1913] SHULL—SEMIPERMEABILITY OF SEED COATS 177 
and most of the acids, except perhaps hydrochloric and tartaric. 
The entrance of sulphuric is very slow. The triatomic alcohol. 
glycerol, does not enter, nor do the sugars. 
The semipermeability is not dependent on any living cells in the 
membrane, but is purely a physical phenomenon. Boiling the seeds 
does not destroy this character of the coat, nor do such poisons 
as HgCl, AgNO,, iodine, the alcohols, ether, etc. Solutions of 
sodium chloride extract water rapidly from seeds which have 
imbibed almost 70 per cent of their weight in 5 per cent HgCl.. 
The peculiarity of the testa must therefore de- 
pend upon the physical structure and chemi- 
cal composition of the dead cell walls, and upon 
the relations it may assume toward the various 
solutes and solvents, not upon any vital con- 
ditions. 
The seed coat of Xanthium possesses a great 
advantage over the membranes discovered by 
Brown and Scur6per in barley and wheat as 
an object of research; for in Xanthium the entire 
coat is easily removed after soaking for a few 
hours in water, and can then be used readily as 
an osmotic membrane. 
For this purpose I have used the apparatus Fic. 3.—Osmotic 
illustrated in fig. 3. A glass tube about 6cm.in apparatus used in 
length and 1.5 cm. in diameter is closed at both testing ete - 
ends with short rubber corks, each perforated by Yuidiscia tial coats: 
a hole 3 mm. in diameter. Since the corks must description in text. 
fit tightly into the glass tube, the perforation of 
one of the corks is prevented from collapsing by inserting a short 
piece of glass tubing 3 mm. in diameter. The testa is prepared for 
use by cutting it longitudinally along one side, and cutting off the 
ends. The testa can then be opened out and dried under a weight, 
thus producing a rectangular membrane about 6X8 mm. 
This membrane is cemented carefully to the rubber cork which 
has been prevented from collapsing by the narrow glass tubing. 
It is best to place a thin layer of wax over the cork first, then press 
the membrane firmly into the wax, after which the edges of the dry 


