THE LEAD ARSENATES. 7 



A second source of error was the necessity of using different varieties of 

 the fruit trees, in some cases, in different seasons. If different varieties 

 of the same kind ^■ary in their degi-ee of resistance, the results might be 

 expected to vary also, to some extent. This could not hold for the plum, 

 all tests with this being of the Bradshaw variety, and for the elm, wliich 

 was always the American elm. With the other trees the results do not 

 indicate, at least, that varietal difference was a factor, though it is generally 

 believed that the Bald\nn, for example, burns more easily than the Mc- 

 intosh. How much the results of these experiments were affected by 

 varietal differences cannot be determined. 



The third source of error is the possibility of a difference in the leaves as 

 the season progresses, the later sprays having, perhaps, been applied to 

 leaves which had already begun to "harden." Here, too, the results fail to 

 indicate that this was a factor. Burning occurred as frequently after the 

 late July and early August sprays as following the earlier ones, under 

 similar conditions of temperature and humidit}', and it would seem that 

 this possible source of error was of little if any importance. 



General Results. 



Some general conclusions from the investigation were : — 



1. The difference in sensitiveness between the upper and under surfaces 

 of the leaves is so slight as to be negligible. Not more than a dozen cases 

 of difference were observed out of nearly 1,600 aiDplications. In these few 

 the under surface showed the greater injury. Apparently in cases of 

 spray injur>^, it is not caused by the poison entering the leaf through the 

 stomata. 



2. Where insects or fungi had produced holes in the leaves, spray injury 

 was frequently observed around the edges of these holes, while the rest of 

 the leaf was not affected. "Wliether this injury- resulted from a freer 

 access of the poison to the inner leaf cells, or would have resulted in any 

 case, is perhaps uncertain. Such injury was not rated as injur}'- by spray- 

 ing where the unattacked remainder was not affected. 



3. It was frequently the case that ihjury did not appear until nearly a 

 week after spraj-ing (longer in some cases), and increased in severity later. 

 A branch graded as showing a trace of injury at the end of the first week 

 often increased to "slight" after two or three days, and even to "bad," in 

 a few cases, by the end of the second week. In general, though, the final 

 degree of injury had been reached after about twelve or thirteen days. 



4. It is well kno\\Ti that some kinds of foliage are more sensitive to ar- 

 senical sprays than others, but details as to this have hitherto been lacking. 

 These tests show that the pear and elm are the most resistant of the trees 

 used in the experiments; that the apple comes next, but is much less 

 resistant; that the cherry comes considerably below the apple in this 

 regard; and that the Bradshaw plum and the peach come some distance 

 below the cherry, and are about equally sensitive, the peach being prob- 

 ablv rather the more sensitive of the two. 



