jan. 20.1923 Structure of the Pericarp of Johnson Grass Seed 
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SUMMARY 
(1) Johnson grass seed is markedly dormant when first matured under 
ordinary conditions of storage, requires a number of months for com¬ 
plete after-ripening, and even when fully after-ripened will not germinate 
completely except with the use of alternating temperatures in a very 
warm temperature range. Seeds of its close taxonomic relative, Sudan 
grass, germinate freely with a wide range of temperature either constant 
or alternating, and without the intervention of any considerable period 
of after-ripening. 
(2) Johnson grass caryopses are invested in the fruit with very hard, 
tight, usually darkly pigmented scales (glumes), the removal of which 
accelerates the germination of the caryopses and increases their ger¬ 
minating capacity. 
(3) The coverings of the naked mature caryopses of both grasses con¬ 
sist of the fused pericarp and inner integument, the outer integument 
and nucellus having entirely disappeared, with the possible exception 
of a portion of the latter over the micropyle. The complete or partial 
removal of these coverings over Johnson grass embryos induces prompt 
and complete germination even of freshly matured caryopses, and under 
the temperature conditions for the germination of Sudan grass seed. 
(4) The pericarp of both kinds of caryopsis consists of a continuous 
outer epidermis, a fragmented inner epidermis, and several intermediate 
layers of loosely arranged cells. One of these layers frequently contains 
starch. The outer epidermis does not have especially thick walls and is 
easily broken. The pericarp tissue breaks jaggedly at the pedicel when 
the caryopsis is removed from the scales, thus opening a passage for 
solutes into the pericarp tissue clear to the inner integument. 
(5) The inner integument of both kinds of caryopses is highly developed 
and of large cells with very thick, darkly pigmented inner walls. 
(6) The micropyle is usually completely closed by a massive, recurved 
development of the inner integument. 
(7) The large circular hilar orifice contains no vascular elements, con¬ 
duction from the vascular bundle of the pedicel and outer layers of the 
pericarp over the hilar region being by means of parenchymatous pericarp 
tissue which entirely fills the hilar orifice, and is fused with the inner integ¬ 
ument at the hilar margins. 
(8) A zone of this conducting pericarp tissue lying just outside the 
hilar orifice and including the elements which are fused with the inner 
integument becomes greatly contracted radially and darkly pigmented 
during the maturation of the caryopsis. This pigmented zone of the 
pericarp and the inner, integument together constitute for the caryopsis 
an unbroken investment which is extremely resistant to the action of 
Javelle water and of chromic acid and has the quality of selective per¬ 
meability, though it probably does not exclude any solute entirely. 
Penetrant solutes (iodin solution) enter much more readily at the proxi¬ 
mal end of the caryopsis, probably through the hilar orifice, than else¬ 
where, but they also frequently enter locally in other places. 
(9) The coverings of Sudan grass caryopses are more fragile and their 
embryos are less tightly inclosed and are so situated as to be more ex¬ 
posed to mechanical injury than is the case with Johnson grass caryopses. 
(10) The coverings of the caryopses are thicker over the hilar and mi- 
cropylar regions and in front of the point of the radicle in Johnson grass 
