338 BOTANICAL GAZETTE [may 
general considerably the more sensitive. Two varieties of these, 
under the trade names Earl Cromer and Gladys Unwin (Vaughan’s 
Seed Store), were especially sensitive. 
II. Methods 
GROWING THE SEEDLINGS 
Gladys Unwin and Earl Cromer are both hard-coated varieties. 
After 24 hours’ soaking in water, about 10 per cent swell, and 10 
days are required for all to swell. Consequently, in order to get 
uniform growth in the seedlings, the coats are scratched with a 
file and the seeds soaked for 15 hours in distilled water. They are 
then thoroughly shaken up with several changes of distilled water 
and placed in a thin layer between sterile wet filter papers and 
allowed to germinate. When the longest hypocotyls have reached 
a length of 5 mm., the seeds are transferred to wet sterile absorbent 
cotton in large petri dishes and allowed to grow in total darkness 
at 20°-24° C. until the epicotyls have an average height of 2-3 cm. 
This method gives cultures fungus-free and of far more nearly uni- 
form growth than can be obtained by less careful methods. It also 
gives cultures bearing only the more vigorous seedlings. This is 
especially important, for sensitiveness to atmospheric impurities 
rises with the vigor of the seedlings. With all these precautions 
there is considerable irregularity in the rate of growth of the 
various epicotyls in a culture. The entire process must be carried 
on in what the German workers have called pure air, which means 
air practically free from the heavy hydrocarbons, especially 
ethylene. A quantity of this substance equaling one part in ten 
million of atmosphere interferes with the growth of the seedling. 
In case the gas to be tested is very soluble in water, it is neces- 
sary to protect it against contact with the moist cotton bearing 
the roots. This is done by covering the substratum with low- | 
melting paraffin mixed with pure paraffin oil. In many cases it 
was also found desirable to have a desiccating agent in the experi- 
mental chamber to keep its walls free from water given off by trans- 
piration. This-especially holds for gases like SO, and NH. 
