jan so. 1933 Structure of the Pericarp of Johnson Grass Seed 
205 
coverings, 100 each of the light-colored and dark-colored Sudan grass 
caryopses and of large and small Johnson grass caryopses were sub¬ 
merged in water in air-dry condition and slowly heated for three 20- 
minute periods with slow cooling between the periods of heating. The 
rate of heating was such that the water was just beginning to boil at 
the end of 20 minutes. 
Scarcely any caryopses broke during the first priod of heating, but all 
but a very few had broken by the end of the third period. The majority 
broke first over the endosperm. At the end of the third period of heat¬ 
ing 42 small and 26 large Johnson grass caryopses and 9 dark and no 
light Sudan grass caryopses had broken over the embryo. The breaking 
of the coat structures of Johnson grass over the embryo and the failure 
* of these to break in Sudan grass caryopses is evidently the result of 
their looser arrangement in the latter case and not of differences in 
resistance to pressure from within. The looser coverings of the Sudan 
grass embryo, and especially of those of the light-colored caryopses, 
allowed greater swelling of the embryos before these coverings were 
distended to their full capacity and at the same time gave better oppor¬ 
tunity for the escape of gases. A natural corollary of this fact is that, 
under germination conditions, the Sudan grass enbryo can imbibe a 
greater percentage of water without breaking the coverings than can 
Johnson grass embryos. If the force of imbibition of a Johnson grass 
embryo is insufficient to overcome the resistance of coat structures, the 
embryo will remain dormant on account of incomplete satisfaction of 
its water requirements. As a matter of fact actual tests with thick 
sections made with a freezing microtome and at once immersed in 
different solutions and examined with a microscope indicate that dormant 
Johnson grass embryos, even after long incubation under germination 
conditions, are about in equilibrium with 2-molar salt solution. The 
sections of the embryos contracted considerably in 4-molar salt solution 
and swelled very appreciably in water but underwent no appreciable 
change in 2-molar salt solution. Coat restrictions to water intake may, 
therefore, be important in imposing dormancy and resistance to germi¬ 
nation here as with the seeds of many water plants (11,12.) 
COMPOSITION OK PERICARP AND INNER INTEGUMENT OF JOHNSON GRASS AND SUDAN 
GRASS CARYOPSES 
Only a few microchemical tests were made by the authors. The 
results of these tests were verified and additional tests were made by 
Dr. Sophia Eckerson, of the University of Chicago and the Bureau of 
Plant Industry of the United States Department of Agriculture, for 
whose generous assistance we are greatly indebted. Table II sum¬ 
marizes Doctor Eckerson’s results so far as they are significant in the present 
comparison. 
The cell walls of the aleurone layer were of hemicellulose in both kinds 
of caryopsis. Although these walls are comparatively thick, it does not 
seem possible, in view of their composition and the much more resistant 
character of the integument and pericarp, to attribute to the aleurone 
layer any special protective function in uninjured caryopses. 
The pericarp tissues contained the same elements in both kinds of 
caryopsis, except iron, which was present in the outer epidermis of 
Johnson grass and absent in that of Sudan grass, but pectic substances 
were much more abundant and the suberization was less in Sudan grass 
22329—23-6 
