296 University of California Publications in Agricultural Sciences [Vol.3 



water content, is sufficient in certain plants to cause abscission. In 

 the light of these observations abscission of plant parts when exposed 

 to smelter fumes is explainable purely on the basis of abnormal water 

 relations. 



In an effort to ascertain whether in the case of illuminating gas any 

 such relation holds true, we have made a careful study of the stomata 

 of citrus leaves and have to report that at an early period in the life 

 of the leaf they lose their power of functioning and remain practically 

 closed thereafter. This is significant in view of our findings mentioned 

 above, namely, that illuminating gas is not a direct stimulus to 

 abscission in Citrus, at least with excised shoots. In the case of potted 

 plants it seems probable that it works in an indirect manner through 

 disturbances in the physiological balance. In connection with the 

 question of the effect of illuminating gas upon the chlorophyll of the 

 guard cells, it should be mentioned that II. M. Richards and D. T. 

 MaeDougal 28 have reported that chlorphyll formation is greatly 

 retarded when the plant is subjected to an atmosphere containing 

 traces of this gas. 



The fumigation of citrus trees with hydrocyanic acid gas for the 

 control of scale insects is practiced quite generally and with marked 

 success in California. It is the general experience that under certain 

 conditions heavy dosages of this gas result in abscission of the older 

 leaves. 29 Researches by Osterhout 30 and Moore and Willaman 31 have 

 shown that when subjected to traces of this gas the permeability of 

 cytoplasmic septa is markedly altered, causing an increased loss of 

 water. In the light of these observations it is entirely possible to 

 explain dropping of citrus leaves due to fumigation on a purely water 

 relation basis. 



Fumigation injury to the blossoms or fruit, whether large or very 

 small, consists of pitting and burning which results in scars on the 

 fruit. Apparently in no case does fumigation of young Navel oranges 

 with hydrocyanic acid gas furnish a stimulus to abscission. 



The whole subject of the effect of gases in causing abscission of 

 plant parts is in a very unsatisfactory state at the present time. In 

 view of the mass of conflicting data, as well as the fact that abscission 



28 The Influence of Carbon Monoxide and other Gases upon Plants, Bull. Torr. 

 Bot. Club, vol. 31 (1904), pp. 57-66. 



29 Woodworth, C. W., and others, School of Fumigation, Pomona, California, 

 pp. 162-64, August, 1915. 



80 Similarity in the Effects of Potassium Cvanide and of Ether, Bot. Gaz., 

 vol. 63 (1917), pp. 77-80. 



3i Studies in Greenhouse Fumigation with Hydrocyanic Acid: Physiological 

 Effects on the Plant, Jour. Agr. Ees., vol. 11 (19J7), pp. 319-38. 



