2 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. 
as well as environmental, which prevail during the actual ex- 
posure of plants to the gas. The prefumigation and postfumiga- 
tion environments have been given scant attention. The writer 
early in his fumigation studies observed types of injury not fully 
explainable by influences during the gas exposure, and subsequently 
it developed that certain factors must be considered, not only during 
but also before and after the gas treatment. Accordingly, a series 
of experiments was performed to determine the prefumigation and 
postfumigation influence, if any, of the two very important factors, 
heat and hight. This paper presents the results of these experiments, 
and furthermore interprets the results in the light of field experience. 
In the discussion it has been found necessary to touch on other 
factors which also bear on the subject of fumigation injury. 
THE EFFECT OF HYDROCYANIC ACID ON PLANTS. 
The modification of plant injury by most external factors can be 
ascertained with sufficient accuracy and comprehensiveness to guide 
field work without attempting to determine the actual physiological 
action which occurs within the plant tissues when these are exposed 
to varying concentrations of hydrocyanic acid. Studies of the effect 
of this gas on plant metabolism have been made, however, and some 
very important papers have appeared setting forth the results. of 
careful research on this subject. One of the earliest comprehensive 
papers confined to this subject was issued by Schroeder (17), in 
which he concluded, as the result of a long series of determinations on 
the effect of potassium cyanid on the fungus Aspergillus niger, that 
injury arises through paralysis of respiration, but that the reduced 
respiration is followed by complete recovery when the poison period 
does not last too long. Moore and Willaman (/2), working with 
greenhouse plants, similarly conclude that the absorption of more or 
less hydrocyanic acid by plants results in a reduction of respiratory 
activity, and show that this inhibitory effect on respiration is due 
primarily to disturbance of the respiratory enzymes, oxidases, and 
catalase. Various other physiological effects resulting are the inhi- 
bition of photosynthesis and translocation of carbohydrates; also an 
increase in the permeability of the leaf septa. 
Since the passage of gases takes place between the open air and the 
intercellular spaces of leaves through the stomata, it has been 
believed by most investigators of fumigation that hydrocyanic acid 
gains entrance into the tissues of fumigated plants through these 
openings. Researches by Moore (//) led to the conclusion that during 
fumigation hydrocyanic acid not only does enter plants through the 
stomata, if they are open, but also through the cuticle, depending 
upon its thickness and degree of cutinization. Im a recent paper 
Clayton (1) emphasizes that the stomata seem to be the most impor- 
tant single factor in determining the amount of injury resulting from 
