494 



THE GYPSY MOTH. 



The lime is understood to be quicklime^ the soda to be crystal- 

 lized neutral soda carbonate. 



It is thus seen that either with the use of lime or of soda a 

 certain quantity of arsenious oxide is removed from Paris .green 

 beyond that which water alone would remove. The actual amount 

 removed depends upon the proportions of the substances mixed, 

 as well as the time allowed for solution. In every case this amount 

 is in the form either of lime or soda arsenite, and is in solution. 



The ' ' burning " of foliage which occurs as the result of 

 spraying with arsenical poisons is due to the presence of 

 soluble arsenic, which is found to some extent in Paris 

 green and to a greater degree in London purple. Other 

 poisons which contain arsenic- in a practically insoluble 

 form, as, for instance, arsenate of zinc, when sprayed on 

 the foliage and fully exposed to atmosj^heric influences as 

 well as to the physiological action exerted by the leaves 

 themselves, sometimes decompose to a certain extent, and 

 give rise to soluble compounds of arsenic. The efi'ect of 

 soluble arsenic on leaves is to kill the protoplasmic contents 

 of the cells which compose the living or assimilative part 

 of the leaf,' as is shown by the illustrations given below : — 



Fig. 1. Section through normal leaf of 

 pig-nnt hickory (Carya porcina) , show- 

 ing structural elements. epidermis; 

 PC, palisade cells; SP, spongy paren- 

 chyma; S, stoma or breathing pore. 

 {After Kirkland.) 



Fig. 2. Section through leaf of same 

 tree, •'burned" by excess of Paris 

 green. Cell contents {protoplasm) of 

 palisade cells and spongy parenchyma 

 dead and dried up ; cell walls shrivelled. 

 {After Kirkland.) 



Leaf Area or Trees. 

 To gain a better knowledge of the surface which needs to 

 be covered in spraying operations, during the summer of 



