Formation of Gum Ducts in Azadirachta indica A. Juss

Abstract

Lysogenous development of traumatic gum ducts in the secondary xylem of the stem of Azadirachta indicaA. Juss. (Meliaceae) was studied histologically. The ducts developed close to the cambial zone from the cambial derivatives of secondary xylem in response to injury subsequently followed by fungal infection. Some distance away from the infection site, several layers of traumatic parenchyma cells developed from the derivatives of xylem mother cells. Lysis of some of these parenchyma resulted in the formation of gum ducts in one or two tangential rows of ducts. Darkly stained epithelial cells bordered these cavities. Some of these epithelial cells underwent lysis and eventually they contributed to enlargement of gum ducts. Along with the formation of epithelial cells, their lysis and development of gum ducts, the traumatic parenchyma surrounding these ducts became meristematic and cambiform. As the lysis of epithelial cells progressed further, the adjacent cambiform cells underwent periclinal divisions and formed additional epithelial cells. This process continued until all the traumatic parenchyma broke down and formed a tangentially anastomosing network of gum ducts. Some of the epithelial cells also accumulated phenolics and lysis of these cells released the phenolic compounds into the gum ducts. In mature ducts, the formation and lysis of epithelial cells ceased but divisions in the cambiform cells continued and daughter cells differentiated into axial parenchyma.