196 BOTANICAL GAZETTE [SEPTEMBER 
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, CuSO,, 
K.CrO,, Na.S.O;, glycerol, sugars, HCl, tartaric acid. 
5. The following enter, either slowly or rapidly: NH,NO,, 
AgNO;, NaNO,, KNO,;, KCl, HgCl., FeSO,, alcohols, ether, iodine, 
KOH, NaOH, H,SO,, HNO,, acetic acid, lactic acid, citric acid. 
6. The selective sg ae 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 leit 
together. The impairing of the membranes may be due to stretch- 
ing, or to the effects of the macerating agent. 
11. The inner layer i is nearly pure cellulose, cal but 
perhaps containing some tannin. The era coat contains more 
tannin than the inner coat. 
12. The tannin does not form a continuous i in the seed coat. 
Moreover, treatment with tannin solvents does not destroy semi- 
permeability. The evidence is adverse to REICHARD’s view that 
semipermeability i 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 Xam- 
thium 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 pet 
cent of its air-dry weight reduces the internal forces by 59° 
atmospheres. 


