no 
WM. CROCKER 
by the fact that they can lie in a saturated solution of copper sulfate or 
47V sodium chloride for weeks without marked injury. Examination 
will show whether leaching is further prevented by stored foods being 
held in the condensed condition. A thorough study of respiratory 
rate under conditions of temperature and moisture approaching those 
of soil dormancy will give an idea of food loss from this source and 
with leaching losses will indicate whether the longevity of these seeds 
in nature is determined by starvation. Life duration experiments 
such as Groves and I have conducted upon wheat may offer evidence 
as to whether these seeds will persist longer in dry storage or whether 
some process similar to Lepeschkin's postulated redispersal theory 
increases their longevity in the imbibed condition. 
4. Of the seeds that are delayed in germination by seed or 
fruit coats reducing the oxygen supply below the minimum for ger- 
mination, various species of Xanthium have ^ been most thoroughly 
studied. The character appears in both the upper and lower seeds of 
the bur but is more marked in the former. The seeds grow readily 
when the testas are removed. Increased oxygen pressure or hydro- 
gen peroxide induce germination of the intact seeds. Under normal 
oxygen pressure high temperature acts as a forcing agent. This gives 
the odd phenomenon of two temperature minima for germination — 
one with coats removed and a much higher one with coats intact. In 
nature the lower seed generally germinates the year after ripening and 
the upper the following year. This regularity is often broken up 
by agencies that modify the testa or by high temperatures during the 
first summer. 
Shull24 in published articles and Denny in unpublished work have 
done much to clear up the mechanics of the behavior of Xanthium 
seeds toward oxygen. The naked embryos absorb much more oxygen 
from the air than embryos in the testas. Embryos in intact seeds show 
an increase in oxygen absorption with increase in the partial oxygen 
pressure of the atmosphere. These facts establish the important point 
of increased consumption of oxygen under oxygen supplies favoring 
germination. The naked embryos of both seeds have rather definite 
oxygen pressure minima for germination ; this is considerably higher 
for the upper than for the lower seed and it falls for both as the tempera- 
ture rises. These facts explain in part both the differences in behavior 
23 Crocker. Bot. Gaz. 42: 265. 1906. 
24Shull. Bot. Gaz. 52: 453. 191 1. 
