204 
Journal of Agricultural Research voi. xxm. no. 3 
structures over various parts of the caryopses in 25-micron median sagittal 
freezing microtome sections of a large number of caryopses. Table I 
summarizes these measurements for several portions of the caryopses, 
each entry being the average of the minimum thickness for five or six 
caryopses. 
Table I. —Minimum thickness in microns, of coat structures of Johnson grass and 
Sudan grass caryopses 1 
Area measured. 
Small 
plump, 
dark 
Sudan 
grass 
caryop¬ 
ses. 
Small 
well- 
matured 
Johnson 
grass 
caryop¬ 
ses. 
Large, 
very 
light- 
colored 
Sudan 
grass 
caryop¬ 
ses. 
Large, 
very 
dark 
Sudan 
grass 
caryop¬ 
ses. 
Large, 
well- 
matured 
Johnson 
grass 
caryop¬ 
ses. 
Integument and pericarp: 
Front of coleorhiza. 
34 
29 
3 2 
28 
34 
Over radicle. 
3 1 
22 
28 
23 
24 
Over coleoptile. 
Over scutellum. 
29 
21 
3® 
26 
21 
36 
2 3 
3i 
26 
25 
At micropyle 2 . 
78 
66 
79 
52 
96 
At hilar orifice. 
80 
7i 
65 
64 
83 
Over middle of endosperm side.... 
40 
33 
43 
38 
28 
Integument at micropyle. 
35 
42 
52 
48 
58 
Dark area over hilum... 
11 
13 
9 
12 
10 
1 Each entry is the average for several caryopses. 
2 The thickness at the micropyle was very variable, depending upon the manner in which the caryopses 
broke from the pedicel. 
In front of the point of the radicle and over the hilar orifice and 
micropyle the thickness of the pericarp and inner integument combined 
was greater in Johnson grass caryopses than in Sudan grass caryopses. 
These differences were related to the more forward position of the em¬ 
bryo in Sudan grass caryopses. 
Over all other portions of embryo and endosperm where measure¬ 
ments were made the coverings of the Sudan grass caryopses were 
thicker than those of Johnson grass caryopses. They were thicker also 
in light-colored than in dark-colored Sudan grass caryopses, this being 
the result of a looser arrangement of the different layers, especially the 
starch-bearing layer, of the pericarp and a less contracted condition 
of the inner integument in the light-colored caryopses. Only in the 
very darkest Sudan grass caryopses were the coverings structures as 
compact as in Johnson grass caryopses. 
The tendency for the pericarp tissues to fall loosely apart along the 
layers betweeen the outer and inner epidermises was greater in sections 
of Sudan grass caryopses than in sections of Johnson grass caryopses. 
RESISTANCE OR SEED COVERINGS TO RUPTURE PROM HEATING IN WATER, AND TO 
IMBIBITION BY THE EMBRYO 
The differences in compactness of the coverings of Johnson grass and 
Sudan grass caryopses suggested that differences in mechanical resistance 
to rupture from the pressure of the expanding embryos might exist and 
might be significant in relation to the germination behavior of the 
caryopses. In an attempt to test the mechanical resistance of these 
