STRUCTURE, PHYSICAL CHARACTERISTICS, AND COM¬ 
POSITION OF THE PERICARP AND INTEGUMENT 
OF JOHNSON GRASS SEED IN RELATION TO ITS 
PHYSIOLOGY 1 
By George T. Harrington, formerly Scientific Assistant , Seed-Testing Laboratories, 
and William Crocker, formerly Plant Physiologisty Drug Plant , Poisonous Plant , 
Physiological and Fermentation Investigations , Bureau of Plant Industry , United 
States Department of Agriculture. 
INTRODUCTION 
These investigations were undertaken in hope of explaining some fea¬ 
tures of the behavior of Johnson grass seeds during their initial dormancy, 
their period of after-ripening, and their germination. As has been shown 
elsewhere (15) , 2 Johnson grass seeds belong to that group whose well- 
matured embryos are never'appreciably dormant, the dormancy of the 
intact fruit being imposed by its outer, nonliving structures. These 
include (1) the closely adhering, hard, brittle scales, (2) the fused pericarp 
and inner integument. Removal of the former hastens the germination 
and increases the germination capacity of the caryopses, whether these 
are freshly gathered, are fully after-ripened, or are in process of after¬ 
ripening, but does not appreciably affect the rate of after-ripening. 
Furthermore, removal of the fused pericarp and integument by corrosion 
with concentrated sulphuric acid or even its removal over one side of the 
embryo by means of a sharp needle induces the complete germination 
within three or four days even of freshly gathered grains which, without 
such treatment, would scarcely germinate at all in weeks or months. 
It has also been shown (15, 16) that the germination of Johnson grass 
seeds is highly dependent upon the maintenance of alternating temper¬ 
atures, that this sensitiveness to temperature conditions disappears upon 
the removal of the seed coverings, and that certain chemical substances 
exert a stimulating action upon their germination, particularly after the 
removal of the caryopses from the inclosing scales. These facts suggested 
that a study of the physical and chemical characteristics of the seeds, and 
especially of their pericarps and integuments, might help to explain their 
physiological behavior. 
In marked contrast to the dormancy and germination physiology of 
Johnson grass seeds, Sudan grass seeds germinate very readily at ordinary 
temperatures, constant or alternating, without the lapse of any consid¬ 
erable period of after-ripening, and without any special preliminary 
treatment. Sudan grass seed has therefore been used for comparison 
with Johnson grass seed in the studies herein reported. The greater part 
of the work was done in the Hull Botanical Laboratory of the University 
of Chicago, and the remainder in the Seed-Testing Laboratories of the 
United States Department of Agriculture in Washington. 
Both Johnson grass and Sudan grass have been described under several 
different names, and they have frequently been assigned to different 
species. Mr. A. S. Hitchcock considers Johnson grass as Holcus halepensis 
1 Accepted for publication July 2, 1921. 
3 Reference is made by number (italic) to “literature cited,” p. 221-222. 
Journal of Agricultural Research, 
Washington, D. C. 
abg 
(193) 
Vol. XXIII, No. 3 
Jan. 20, 1923 
Key No. G-268 
