705 



m which production of the gum centres was suppressed, show that a supply 

 of oxygen seems to be an absolute necessity. 



In our opinion, the necrobiosis theory of Beijerinck and Rant is unten- 

 able, since gummosis may be found with(Kit any previous presence of dead 

 cells in very young branches and one-year-old seedlings in places which 

 represent still intact cell centres such as in Fig. 155 2 p. Therefore, the 

 wound stimulus does not enter into the ([uestion here. We believe rather 

 that all embryonic and mature cells are capable of forming gum as soon as 

 certain processes of cell wall formation, or maturation, are suppressed. This 

 prevention of the normal maturing of the cell wall can be caused very well 

 by an increased supply of oxygen. This oxygen, however, can be directly 

 atmospheric oxygen only in case of injury, but probably is only rarely 

 actually such, being furnished rather by substances which carry oxygen as 

 Griiss explains. Substances of this kind are present in the iionnal ijroivtli 

 of trees. In an exudation of gum only an abnormal increase in the amount, 

 or the length of action of these substances is involved^ This increase can 

 take place because of wound stimulus. It can also be produced by different 

 parasites and, finally, developed by inorganic poisons. In the latter connec- 

 tion. I would mention my experiments in introducing a weak oxalic acid 

 solution under the bark of perfectly healthy cherry trees. In the course of 

 the summer profuse streams of gum were produced which gradually ceased 

 because of the dying out of the oxalic acid action and they did not continue, 

 for example, in wounds which had received only distilled water instead of 

 the oxalic acid. 



In regard to the manner in \\hich gum exudations can develop we will 

 take as a basis the theories formulated by Griiss'-. 



In his investigations, this scientist has come to the conclusion that the 

 hemi-celluloses. Mannan, Galactan and Araban are deposited directly, or 

 indirectly, as reserve substances. This takes place directly in the form of 

 thickened cell walls in the endosperm of the seed (Phoenix, Phytelephas), 

 or in the form of secondary thickening layers in libriform or wood paren- 

 chyma cells (different varieties of Astragalus, Prunus, Acacia, etc). They 

 can be considered as indirect reserve substances if they compose the cell 

 walls of cells containing starch, such as those in the endosperm of the 

 Gramincae. The hemi-celluloses, Galactan and Araban, are changed by 

 enzymes into the gums Galactin and Arabin and can migrate in the tissue 

 even before they have l)ccn transformed into the sugars galactose and 

 arabinose. 



1 These substances arc found in varying amounts in the tree according- to the 

 individual, the place of growth, the time of year, etc. This explains also the different 

 results when the gum exudation is produced by injuries. Thus, for example, the 

 youngest tips of the branches are not the ones most endangered in this, but the 

 region in which the tissue elongates most, i. e. the region beneath the apex. In 

 vegard to the influence of the different sides of the tree and the seasons, I found in 

 incisions made monthly that the late spring and the southern to. western sides of the 

 tree are most favorable for the development of gummosis. 



- Loc. cit. 



