INSECTICIDES, FUNGICIDES, ETC. 191 



and, in extreme cases, they may split, owing to the growing cells 

 tearing themselves away from the dead ones. The spotting of the 

 leaves is most conspicuous in parts situated under patches of the 

 Bordeaux mixture, and seems to arise from the copper entering 

 at the stomata, or at the base of the fine covering hairs ; but the 

 yellowing of the leaves may occur when no spotting is observed, 

 and, in that case, the toxic substance has probably entered the 

 leaf at the axil, where it is united to the branch. The yellowing 

 would, on that view, depend on the strength of copper solution 

 which happened to reach the axils, and, hence, on the weather 

 conditions. This tallies with results obtained at Woburn : thus, 

 in 1908, a number of trees of Cox's Orange Pippin, sprayed with 

 various copper fungicides, were so much affected that most of 

 them become defoliated, whereas in the following season, the 

 injury under similar treatment affected little more than I per 

 cent, of the foliage (XI, 100). 



The erratic character of Bordeaux injury is always noticeable, 

 but it is certainly greater in damp, muggy weather, and as 

 regards the fruit, it is more marked when this is young. Accord- 

 ing to Hedrick, the injury does not become apparent till about 

 two weeks after the spraying, often not till after several months ; 

 but results obtained at Woburn (XI, 117) indicate a shorter 

 interval as usually elapsing, namely, about 14 days in the case 

 of ordinary Bordeaux mixture, and only 3j days in cases where 

 the spray- fluid contains nothing but soluble copper. It was also 

 found that the effect occurred two or three times more rapidly 

 in October than in August. This is not inconsistent with the 

 effect being greater in spring than in summer, for both in spring 

 and in autumn the leaves are in a susceptible condition, owing 

 to extreme youth in the one case, and to extreme age in the other. 

 Hedrick found no evidence that the lime in ordinary Bordeaux 

 mixture produced any scorching effect, but evidence that it has 

 such an effect has been afforded by the Woburn work (see p. 194), 

 though the main injury done by the mixture is, undoubtedly, 

 due to the copper. 



The connection between fungicidal and scorching action is a 

 very intimate one. In the investigations on scorching action 

 at Woburn, two methods of procedure were adopted: in the 

 one, leaves were immersed for (generally) 24 hours in the liquid, 

 taken out, washed, and the percentage of leaf -surf ace affected 

 estimated by an observer kept in ignorance of the treatment 

 applied; this immersion method was applicable only in cases 

 where the substance investigated was soluble : in the other, the 



