Development and structure of resin glands on tissues of Betula pendula Roth. during growth

Betula pendula Roth, and a number of other species of the genus are known to excrete resinous chemicals on juvenile shoots and twigs as deterrents against herbivores. The present studies show that the resin is produced by specific multi cellular glands that are of the same ontogenetic origin as the glands covering developing tissues in buds during winter dormancy. It was possible to deduce from observations over a season that the resin production may be regulated at two levels. Density of the glands on the shoot surface is determined at the time of tissue differentiation in the beginning of primary growth, and the excretory activity is related at least in part to gland size. Towards the end of the growth period the cores of glands were gradually filled with bark cells and the interior of the columnar excretory cells of their epidermis appeared pale and hollow instead of the dense blue in younger glands, when stained with haematoxylin. The excretion of resin by B. pendula appears to be a primary defence mechanism that is active during that part of the season, when apical growth of a juvenile shoot takes place.