36 S. E. A. McCallan 



the controls. However, when the leaves were injured or ground up, as 

 much as 0.009 per cent of copper was found in solution. These investi- 

 gators conclude that there is no evidence that leaf excretions ordinarily 

 are agencies in the solution of copper. The results do not appear to war- 

 rant such an unmodified conclusion. Unfortunately the leaves were detached 

 in all cases. 



Barker and Gimingham (1911) placed healthy shoots of gooseberry, 

 pear, currant, and apple, while still attached to the trees, in jars containing 

 a small quantity of bordeaux mixture in distilled water, and afterward, on 

 analysis of the water, could detect no soluble copper. However, when 

 no-excess-lime bordeaux was used, very appreciable quantities of soluble 

 copper were detected. The controls showed none. Parallel tests in the 

 laboratory with detached leaves gave the same results. Leaf scorching 

 was correlated with the presence of soluble copper. Later experiments by 

 these investigators (1914 a, 1914 b, 1914 c), on bordeaux injury to apple 

 leaves, showed that, in the case of leaves from summer foliage, scorching 

 by bordeaux resulted only at spots where an injury, naturally or purposely 

 made, allowed contact between the copper compound and the inner thin- 

 walled cells. With leaves from late autumn foliage, the scorching was 

 more general. The injury from no-excess-lime bordeaux was much more 

 pronounced in all cases. When uninjured apple leaves were immersed 

 directly in copper-sulfate solutions, very little general injury resulted. 

 Barker and Gimingham concluded that injured foliage possesses a solvent 

 action not exhibited by uninjured foliage. Presumably this is because the 

 interior cells have readily permeable walls unlike the heavily cuticularized 

 outside walls of the epidermis. 



The experimental evidence on the hypothesis* of solvent action by 

 suscept excretions is meager. There seems to be no satisfactory proof 

 that uninjured leaves exercise any solvent action. Injured leaves, on 

 the other hand, appear in all cases to exert some solvent action. How- 

 ever, it must be borne in mind that fungicidal toxicity is definitely 

 exhibited in vitro, so that even though the excretions from injured leaves 

 may be contributory to the solution of copper, they cannot be the sole 

 agents. 



SPORE EXCRETIONS 



Since the present investigations have been primarily directed toward 

 a study of the hypothesis that spore excretions exert the necessary solvent 

 action on the compounds, a fairly comprehensive review of the literature 

 is presented. 



Swingle (1896) was apparently the first to suggest the action of the 

 fungus spores themselves. He states (page 21 of leference cited) : 



Finally, it is possible that substances secreted or excreted by cells of parasitic fungi may 

 effect the solution of a small amount of copper. The ferments and cements secreted by the 



