1913]. SHULL—SEMIPERMEABILITY OF SEED COATS 173 
diffuse through the thoroughly dried seed coats of certain seeds. 
CROCKER (14) was inclined to believe, from his study of the respira- 
tory ratio in Xanthium seeds, that more oxygen diffused through the 
dry seed coats than through those which had not been thoroughly 
dried. This conclusion is probably shown now to be incorrect. 
The gas exchange noted by him in the case of dried coats may have 
occurred at least partially through defects in the coats; for, as I 
shall show later, there are a good many seeds which prove to have 
defects which are invisible, even on microscopic examination, and 
these defects may — be increased in number by the process 
of drying. 
2. SELECTIVE SEMIPERMEABILITY OF XANTHIUM SEED COATS 
A few preliminary tests to determine the amount and rate of 
imbibition of Xanthium seeds in solutions of sodium chloride of 
various strengths indicated the exist- 
ence of a very efficient semipermeable 
membrane in the seed coat. Water 
enters the seed very rapidly under the 
powerful forces of capillarity and imbi- _,. 
bition until more than 50 per cent of 
the air-dry weight of the seeds has been» 
taken up. Nearly all of the water is 
taken up in 12~15 hours, as shown by ~* 
the curves in figs. 1 and 2. 
This rapid intake of water is in 
sharp contrast with that which occurs 
in the Gramineae. In the seeds of the Pitdiiiiiiipiiig 
latter the rate of water intake is con- fic, i.—Curves of imbibition 
siderably slower, and extends over 7 or for seeds of Xanthium in salt 
8 days or even longer, before saturation Solutions of various strengths, 
: : . and in water; time element 
is reached. Evidently the protective 
structures influence the rate at which sae, percentage increase over 
water passes into the seeds. air-dry weight on the axis of 
When strong salt solutions instead — 
of distilled water are used with Xanthium seeds, the powerful 
internal forces, capillarity, imbibition, and perhaps the chemical 
tug 
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