34 BULLETIN 907, U. S. DEPARTMENT OF AGRICULTURE. . 



This shows the comparative resistance of the two types of leaves. 

 Mature citrus leaves are so resistant to cyanid that it appears that the 

 concentration needed to produce injury is so great that the gas pene- 

 trates the tissues whether the stomata are open or closed 7 and the 

 amount which enters while the stomata are open over that entering 

 while closed 1 is insufficient to modify to any great extent the degree 

 of injury. Of particular interest in this connection are the obser- 

 vations of Coit and Hodgson {2) that early in the life of the leaf the 

 stomata lose their power of opening and closing and remain for the 

 most part practically closed thereafter. On the other hand, such 

 tender plants as Tradescantia react to such small amounts of cyanid 

 that the stomata seem able to exercise an influence on the passage 

 of gas sufficient to modify the effect on the plant. 



SUNSHINE. 



The earliest fumigation of citrus trees was performed during the day- 

 time and accompanied by much injury. Coquillett (3) offered as an 

 explanation of this result that "in the daytime the light and heat 

 decompose the gas," the assumption being that the products of 

 decomposition are more injurious than the hydrocyanic acid gas 

 itself. In an effort to correct the action of sunlight blackened tents 

 were used but without marked success. The final solution was night 

 fumigation. American writers on fumigation who have experimented 

 with sunshine work have been unanimous in proclaiming its impracti- 

 cability as a general practice. 



The results of the many experiments presented in this paper show 

 that sunshine is one of the most important factors influencing injury 

 and that its effects are not confined solely to the period of exposure, 

 but are also exerted immediately before and immediately after the 

 treatment. The postfumigation influence appears to be somewhat 

 greater than the prefumigation influence and its effects are sometimes 

 so injurious as to discolor fruit (PL IV, B, a) and destroy the foliage of 

 plants which, if protected from the direct sun, would have been but 

 slightly affected (PL I, B). This action on the foliage is most con- 

 spicuous through burning which, if including the entire petiole as 

 well as the blade, results in the destroyed foliage clinging to the tree 

 until exfoliation takes place by mechanical means. (PL II, B.) 

 This postfumigation sunshine influence in such experiments as No. 10, 

 in which plants in the dark were fumigated in the dark and placed in 

 the sunshine with the result of severe injury, would appear to disprove 

 Coquillett's explanation of decomposition of the gas, for in this case 

 the sunshine did not reach the plants until they had been removed 

 from the fumigatorium. 



i According to MacDougal (10) stomata do not close their pores so tightly that some gaseous diffusion 

 may not take place through their diminished opening. 



