses absorb water when immersed in it so that their total moisture 
content is about 50 per cent of their dry weight at the end of 24 hours, 
after which small amounts are absorbed. So far as water intake is con¬ 
cerned, therefore, the coat structures need be considered only as possibly 
limiting by their physical resistance, the total amount of water imbibed 
by the caryopses. 
It is probable, however, that the substances which Denny (13) found 
limiting the permeability of membranes to water would also decrease 
