Growth Periodicity and Structure of the Shoot Apex of Picea smithiana(Wall.) Boiss.: An Anatomical and Histochemical Study.

Abstract

The organization of the shoot apex of Picea smithiana (Wall.) Boiss. and its seasonal variations have been investigated by studying anatomically and histochemically apices collected every month. The apex shows that the outer layer can be distinguished into a few larger cells at the top of the dome — the apical initials — which divide both periclinally and anticlinally, and the rest of the surface layer, where the cells divide mainly anticlinally. Covered by these are the subapical initials, the flanking and pith rib meristem zones. The shoot apex exhibits growth periodicity. Four periods, viz., periods of dormancy, bud elongation, cataphyll formation and foliar primordial development, are distinguished. But, at no period is the bud completely inactive. During the dormant period, the zonation in the apex is not very sharp. The zones show variations in size during these growth periods. As the period of dormancy approaches a “crown” is formed below the apex separating the older portion from the younger. The anatomical and histochemical characters of the cells of the crown have been studied. Lignin is deposited in the cell walls and their nuclei are deformed and sickle shaped. The function of the crown appears to be to prevent the movement of metabolites to the apex above it, as the vascular elements stop below it.