813 



Slimmer the camhium of' the still h)ngitiKliiiaIl)- growing' pari of 

 young green hranclios may be caused to form giim by |)un('liires or 

 incisions, bul these wounds lieal <|ui('kly, exceni when "'kepi o|)en" 

 by Coripieuni or other jtai-asites. 



As to thicker branches, wounded in spring, the microscope shows 

 tlie following. 



Around the wound a great number of gum canals are formed in 

 the cambium, aboul parallel witii the axis of the branch, some centi- 

 meters long, which become the thimier and shorter as they are 

 more remote from the wound. The canals are separated by the 

 medullary rays, which are with more difficulty converted into gum than 

 the phloeoterma. All the gum canals together form a kind of net- 

 work, whose meshes are filled by the medullary rays. The whole 

 network has the shape of an ellipse, the "gum ellipse", the wound 

 lies in the lower focus towards the base of the branch. The stimulus 

 extends over the ellipse, evidently farthest in the direction of the 

 branch, less far towards the base and sideways. So it may also be 

 said that the w^ound stimulus extends farthest opposite to the "de- 

 scending" current of nutrient matter, following the phloem bundles, 

 or along with the "ascending" water-current, following the wood. 

 Evidently the gum canals are more easily formed in the better fed 

 cells above the wound than in those beneath it, where the nutrition 

 must be worse. This is especially obvious in ringed branches. Wounds 

 in the cambium, directly above the ring produce much more gum 

 than those immediately below. ') 



Under ordinary circumstances the branches, after simple mechanical 

 wounding, are soon completely healed, and if the cambium at the 

 outside of the gum canals then again begins to produce normal 

 secondary wood, the gum canals may later be found back in the 

 wood itself. ^) Evidently the healing takes place as soon as the 

 stimulus ceases, and so it is not strange that when it continues 

 by poisons or parasitism the gum production also continues. 



1) The nature of the power, by which the food transmitting, "descending" 

 sap current moves tln-ough the phloem bundles, is not known. It is thus not 

 impossible, that if the cause of gummosis is of a material nature, a lysine, moving 

 through the tissues, it is able to run in opposition to the "descending" current. 

 I think, however, that the extension of the stimulus does not go along the phloem 

 but along the xylem bundles and the young wood, with the "ascending'' sap. 



~) I have never seen dislinct gum canals in the secondary wood, but accord'ng 

 to the descriptions they occur eventually. 



