16 Bordeaux Mixture and Plants 



after immersion for one lionr in a 5 % solution or for half an hour in 

 a 10 % sohition the general surface of healthy leaves is not seriously 

 injured. It must be mentioned, however, that with all leaves, damaged 

 or undamaged, treatment with copper sulphate affected the hairs on 

 the under surface, resulting in slight yellowish discolouration, which on 

 close examination were found to be due to the staining of the cell walls. 

 It also hastened the death of late autumn foliage. 



By coating one or other of the surfaces of the leaf with vaseline, it 

 was possible to compare their behaviour towards copper sulphate solu- 

 tion, and it was thus found that the upper surface, where quite free from 

 damage, possesses a remarkable power of resistance to the penetration 

 of the solution. Even when the liquid was allowed to dry on the leaves, 

 mjury to the upper surface was confined to certain areas, usually 

 evidently arising from some original damage. The under surface is 

 more easily affected : possibly the presence of stomata (in the apple) 

 on the under surface of the leaf only may have some bearing on this 

 point. 



These observations apply, however, only to summer foliage. 

 When similar experiments are tried in the late autumn the 

 results are different. The effect of covering autumn leaves, whilst 

 still on the trees, with " no-excess-lime " Bordeaux mixture is 

 to cause considerable and apparently general scorching over most of 

 the leaf surface, accompanied by premature defoliation. When ordinary 

 Bordeaux mixture (containing excess lime) is used, there is more scorch- 

 ing than is noticed in the summer, but the action is not severe. With 

 5 % copper sulphate solution the leaves very soon shrivel up and drop, 

 and the presence of copper can be traced inside the stem lower down 

 than the parts actually immersed^. There is in these cases apparently 

 a general scorching independent of the presence of visible injuries, and 

 of a somewhat different character to that which occurs in the summer. 

 The cuticularised walls of the cells are found to be stained a pale greenish 

 colour, in a manner similar to the leaf-hairs already mentioned. 



Possibly under autumnal conditions changes take place in the nature 

 of the cuticle, which lead to the production and absorption of soluble 

 copper over the general surface of the leaf, the Bordeaux mixture thus 

 damaging underlying cells in spite of the really uninjured leaf surface. 

 On the other hand, the possibility of the presence of small injuries was 

 not entirely excluded in these experiments, though the foliage was chosen 

 for its generally sound appearance. 



^ Sec also p. 18. 



