13 



the tension becomes so great that the cells can no longer withstand the 

 strain put upon them, and consequently burst. As soon as the cell is 

 ruptured atmospheric oxygen gains access to the cell and, together with 

 the enzymes present, act on the tannin, producing dark-coloured oxy- 

 compounds, which are precipitated on the walls as a gummy substance, 

 while any further diastatic action is inhibited, with the result that the 

 starch grains remain unaltered, and are always abundant in the vicinity 

 of the necrotic areas. As soon as a few of the cells bordering on the vas- 

 cular bundles or those situated at their ends are ruptured, their neigh- 

 bours surrounding them and depending upon them for their water supply 

 are immediately aSected, and co;isequently suffer also, with the result 

 that a nest of unhealthy arid dried-out cells becomes one of the dry and 

 tough bitter-pit spots. The collapse of these cells in the interior of the 

 fruit eventually leads to the characteristic depression or pit on the 

 exterior. 



Possible Causes of Bitter-Pit. 



Various theories have been advanced from time to time to account 

 for this trouble, and since it occurs in so many different parts of the world 

 under such diverse conditions of soil, each has in turn been considered 

 in so far as it was possible to test or disprove them, although it is extremely 

 probable that no one factor alone is responsible for the spotting. 



The following causes have been suggested : — 



(1) Bacteria and fungi. 



(2) Insects. 



(3) Unfavourable soil and atmospheric conditions. 



(4) Unfavourable grafting. 



(5) Mechanical injury. 



Bacteria and Fungi. 



An exhaustive examination of many hundreds of spots by the most 

 modern methods of microscopical technique failed to demonstrate the 

 presence of either bacteria or fungi, and all attempts to obtain any growth 

 of such organisms from the diseased areas by means of culture media 

 entirely failed. Trees which bore affected fruits were to all intents and 

 purposes perfectly healthy. Root, shoot, and leaf system, when examined, 

 showed no evidence of harbouring fungi or bacteria, but appeared in every 

 instance to be performing their normal functions. 



Insects. 



One of the earhest theories brought forward to account for the spotting 

 was that the injury was due to insect puncture. This was soon afterwards 

 disproved by the original proposer, but since then similar explanations 

 have been frequently advanced. However, the most careful microscopic 

 examination fails to reveal any trace of external injury, and to one well 

 acquainted with the trouble, no such explanation would account for the 

 isolated and deap-seated spots, such as are seen in Plate II. 



