1913] SHULL—SEMIPERMEABILITY OF SEED COATS 179 
formed in the same way as in another of the Compositae, Helianthus, 
the structure of which has been reported by BRANDZA (9). 
In Helianthus the inner layer of the 3-layered seed coat is formed 
from the nucellar epidermis, and is composed of just one layer of 
cutinized cells. Xanthium likewise has a 3-layered testa, the inner 
layer of which is a single layer of cells excepting over the hypocotyl, 
where, as CROCKER has shown, it is several cells thick. It is probable 
that this inner layer, with the exception of its chalazal portion, 
which lies immediately over the hypocotyl, is derived from the 
epidermis of the nucellus. 
At first it was suspected that the inner stratum might be the 
semipermeable layer, although Crocker had suggested in 1906 
that the thicker middle one probably excluded oxygen. While 
attempt was being made to locate the seat of the semipermeability 
it was discovered that certain substances, the higher alcohols, 
acetic acid, and the alkalies, occasionally cause a bursting of the 
middle layer without injuring the single-celled inner lamina. The 
outer layer, of course, is too loose and chaffy to function osmotically. 
In the case of M and M/ro solutions of KOH and NaOH, the 
bursting of the middle layer is followed by a periclinal separation of 
the inner layer from the middle one, which is probably the ovular 
integument, so that the embryo, surrounded by the intact, thi 
delicate, one-celled inner layer can be removed from the extecial 
coats. This discovery was a very important aid in studying the 
semipermeability of the different layers of the testa, and illustrates 
the advantage which removable coats have over such as have been 
found in the grains. In the barley and wheat all such investigations 
had to be indirect, because the membrane could not be removed or 
separated into distinct layers. 
The thin inner coat was shown to be a very efficient semi- 
permeable membrane by treating the much swollen seeds sur- 
rounded by this membrane with 4M—NaCl. At the time one such 
seed was introduced into the salt solution it weighed 110 mg. After 
one hour the membrane was completely collapsed and pressed very 
tightly against the embryo. It weighed 58 mg. In 4 hours it 
weighed 52 mg., in 10 hours 51 mg., after which a rise in weight 
gradually occurred, probably due to a very slow entrance of NaCl. 
