Semipermeability of Seed Coats

Abstract

1. The dry seed coats of Xanthium are impermeable to dry alcohol, ether, chloroform, and acetone. Becquerel'S results with the coats of other seeds are confirmed. 2. Evidence of the diffusion of oxygen through absolutely dry seed coats was not obtained. 3. Selective Semipermeability like that found in Hordeum has been demonstrated for the seed coat of Xanthium. 4. The following substances seem to be excluded: NaCl, CuSO4, K2CrO4, Na2S2O3, glycerol, sugars, HCl, tartaric acid. 5. The following enter, either slowly or rapidly: NH4NO3, AgNO3, NaNO3, KNO3, KCl, HgCl2, FeSO4, alcohols, ether, iodine, KOH, NaOH, H2SO4, HNO3, acetic acid, lactic acid, citric acid. 6. The selective activity is independent of any living substance in the seed coat. 7. The coat of Xanthium can be removed and used as an osmotic membrane, possessing a great advantage over the coats investigated by BROWN and SCHRODER. 8. The testa is composed of three layers, the outer of which cannot function as a semipermeable membrane. The middle layer is several cells thick, the inner layer one cell thick except in the chalazal region. This last layer is probably the nucellar epidermis. 9. By use of strong alkalies the inner membrane can be split loose from the middle layer. Both layers possess osmotic properties,the inner layer in a higher degree than the middle one. 10. Neither layer is as efficient alone as when both are left together. The impairing of the membranes may be due to stretching, or to the effects of the macerating agent. 11. The inner layer is nearly pure cellulose, unsuberized, but perhaps containing some tannin. The middle coat contains more tannin than the inner coat. 2. The tannin does not form a continuous layer in the seed coat. Moreover, treatment with tannin solvents does not destroy semipermeability. The evidence is adverse to REICHARDS'S view that semipermeability is due to tannin compounds. 13. Semipermeability has been demonstrated for the seed coats of a number of plants in six widely separated families. Many dead plant membranes may possess this property. 14. The capillary and imbibition force of the embryo of Xanthium as measured by the osmotic pressure of concentrated salt solutions is about 965 atmospheres when the seed is air-dry. 15. An increase in the moisture of the embryo equal to 7 per cent of its air-dry Weight reduces the internal forces by 590. atmospheres. 16. The unusual intake of water noticed with certain substances, especially with certain acids and alkalies, is due largely to the development of osmotically active substances inside the semipermeable membrane. 17. There is some evidence unfavorable to ARMSTRONG'S hydrone theory of selective semipermeability.