490 BOTANICAL GAZETTE [june 



of stomatal aperture and in connection with photosynthetic activity 

 will be discussed later. 



PLANT FACTORS 



Protective membranes, — Most plants which are highly 

 resistant to cyanide are characterized by having thickly cutinized 

 or suberized epidermal membranes which serve as a protection. 

 As further evidence of the protective power of these, Tradescantia 

 zebrina is made perceptibly more resistant by coating the upper 

 surfaces of the leaves with Blackman's wax. The Tradescantia 

 leaf has no stomata on the upper surface, and the reduction in 

 injury must be due to the wax covering, thus making the thin 

 epidermis relatively impervious. This increased resistance, when 

 unobscured by a large amount of stomatal activity, is very marked. 

 It is by no means possible to explain all differences in resistance on 

 the basis of protective membranes, however. The radish endures 

 without injury 3 times the strength of fumigation which a tomato 

 endures, yet microchemical examination reveals but little difference 

 in the cuticular development. 



Stomata.^ — The stomata seem to be the most important single 

 factor in determining the amount of injury resulting from hydro- 

 cyanic acid fumigation. To ascertain the extent of their influence 

 experiments were conducted in the following manner. Fumigations 

 were run at various times of day and night, using like strengths of 

 cyanide. Each lot of plants exposed included tomatoes and 

 Tradescantia zebrina, the leaves of the latter being painted in various 

 ways with Blackman's wax. After the beginning of each fumiga- 

 tion samples of epidermis were taken from several species with large 

 stomata {Geranium and Tradescantia), and the amount of stomatal 

 opening determined under the microscope. 



Fumigations conducted on a very dark rainy day (0.02 gm. kcn 



PER CUBIC foot) 

 Exposure 1 : 30 TO 3 : 30 P.M. 



Tomatoes Badly injured 



Tradescantia 



Leaves under surface coated . . Uninjured (stomatal surface closed) 

 Leaves upper surface coated . . Badly injured (stomatal surface open) 



Leaves untreated Killed (stomatal surface open) 



Average stomatal opening i : 30 p.m., 3.5/1 



