1 9 1 9] CLA YTON—FUMIGA TION 489 



not only did the free water have no detrimental action, but actually, 

 under certain conditions, gave beneficial results. This was true 

 in the cases where the strength of fumigation was not sufficient to 

 injure the plants (below 0.008 gm. KCN): 



Growth during 12 days following fumigation 



Dry plants Wet plants 



Lot I : . . . ICX3 per cent 127 per cent 



Lot 2 100 per cent 115 per cent 



Lot 3 100 per cent 139 per cent 



With the following data, however, the fumigation was strong 

 enough to injure the plants: 



Dry plants Wet plants 



Lot I 100 per cent 115 per cent 



Lot 2 100 per cent qq per cent 



Lot 3 100 per cent 100 per cent 



Thus wetting the leaves had a beneficial eft'ect if the fumigation 

 was not strong enough to cause injury. With the appearance of 

 injury this beneficial action ordinarily disappeared. The results 

 just given are from experiments performed at wide intervals, and the 

 plants used varied greatly as to size and age. Moisture present in 

 pans as free water surfaces or saturated soil gave negative results. 



Temperature. — The effect of temperature on plant resistance 

 was determined quahtatively only. Excessive temperatures during 

 the period of fumigation very materially increase the amount of 

 injury. A variation of 20° F. has a marked effect; thus plants 

 fumigated at 75° F. suffered much less injury than others which 

 w^ere fumigated at 95°. In what way temperature affects the 

 plant, to cause these changes, has not been determined. One 

 possible explanation is that the effect is through changes in per- 

 meability of protoplasm, such as Rysselberghe, Lepeschkin, and 

 EcKERSON have shown to occur, or even in permeability of wall 

 structures. That it may be due to a change in the rate of chemical 

 reaction is of course possible. 



Light. — Light undoubtedly exerts a direct action on plant 

 resistance, but the conditions did not permit accurate observation 

 on this point. Here permeability changes as well as catalytic effects 

 of Light appear possible. The indirect action of Light as a regulator 



