34 BULLETIN 844, U. S. DEPARTMENT OF AGRICULTURE. 



suberin, but a considerable amount of cellulose and pectose. All 

 cell walls below the light line are mainly cellulose but contain some 

 pectose. The upper portion of the light line contains callose, but 

 the remainder of the light line appears to be chemically different 

 from all other parts of the seed coat or else so dense as to resist the 

 attack of the reagents. 



THE SEED COAT IN RELATION TO THE ABSORPTION OF WATER. 



A study of permeable seeds soaked in water containing stains 

 showed that there were no local regions through which the water 

 passed. The stains passed through all regions of the seed coat. 

 Coating the micropylar region with vaseline retarded germination, 

 but had no effect upon the percentage of germination at the end of 

 three days. In seed coats through which the stain had passed, the 

 light line was not stained. Some stain was found in the canals which 

 crossed the light line, and much more in the cell cavities. There was 

 no evidence that the stain had permeated the substance of the light 

 line. It was able to cross the light line only when pores were present. 



In impermeable seeds the stains passed readily to the light line. 

 (PL V, fig. 7.) It was evident that the absorption of water was not 

 prevented by either the cuticularized layer or the cone-shaped struc- 

 tures of the Malpighian layer, but by the light line. The region out- 

 side of the light line became stained in a few hours, but there was no 

 trace of the stain within the light line after the seeds had remained 

 a week in the stains. Alcohol did not penetrate the seed coat more 

 readily than water. 



A COMPARISON OF PERMEABLE AND IMPERMEABLE SEED COATS. 



No difference in chemical structure was found between the coats of 

 permeable and impermeable seeds. The principal differences were 

 in the character and amount of thickening of the cell walls. 



In many of the permeable seeds some of the canals were found to 

 extend across the light line, but this was not true for all permeable 

 seeds. (PL V, fig. 8.) Oblique sections of permeable seed coats 

 showed that the cell cavities, although reduced to mere pores by the 

 thickening of their radial walls, extended across the light line into 

 the base of the cones, thus forming a passageway through which the 

 stains passed to the larger portions of the cell cavities below the light 

 line. (PL V, fig. 4.) 



In the coats of the impermeable seeds the light line was usually 

 broader, the Malpighian cells thickened more below the light line, 

 and the main cavities of the Malpighian cells were more reduced and 

 farther below the light line than in the coats of permeable seeds. 

 (PL V, fig. 6.) No canals except occasionally a few very small ones 

 were seen crossing the light line in impermeable seeds. Cross and 



