820 



seed and is yet too labile to be demonstrated by infection of bark 

 wounds with gnm. Experiments in this direction may perhaps be 

 effected with the peachalniond. 



Wound stimulus as factor of development. 



Formerly I thought that tiie presence of gnm canals in tlie fruits 

 was accidental and should be explained by parasitism, although I 

 could not find any parasites. 



In later years, with better knowledge. 1 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 diftei-ent gardens. In specimens of sandy 

 grounds they can sometimes only be found with the microscope. 

 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. ^) 



The great ease wherewith mechanical tension causes wounds in 

 the fruit-fiesh 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 w^ord. 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-fiesh. 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, 



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

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

 eventually occur as gum parasites, as is stated by some authors, 1 do not tliink 

 impossible, although till now I only found caterpillars and Fungi as active agents, 



