J Integr Plant Biol. ›› 1965, Vol. 13 ›› Issue (3): -.

• Research Articles •    

Structure and Cytogenesis of the Shoot Apex of Syringa oblata var. affinis Lingelsh

C. Chu   

Abstract: The structure, growth and mitotic activity of 211 shoot apices of developing sprouts of Syringa oblata var. affinis Lingelsh. in longitudinal sections and 67 in transverse sections have been studied with the view to understanding the nature of zonation patterns and cytogenesis of the apical meristems during a double plastochron. The external morphology and the anatomical structure of the apices in 4 plastochronic stage-early, middle, late Ⅰ and late Ⅱ stages are described. In the shoot apices examined, especially those at late plastochronic stage, the following zones may be delimited: Zone of tunica initials, zone of corpus initials, peripheral zone and zone of rib meristem. The location and orientation of mitotic figures observed in longisections of the apices in 4 plastochronic stages are plotted in diagrams and the mitotic frequency has been calculated. Information obtained from these investigations reveals that the tunica and corpus inititals constitute an active region of the apex, but their mitotic activity changes periodically within the double plastochron. In the middle plastochronic stage when the apex is at its minimal area and the cells of peripheral zone and rib meristem zone have been completely transformed into constituent parts of foliar primordia and the subjacent tissues of the stem and the pith mother cells respectively, the mitotic frequency of the initials is at its maximium and its intensity of mitotic activity is not much lower than that of any other meristematic zone at any stage. When the apical dome is reformed by the activity of these initials in late plastochronic stages, the mitotic frequency of the initials gradually drops and the region of high mitotic frequency shifts to the flank of the apex, the peripheral zone. Anticlinal divisions are predominant in this zone. On the other hand, those cells directly left behind by the corpus initials, which constitute the rib meristem, are vacuolated and marked by the pre- dominance of transverse divisions. Thus the entire zonation pattern reappears. In the next early plastochronic stage, the mitotic frequency of the tunica and corpus initials drops to its mimimium, but other regions of the apex still maintain a high mitotic frequency. It may be concluded that the tunica and corpus initials form a cytogenerative center of the shoot, and the cytohistological zonation is actually a result of the fact that different regions of apical meristems are different in mitotic activety, different in state of cell differentiation and different in their function in morphogenesis.

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