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 ga's 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 light. 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 winch occurs within the plant tissues when these are exposed 

 to varying concentrations of hydrocyanic acid. Studies of the effect 

 of tliis 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 tins 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 (12),' 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 (11) 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. In 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 



