94 GRKENHOl'SE FL'MICATION 



and studied (3). Idle a\'erag"e of twenty measurements of a pea is 

 given as an example. 



Control at Fumigated Fumigated Fumigated Control 



start 5 min. 30 min. 60 min. 60 min. 



2.15 m 2.21m 2.32 m 2.72 m 2.61 m 



Pfelifer states that "'Filtration and gaseous ditiusion 

 takes place through the stomata, lenticels, etc.. and their 

 rapidity varies inversely as the square root of the densit}' 

 of the gas concerned" (4). Inasmuch as hydrocyanic acid 

 does not cause the stomata to close/" it would therefore gain 

 entrance inversely as the square root of its density. The 

 rate of diffusion of carbon dioxide known to enter the 

 stomata. compared with air is .8087 while hydrocyanic acid 

 is 1.0276. 1 lvdroc_\anic acid could therefore more easily 

 enter the plant \)y means c^f the open stomata than carlion 

 dioxide. 



This conclusion agrees with the results obtained in 

 fumigation. Plants fumigated during ])right sunlight when 

 the stomata are open will show more injury than similar 

 plants fumigated at night when the stomata are closed. 

 hTirther. the nature of this injury is a spotting of the mature 

 leaves, the stomata of which are abundant and active while 

 injury produced by night fumigation is in general confined 

 to the youngest leaves having the thinnest cuticles. 



According to Pfefifer the passage of a gas through the 

 cuticle of a plant depends upon its solubility in water and 

 the extent to which the cuticle is impregnated with cutin. 

 or suberin. The following experiments were devised to 

 show if hydrocyanic acid could pass through the cuticle of 

 greenhouse plants. 



Lettuce plants exposed to the vapor of osmic acid in 

 the sunlight when the stomata are open will show a spot- 

 ting or mottling of the mature leaves. Examination of the 

 cuticle reveals that the si)Otting is due to the penetration of 

 the gas through the stomata. A similar burning of the 

 leaves is produced by hydrocyanic acid fumigation under 

 similar conditions ])ut the injured areas are not as sharplv 

 defined, due to the great solubility of the hydrocvanic acid 

 and its ditTusion through the tissues of the leaves. If a 

 lettuce plant with closed stomata is exposed in the dark to 



Fie:. 22 

 /If the plant takes in sufficient hydrocyanic aciil tcj be injured the stomata close after the 

 funngation or during the fumigation if it exceeds an hour exposure or if the dose is very strong 

 resultmg ni the immediate injury of the i)lant. 



