Morphophysiological studies of the shoot apex. I. Recurrent growth flushes and their relationship with structural changes in the growing apex of Callistemon viminalis
The annual growth cycle of Callistemon viminalis consists of three phases: (1) a reproductive phase that continues till the end of February; (2) a vegetative phase that lasts from the end of February till the end of August; and (3) a long dormant phase which lasts till the end of December. The cycle exhibits four growth flushes with alternating periods of active growth and rest. The duration of the successive rest periods increases, leading eventually to the onset of the long dormant phase.The growing apex exhibits more or less similar histological changes in different growth flushes. An increase in divisional activity of the cells of flanking meristem, accompanied by a decline in the elongation of cells of pith rib meristem, characterizes the rest period. During the active period of growth, the cells of the pith rib meristem elongate but the activity of the flanking meristem decreases. The number of leaves produced in each growth flush, therefore, is dependent upon the duration of the preceding rest period, and their size and nature on the duration of active growth. In growth flush 1 the cells of the rib meristem show elongation twice during the active period corresponding with the separation of floral buds and of foliage leaves respectively. In the long dormant phase, on the other hand, the cells of the flanking meristem show high divisional activity twice, first during September when new leaf primordia are initiated, and again during November when floral buds are initiated. Events of great significance thus occur during the long dormant phase. The reproductive development is characterized by a marked increase in the rate of cell division in the flanking meristem accompanied by cessation of growth, the features that characterize the rest period as well.