272 BOTANICAL GAZETTE [OCTOBER 
Effect of increased oxygen pressures.—Since the delay is secured 
by the seed coat, it must exclude either water or oxygen. As ARTHUR 
states, both seeds seem to take up water with equal readiness. I found 
that in eighteen hours of soaking, the upper seeds of X. canadense im- 
bibed 51 per cent. of their dry weight, wliile the lower ones imbibed 62 
per cent. In the same time the upper seeds of X. echinatum imbibed 
48 per cent. and the lower ones 47 per cent. The difference then 
in water imbibition will not serve to explain the difference in the 
germinative characters of the two seeds. 
It was found best in testing increased oxygen pressures to soak 
the seeds 12-18 hours and then allow them to stick to the walls of a 
flask or bottle containing the oxygen or (in the check) air. After 
being thus treated and kept at 21-23° for six days, the upper seeds 
of X. canadense gave too per cent. of germination in pure oxygen 
and o per cent. in air. The growth in the seeds germinated in oxygen 
at these relatively low temperatures does not take place in the normal 
way. It begins in the distal 
region of the cotyledons and 
works down toward the 
radicle, as was described 
for seeds wounded at the 
distal end of the cotyledons. 
This peculiarity seems 10 
be related to the structural 
character of the seed coat. 
The seed coat consists 
of three distinct layers 
Fic. 2.—Cross section of the seed coat of (fig. 2). The outer layer con- 
Xanthium, showing the three layers. sists of shell-like cell walls 
which are more and more 
collapsed as the inner portion of the layer is-approached. This layer 
is traversed by several groups of tracheae which are parallel with 
the long axis of the seed. The middle layer is very dense, apparently 
consisting of collapsed cell walls and staining very deeply with saf 
franin, The inner layer consists of 1-5 layers of cells containing 
protoplasm and nuclei. Each layer is thickest at the radicle end 
and gradually becomes thinner toward the distal end of the cotyledons. 
