821 



it is conceivable, tliat by great leiisioii ofllie (issue in tlie suiToundiiig 

 parenciiynia, they undergo strain and pressure causing mechanical 

 rupture and necrobiose, centre and prey of the wound stimulus 

 being the pliloem bundles themselves. 



This conception is in accordance with the tact that the gum 

 canals are broad in the fruits of well-fed trees on rich grounds, 

 which have a hard and solid tlesh, wherein stress and strain are 

 certainly very great. Oidy here and (here remains of the protophloeni 

 along (he gum canals are s(ill to be found in such fruits. But in the 

 softer fruits of sandy soils, along the much narrower gum canals 

 not only the protophloeni is still [)resent, but also stripes of the 

 secondary phloem. 



Summarising we come to the following conclusions. 



xA'Iechanical wounds in growing tissues of Amygdalaceae will some- 

 times heal dii'ectly, sometimes after previous gummosis. 



The chief tissue, which is transformed into gum is the young 

 secondary wood newly spruiig from the cambium and not yet 

 ditfei-entiated. By the wound stimulus a netwoi-k of gum canals 

 is formed around the wound. In thick l)ranches, with a bark wound, 

 this network has an elliptical circumference, the wound being in the 

 lower focus of tlie ellipse, 



If the stimulus is removed by the cure of the wound, the cam- 

 bium again continues to produce normal secondary wood, so that 

 aftei'wards the gum canals may be found in the wood itself. 



If the stimulus continues (he gum formation also becomes lasting. 



The stimulus issues from the cells that die slowly by wounding, 

 poisoning or parasitism. Probably a cytolysine flows from these cells 

 into the young wood or the procambium ; these bind the lysine and 

 liquefy to gum. Hence, gummosis is caused by necrobiose. 



Young medullary rays and phloembundles are with more diftlculty 

 converted into gum than the young secondary wood. But in the 

 fruit-tlesh of the almond and the peachalmond it is the phloem 

 which changes into gum. The protophloem of the bundles often 

 remains unchanged. 



Although gummosis in these fruits belongs to their normal develop- 

 ment, a wound stimulus is nevertheless active. This stimulus springs 

 from the strong tension in the parenchyma of the fruit-wall, which 

 gives rise to tearing, necrobiose and gum formation in the delica(e 

 tissue of the phloem bundles. Consequently the wound stimulus is 

 here a iiormal factor of development. 



It might also be said that the almond and the peachalmond are 

 pathological species, but thereby nothing would be explained. 



