RUST OF ANTIRRHINUM. 



53 



Table 5. — ■ Average Number of Stomnta ■per Unit Area of Leaf. 



The averages of these figures show that there are 3.25 stomata on the 

 susceptible varieties to 1.46 stomata on the resistant varieties. Or, 

 stated differently, the resistant varieties have only 45 per cent as many 

 stomata as the susceptible varieties. The susceptible varieties showed 

 approximately 200 per cent as many uredinia as the resistant varieties. 

 This would indicate that in the snapdragon susceptibility is directly 

 proportional to the number of stomata; that is, doubUng the number of 

 stomata doubles the number of uredinia or the amount of infection. 

 Such a relation is, of course, relative rather than absolute. 



It may be added that the stomata on resistant and susceptible varieties 

 are present in the same numerical relation if both upper and lower epider- 

 mis are considered. The figures in Table 5 are for the upper epidermis 

 only, because, owing to the fact that but little water clings to the lower 

 epidermis, infection is mostly through the upper epidermis. 



The stoma is the gateway through which the fungus enters. The 

 fewer stomata there are, the fewer infections there will be, and the plant 

 will appear correspondingly resistant. 



Control. 

 Laboratory Toxicity Tests with Copper Fungicides. 

 In all the toxicological experiments here described the general method 

 used was that of Reddick and Wallace (1910). The fungicides used in 

 these toxicity tests were prepared by Dr. 0. R. Butler of the New Hamp- 

 shire Agricultural Experiment Station. Glass slides were cleaned in 

 potassium bichromate cleaning solution, rinsed in distilled water and 

 dried between filter papers. The solution, the toxicity of which was to 

 be tested, was sprayed on the slide by means of an atomizer, and the 

 slides were then dried for twenty-four hours. Fresh urediniospores 



