270 BOTANICAL GAZETTE [OCTOBER 
TABLE II (X. echinatum). 
PER CENT. GERMINATED AFTER 
Temp. °C, SEEDS 
1 day 2 days 5 days 8 days 
Zoe 5, Gaara Sea upper ° fe) ° oO 
lower 3 31 87 99 
5 Var Pe upper 8 55 99 99 
lower 23 100 100 100 
is 23°. In X. canadense, X. glabratum, and X. speciosum the upper 
seeds germinate only sparingly at a constant temperature of 35°, but 
to a considerably larger per cent. at a temperature fluctuating between 
25 and 40°. The lower seeds of X. canadense germinate readily at 
18-21°, while the lower seeds of X. glabratum have a minimum 
germinative temperature of about 23°. The highest minimum ger- 
minative temperature yet reported, 15.6-18.5°, is recorded by 
DeETMER (2) for the cucumber and watermelon. 
From the above data it may be seen that in the cocklebur there 
are remarkably high minimum germinative temperatures. *- 
echinatum, the least remarkable of the four species studied in this 
respect, has this critical temperature 15° higher in the upper seed and 
5° higher in the lower one than that of the watermelon and cucumber. 
Effect of wounding. —In removing seeds from the burs the knife 
often clipped off a small portion of the distal ends of the cotyledons. 
It was observed that the upper seeds so wounded begin a marked 
growth in the wounded region even at the temperature of 20-22°.. 
The growth gradually moves down the cotyledons until it reaches. 
the radicle. This reverses the normal method of germination. Nor- 
mally the radicle first pushes out, sets itself in the ground, and lifts. 
the cotyledons above the soil, after which they begin their growth. 
When the upper seeds are wounded at the radicle end, either by a 
slight cut or a pin prick, germination takes place in the normal way- 
This observation suggested complete removal of the seed coat. 
Effect of removing the seed coat,—After the seeds have been soaked 
six hours the extremely delicate seed coats can be removed, without 
the least injury to the embryo, by merely pinching the seed between. 
thumb and finger. The coat-free upper and lower seeds of any one of 
the four species studied germinate with almost equal readiness at ay 
