SEASONAL-CHANGES IN THE ULTRASTRUCTURE OF THE VASCULAR CAMBIUM OF ROBINIA-PSEUDOACACIA

The ultrastructure of the vascular cambium of Robinia pseudoacacia L. was examined in trunk tissues collected over a 2 1/2 year period. Duri ng dormancy, fusiform cells are densely cytoplasmic with many small va cuoles and centrally located nuclei. Mitochondria are round to oval in sectional view. The plastids are variable in shape, have few internal membranes, and generally lack starch grains. The plasmalemma is smoot h in outline. Proteinaceous material occurs in the vacuoles and many l ipid droplets are scattered throughout the ground substance. Smooth tu bular ER, often highly dilated, predominates, but short segments of ro ugh ER are also present. Abundant free ribosomes are evenly distribute d throughout the ground substance and the dictyosomes are inactive. Mi crotubules are parietal and have various orientations. During reactiva tion, the plasmalemma becomes irregular in outline and begins to form invaginations. Concurrently, the proteinaceous material disappears, th e vacuoles begin to fuse, polysomes appear, and the dictyosomes begin to produce vesicles. During the period of cambial activity, fusiform c ells are highly vacuolate, and the nuclei are centrally located. The m itochondria are round, oval, or elongate. Now the plastids contain phy toferritin, starch grains, or both. Many large invaginations of the pl asmalemma intrude into the vacuole, pushing the tonoplast inward and p inching off into the vacuole, which lacks proteinaceous material. Lipi d droplets are scarce. Most ER is rough, and ribosomes are generally a ggregated as polysomes. Dictyosomes are actively producing vesicles. D uring the transition to dormancy, the fusiform cells gradually assume the appearance typical of the dormant cambium.