GREENHOUSE FUMIG.\TION 95 



the vapor of osmic acid the youngest leaves turn uniformly black, 

 and if the time of exposure is extended the other leaves take on a 

 similar appearance from the youngest to the oldest. One does not 

 obtain a spotting as in exposure in sunlight. x'Vn examination of 

 the cuticle shows a uniftirm l)lackening due to the penetration of 

 the osmic acid vapor through the cuticle and not by way of the 

 stomata. Exposure to hydrocyanic acid under similar conditions 

 results in a burning and death of the yoimgest to the oldest leaves 

 according to the length of exposure. Since the penetration of a 

 thin cuticle, such as lettuce, depends upon the solubility of the gas 

 or vapor concerned, naturally hydrocyanic acid, which is soluble to 

 infinity in water, could pass where osmic acid, which is but slightly 

 soluble, gained entrance. 



To test the passage of hydrocyanic acid through cuticles of 

 different thicknesses and degrees of cutinization ,an apparatus 

 shown in figure 1 was devised. The upper end was closed with a 

 piece of cork through which a hole about one-third of an inch in 

 diameter was made. A very fine mesh wire was placed over this 

 opening and fastened to the cork with sealing- wax. Leaves without 

 stomata on the upper surface were selected and a piece cut from 

 the desired leaf was placed with the upper surface next to the wire. 

 The leaf w^as then carefully sealed down with wax, making the 

 chamber air tight. The lower portion of the tube was filled with 

 hydrocyanic acid gas by collection over mercury, and while the 

 lower end remained in the dish of mercury, the dividing stop-cock 

 was opened. The upper surface of the leaf was thus exposed to the 

 hydrocyanic acid gas and as it was absorbed by the cuticle the mer- 

 cury rose in the tube. The accompanying table gives data obtained 

 by the use of this apparatus. 



Temperature Temperature Average Thickness 



Plant at Start at Close Time Total Rise per hour of Cuticle 



Figure 23 gives the appearance of cross sections of these 

 leaves. An increase in thickness is generally accompanied by an 

 increase of cutin or suberin which will stain with sapranin. Al- 



