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In later years, with better knowledge, I again examined the gum canals in the 

 peachalmond and their surroundings repeatedly. Never did I find a fruit without 

 them, but they were not equally developed in different trees from different gardens. 

 In specimens of sandy grounds they can sometimes only be found with the micro- 

 scope. Neither microscopically nor by experiments has it been possible to detect 

 gum parasites. This makes it quite certain that in the formation of gum canals 

 parasitism is excluded 1 ). 



The great ease wherewith mechanical tension causes wounds in the fruit-flesh 

 of the peachalmond, gives rise to the supposition, that the normal gum canals may 

 be the product of some hidden wound stimulus. 



If this supposition is true, we cannot think of wounding in the common sense 

 of the word. When the flowers fall off, a ring-shaped wound forms around the 

 base of the young fruit, but this is a normal process, taking place in an intercepting 

 layer and soon followed by complete healing. In the flowers of peach, plum, apricot, 

 cherry, we observe the same without any formation of gum canals in the fruit-flesh. 

 Moreover, although the peculiar structure of the layer between the woody peduncle 

 and the stone, along which the ripe fruit detaches, reminds of rent tissue, no gum is 

 formed at that spot and the layer also exists in the other stone-fruits, where no gum 

 canals occur. 



So long as nothing else has been proved it must therefore be accepted that in 

 the phloem bundles of the fruit of the peachalmond, where cytolysis takes place, the 

 same factor of development is active as that, which gives rise to the pathological 

 gum canals in the cambium of the branches. This leads to the conclusion, that the 

 wound stimulus belongs to the normal factors of development of this fruit, although 

 nothing is seen of external wounds. When considering, that the phloem bundles 

 are built up of extremely thin and soft-walled cells, it is conceivable, that by great 

 tension of the tissue in the surrounding parenchyma, they undergo strain and pres- 

 sure causing mechanical rupture and necrobiose, centre and prey of the wound 

 stimulus being the phloem bundles themselves. 



This conception is in accordance with the fact that the : cr um canals are 

 broad in the fruits of well-fed trees on rich grounds, which have a hard and solid 

 flesh, wherein stress and strain are certainly very great. Only here and there 

 remains of the protophloem along the gum canals are still to be found in such 

 fruits. But in the softer fruits of sandy soils, along the much narrower gum car.als 

 not only the protophloem is still present, but also stripes of the secondary phloem. 



Summarising we come to the following conclusions. 



Mechanical wounds in growing tissues of Amygdalaceae will sometimes heal 

 directly, sometimes after previous gummosis. 



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

 newly sprung from the cambium and not yet differentiated. By the wound stimulus 

 a network of gum canals is formed around the wound. In thick branches, with a 



') The supposition, sometimes met with in literature that the gum of the Amyg- 

 dalaceae should consist of bacterial slime is quite erroneous. That parasitic bacteria 

 eventually occur as gum parasites, as is stated by some authors, I do not think impos- 

 sible, although till now I only found caterpillars and Fnngi as active agents. 



