19G A Sfudy of the Capml Brnjafomid on Apple Trees 



dimpled or malformed fruit resulting. In a young apple the injury extends 

 several cells below the cuticle, but in an old apple there is no discolora- 

 tion of the flesh under the cork layer. 



Punctured apples in their late stages show a russetting due to the 

 brown cork scars and in some varieties they crack badly. Practically all 

 the damage to the fruit is done before it reaches an inch in diameter, 

 probably because later on it is too hard to be easily punctured. In one or 

 two cases adults were found dead with their probosces inserted into the 

 tissue of medium-sized fruits. Cork formation follows the punctures in 

 leaves, fruit and young shoots. 



The cause of injury. 



It has been suggested (i, 14) that the injury done by a capsid is due to 

 the mechanical laceration of the tissues by the barbed stylets of its 

 rostrum. There is no doubt that the ends of the stylets are barbed and 

 that the bug does puncture the plant repeatedly, but how far this lacera- 

 tion is responsible for the brown spots or any other external sign of 

 damage is difficult to say. A number of different capsids were found 

 feeding on the same trees but only in the case of P. rugicollis was any 

 visible damage done. 



The stylets of all these capsids are very similar and it seems to us that 

 the cause of the damage is chemical rather than mechanical, the salivary 

 injection from P. rugicollis being lethal to the tissue of apple trees whilst 

 that of the other species is not. This is supported by the fact that it is 

 the mesophyll and not the epidermis which shows the first signs of injury, 

 and dead mesophyll may be present under a healthy epidermis, and 

 again the damaged area may spread for some time after laceration has 

 taken place. 



This brings us to an interesting biological problem. The injury done 

 to a plant may be mechanical or physiological, and although the final 

 result to the plant may be the same it seems necessary to distinguish 

 these two kinds. The term " mechanical injury " might be used to include 

 the wounding of plants by eating away parts of them or by sucking the 

 juices. The term "physiological injury" might mean the injection of 

 some material into the plant which kills the tissue or brings about ab- 

 normal growth. The same species might be responsible for both mechani- 

 cal and physiological damage. 



It seems probable that in the case of Plesiocorus rugicollis the 

 mechanical injury is of little consequence. Psallus ambiguus and 

 Orlhotylus marginalis were sometimes as numerous as that species and 



