2 
Different species and varieties of plants, however, were found to vary 
remarkably in their power of withstanding the poison. This in 
many cases appeared to depend upon the open or closed condition of 
the breathing pores as well as upon peculiarities of the cell contents. 
Fumigation an hour or two after sundown, with the temperature 
as low as practicable, was found to give the best results. In each 
case the proper amount of gas to use and the length of exposure 
must be determined by experiment. It is impossible at present to 
give a general rule applicable to all plants in all stages of develop- 
ment. When the conditions are once determined they must be 
strictly followed to insure the greatest success. Methods of experi- 
menting will be described in the latter part of this circular. The 
quantity of gas in each case is always given in terms of the potassium 
cyanide from which it is made, and on a basis of a cubic foot of space, 

Fie. 1.—Fumigating box showing trays and coleus cuttings (original). 
as will be explained more in detail further on. The cyanide, as well 
as the gas made from it, is exceedingly poisonous, and both must be 
used with the greatest care. 
CROPS AND PESTS ON WHICH THE GAS HAS BEEN SUCCESSFULLY 
USED. p 
Ferns.—For Davallia mooreana infested with a scale insect 
(Chionaspis sp.), 0.075 gram of 98 per cent potassium cyanide 
should be used for each cubic foot of space to be fumigated, not 
deducting the space occupied by the plants. Length of exposure, 
twenty minutes. 
One hundred and fifty to two hundred plants with fronds in all 
stages of development have been thus treated two or three times each 
year for the past four years with no injury to the plants and almost 
complete destruction of the insect. They were treated fifty at a time 
in a fumigating box (fig. 1), described later. 
