Wood anatomy and heartwood formation in Neem (Axadirachta indica A. Juss.)

The wood of Azadirachta indica is diffuse porous. Axial parenchyma is paratracheal banded or vasicentric. Rays are uniseriate to multiseriate and heterocellular with procumbent and upright cells. There is a strong negative correlation between vessel member length and diameter. The vulnerability and mesomorphic values are different in two trees of almost the same age growing in the same locality. The vessel member wall has spiral thickenings on its inner surface. Axial and ray parenchyma cells and sometimes vessels and fibres of the heartwood show the presence of extractives. The necrobiosis of the parenchyma cells occurs at the heartwood boundary. Senescence and death of parenchyma cells are associated with depletion of starch grains and accumulation of extractives. There is a climacteric rise in succinate dehydrogenase and acid phosphatase activities at the sapwood-heartwood interface which may be associated with heartwood formation. Peroxidase activity is greater near the cambial zone, indicating its probable role in lignification.