Jan. 20, 1923 Structure of the Pericarp of Johnson Grass Seed 
199 
the opposite direction it extends only about half a dozen cells and its 
surface forms a regular flat contour with that of the single layer of cells 
of which the inner integument consists farther from the micropyle. 
The pericarp over the micropyle as elsewhere consists of the very 
distinct outer and inner epidermises and between these 3 or 4 cell thick¬ 
nesses of thin-walled, elongated, irregularly arranged cells. The outer 
epidermis (h) is of thick-walled cells with large lumena, rectangular in 
section; the inner epidermis (i) of small, thick-walled, closely crowded, 
heavily pigmented tubular cells, roundish or elliptical in sagittal section, 
but changing between the micropyle and the hilum to compressed, rec¬ 
tangular cells with much 
thinner walls. At this 
point the inner epidermis 
of the pericarp is under¬ 
laid by another layer of 
small, thick-walled cells, 
also heavily pigmented 
(k), which extends unin¬ 
terruptedly over the mi¬ 
cropyle from the hilum, 
thinning out and gradu¬ 
ally disappearing distally. 
The origin of this cell layer 
is not clear, but it may 
be a persistent portion of 
the outer integument in 
which the micropylar 
opening is entirely oblit¬ 
erated. Over the area 
where the reversed outer 
layer of the inner integu¬ 
ment ends next to the 
hilum is a mass of small, 
irregular, closely packed 
pericarp cells (1) whose 
very thick, densely pig¬ 
mented walls are fused 
with the walls of the integ¬ 
ument cells upon which 
they impinge. This group 
of cells around the margin of the hilum is continuous with the cells of 
the “closing tissue” of the hilum to be described in later pages. Thin- 
walled pericarp cells (r) fill the hilum. 
Where the caryopsis breaks irregularly from the pedical (m) the peri¬ 
carp is supplied with a group of scalariform tracheids (n), most of which 
end irregularly only a few cells from the break, while a few extend for a 
short distance in rows in a tangential direction over the hilum. A con¬ 
stant feature of this system of tracheids is its splitting into two branches, 
one of which ends abruptly in a coiled knot, a few elements of which are 
shown (o) in a direction toward the micropyle, while from the other 
branch extend the rows of tracheids over the hilum (p). These latter 
rarely extend over the circular hilum for more than one-third of its 
diameter, and the underlying tissue of the pericarp is entirely nonvascular. 
Fig. 5.—Mean sagittal section through the micropyle and neigh¬ 
boring structures of a Johnson grass caryopsis: a, aleurone layer; 
b, proximal end of scutellum; c, micropyle; d,d inner integu¬ 
ment; e, its inner layer of cells at the micropyle; f, its outer layer 
of cells at the micropyle; g, edge of the hilum; h, outer epidermis 
of the pericarp; i, inner epidermis of the pericarp; k, layer of cells 
locally underlying the inner epidermis of the pericarp, possibly a 
persistent portion of the outer integument; 1, group of closely 
packed, thick-walled pericarp cells whose walls seem to be fused 
with the walls of the inner integument cells; m, edge of irregular 
scar of the caryopsis; n, scalariform tracheids of the pedicel; o, 
coiled branch of tracheid system; p, tracheids in rows parallel to 
the surface of the hilum; r, pericarp cells which fill the hilum; s, 
large, empty, functionless endosperm cells which underly the 
micropyle. X 180. 
