16 BULLETIN 907, IT. S. DEPARTMENT OF AGRICULTURE. 



An examination of this table shows that a dosage which slightly 

 injures merely the tenderest growth of plants placed either in the 

 shade or darkness after fumigation may result in severe burning of 

 even the old, resistant leaves if the treated plants are exposed to 

 sunlight (PI. II, A), while in extreme cases complete defoliation fol- 

 lows, as happened in experiments 13 and 19. Increased injury due 

 to sunshine was apparent even at as low a sun temperature as 65° 

 F., which was the minimum experienced (experiment 11). The 

 plants in this case were comparatively tender, and this condition 

 naturally invited greater injury than would have been the case with 

 more resistant foliage. A study of the data presented in these experi- 

 ments would appear to indicate that, all other conditions being the 

 same, injury increases more or less directly with the sun temperature. 

 The plants in experiment 11 were somewhat tender yet the degree of 

 injury in the case of those exposed to the sunshine (65° F.) was less 

 than to the plants in a hardened condition in experiment 19, in which 

 the postfumigation sunshine temperature was 87° F. In this latter 

 experiment the hardened or resistant growth of plants placed in the 

 sunshine was almost completely destroyed, although plants placed 

 in the shade at the same temperature merely had the tenderest growth 

 injured (PI. II, A). This would indicate that sunshine at such a high 

 temperature is more injurious than sunshine at a low temperature; 

 in short, that the toxic action of hydrocyanic acid on fumigated 

 plants subjected to sunshine increases with the sun's intensity. 



Turning to experiment 12 it is seen that the sun-exposed plants 

 were no more injured than those in the shade at a slightly higher tem- 

 perature. It so happened that in this experiment the fumigatorium 

 contained a piece of ice which had been used to maintain a low tem- 

 perature prior to the treatment. This ice absorbed gas with the 

 result that the strength of the gas was greatly reduced at the end of 

 the hour's exposure. A conclusion to be deduced from this experi- 

 ment is that plants exposed either to a strong gas for a short period 

 or to a dilute gas for a longer period before being placed in the sun- 

 shine are much less injured by fumigation than plants exposed to a 

 gas which maintains its proper strength throughout the period of an 

 hour's exposure, as in the other nine experiments. Data in support 

 of this statement are found elsewhere in this paper and, furtheimore, 

 are supported by other experiments performed by the author but not 

 included in this article. 



The relation of the temperature of fumigation to injury to plants 

 subsequently exposed to sunshine is not well shown by the experi- 

 mental data presented. Plants in experiments 10, 11, 13, 16, and 



17 were fumigated at comparatively cool temperatures (60°-68° F.), 

 yet the injury was about as severe to those plants subsequently placed 

 in the sunshine as to those under similar postfumigation conditions 



