1906] CROCKER—DELAYED GERMINATION 269 
gathered gave no marked germinative variations in the species used, 
the time of storage and the region in which they were collected need 
no further consideration. A seed was considered germinated when 
root hairs appeared, except in Xanthium and Iris, where late develop- 
ment of root hairs made this test worthless and the lengthening of 
the radicle 5™™ was considered the criterion of germination. 
- III. Experiments. 
I, COCKLEBUR. 
Effect of enzymes——When work was begun on the seeds of the 
cocklebur, ARTHUR’s enzyme theory was adopted as a working hypo- 
thesis, on the supposition that it would involve a study of the differ- 
ence in the development and action of the enzymes in the two seeds. 
As WaucH (13), STONE (10), and THompson (11) were able to 
increase markedly the germination of old seeds by soaking them in 
solutions of various enzymes, it was thought that perhaps the upper 
seeds of the cocklebur could be made to germinate without delay 
by merely soaking them in solutions of pepsin, or plant trypsin, or 
in filtered extracts from the germinated lower seeds. Experiments 
in this line gave only negative results, but led to the discovery that 
high temperatures cause the immediate germination of some of the 
upper seeds, The enzyme theory was abandoned after failures to 
detect any differences in the digestive activities of extracts of the 
upper and lower seeds, At this point the results showed that the 
difference in the germinative characters of the two seeds had other 
causes; hence a new line of experiments was begun. 
; Effect of high temperatures.—High temperatures bring about the 
immediate germination of the upper seeds of X. echinatum. Table 
II shows the results of temperature experiments with this species. 
Results were similar whether soil or filter paper was used for germi- 
nators and whether the seeds were in the burs or removed from 
them, 
As Table IT shows, the lower seeds of X. echinatum germinate . 
readily at 22-24° but even more readily at 32-34°. The upper seeds 
do not germinate at all at 22-24°, but respond readily at 32-34°. 
The lowest temperature at which any considerable per cent. of the 
upper seeds grow is at 33°, and the similar point for the lower seeds 
