Anatomical Studies of Wood and Bark of Acacia senegal Willd. Trees with Special Reference to Gum Exudation

The gum of Acacia senegal comes from gum cysts which develop in the inner bark of some trees. These cysts are developed in tangential rows in the axial parenchyma strands of the phloem adjacent to the cambial zone. They are first initiated schizogenously but later on enlarge considerably due to the breakdown of the surrounding cells (lysigenously). These cysts do not have any definite shape or size and in longitudinal sections, they appear as vertically aligned, sinuous and sometimes interconnected passages ending abruptly. The specimens showing spontaneous exudation of gum indicated that the development of these cysts is preceded by certain widespread changes like profuse development of parenchymatous tissues, disappearance of starch, etc., both in the xylem and phloem, as well as disintegration of the pith. The formation of gum ducts and other changes, however, occur only in localized areas of the stem. Blazing the tree also sometimes results in the formation of similar gum cysts in the bark but, unlike the case of natural exudation, the structure of the xylem does not change to any great extent. Rod-shaped bacteria have been found to be conspicuous in all the tissues affected by gummosis. These bacteria have also been noticed in other tissues of the phloem and xylem and the exact significance of their presence is not clear. The probable reasons for the development of gum cysts have been discussed in the light of the anatomical changes observed.