A GRADIENT OF PERMEABILITY TO IODIN IN WHEAT 
SEED COATS 1 
(A PRELIMINARY NOTE) 
By Harry Braun 2 
Assistant Pathologist, Laboratory of Plant Pathology , Bureau of Plant Industry, 
United States Department of Agriculture 
Statements in literature on the relation of semipermeable seed coats to the 
entrance of toxic solutes, expressed as recently as 1923 (Harrington and Crocker), 3 
are to the effect that a solute like iodin enters only to a small degree through the 
general surface of the grain, but largely through the hilum or micropyle whence 
it spreads laterally and in a distal direction under the seed coats. Experiments 
by the writer point to contrary conclusions as follows: (1) there is very little 
spread of iodin under the seed coats from the embryo end either laterally or in 
a distal direction; (2) this is equally true for all other parts of the seed; (3) 
entrance of iodin takes place over the entire surface of the grain radially but 
not uniformly; (4) the apparent lateral and distal spread from the embryo end 
is due to the existence of a gradient of permeability to iodin in the seed coats, 
which permeability is greatest near the embryo end, diminishes to a minimum 
near the distal end and again increases slightly. These conclusions are sup¬ 
ported by experiments with wheat seeds coated in various ways with water¬ 
proof substances like paraffin. The experiments have shown that (1) when the 
embryo end only is exposed, iodin penetrates only as far as the limits of the 
exposed area and fails to spread much further laterally or distally under the 
seed coats protected by paraffin; (2) the same is true for any exposed region of 
the kernel partially paraffin-coated, the stain only goes as far as the limits of 
the exposed part; (3) when the embryo end alone is coated, the entrance of 
iodin manifests itself progressively, beginning always on the exposed part 
nearest the embryo end 180 degrees from the grooved surface and continuing in a 
lateral and distal direction but not toward the coated embryo end; and the 
blackening due to iodin at any point on such a grain corresponds in degree with 
that shown after a similar period of time on the same point of a wholly unpro¬ 
tected grain. 
Ultimately, the entire grain is softened by entrance and spread of water from 
the iodin solution but is not discolored except near the exposed surface. This 
indicates that the failure of iodin to spread rapidly laterally and inwardly is 
due to its removal either (1) by chemical combination with the starch nearest 
the point of entry, or (2) by adsorption (Biltz) 4 on the starch immediately under¬ 
neath the exposed area, while water goes through. 
1 Received for publication Jan. 23, 1924. Presented at the Cincinnati meeting, A. A. A. S., Dec. 31, 
1923 , before Physiology Section, Botanical Society of America. 
a The writer is deeply indebted to Prof. R. A. Harper, of Columbia University, in whose laboratory the 
greater part of the experiments were made. 
3 Harrington, G. T., and Crocker, W.—structure, physical characteristics, and composition 
OF THE PERICARP AND INTEGUMENT OF JOHNSON GRASS SEED IN RELATION TO ITS PHYSIOLOGY. JOUT. AgT 
Research 23: 193-222, lllus. 1923. 
4 BILTZ, W.—UEBER DIE BLAUE ADSORPTIONSVERBINDUNG VON BASISCHEM LANTHANACETAT UNDf}3 
Ber. Deut. Chem. Gesell. 37: 719-724. 1904. 
Journal of Agricultural Research, 
Washington, D. C, 
( 225 ) 
Vol. XXVIII, No. 3 
April 19, 1924 
Key No. G-393 
