358 GROWTH OF PLANTS 



occur from external applications of pyrethrum concentrates under condi- 

 tions where no tracheal penetration takes place. Evidence is presented that 

 pyrethrum can penetrate the integument of insects through the articular 

 membranes into the trichopores and permeate the cells of the hypodermis. 



Histological changes were detected in the central nervous system of 

 insects killed by pyrethrum, piperine, and organic thiocyanates by means 

 of stains. 



The uses of activators \\ath pyrethrum presented new problems in the 

 mode of action of the combined sprays. The histological effects of pyreth- 

 rum and isobutylundecyleneamide, an activator of pyrethrum, were studied. 

 A widespread clumping effect of the chromatin of the nuclei was observed 

 for pyrethrum, while the activator isobutylundecyleneamide caused chro- 

 matolysis or dissolution of the chromatin in preparations stained with 

 Bodian's method. The combination of these two agents showed a histo- 

 logical picture that is a summation of the effect of both. 



Piperine causes destruction of the fiber tracts of the brain of the housefly. 



The physical condition of the pyrethrins at the time of application was 

 found to be the determining factor in their relative toxicity. When extracts 

 high in pyrethrin I were compared with extracts high in pyrethrin II, 

 using acetone as a solvent, the pyrethrin I extracts were considerably more 

 toxic than extracts high m pyrethrin II. When a miscible oil was used 

 as a solvent the differences tended to disappear. 



An improvement in the method of the determination of pyrethrin I was 

 made by Wilcoxon, which with slight modification has been adopted by 

 the Association of Official Agricultural Chemists. 



Various esters of chrysanthemum monocarboxylic acid were prepared by 

 Harvill. The lauryl, myristyl, cetyl, and diethanolamine esters were slightly 

 less toxic than the pyrethrins at the same concentrations. 



A number of organic thiocyanates were prepared. 7-Thiocyanopropyl 

 ether gave excellent control of Aphis rumicis, mealy bugs, and red spider. 

 The toxicity of mixtures of equal parts of rotenone and lauryl thiocyanates 

 was the same as that of a spray containing rotenone alone at a concentra- 

 tion equal to the total concentration of the mixture and greater than a 

 spray containing thiocyanates alone at this concentration. 



A method of fumigation with naphthalene was devised which permits 

 control of the concentration of naphthalene vapor in the air of a greenhouse, 

 which insures that the desired concentration ^vill be maintained throughout 

 the fumigation period. The method involves the continued recirculation 

 of the greenhouse air through a saturator containing a solution of naphtha- 

 lene in a solvent. The concentration of the naphthalene m the solvent 

 determines the maximum concentration which can be reached in the green- 

 house air. It was found that a satisfactory control for red spider could be 

 obtained by fumigation with this method without injury to most plants 

 usually considered sensitive to naphthalene fumigation, if a sufficient time 

 interval elapses between fumigations to avoid hardening of the plant tissues. 



