Cell Growth and Proliferation in Ground Tissue of Developing Terminal Shootin Picea abies (Pinaceae)
–Morphogenesis and growth of Picea abies (L.) H.Karst. shoots, under bud scales and after its burst, occurs by cell proliferation in most of its structures including pith. The rate of pith cell doubling is positively related to the average daily air temperature from the beginning of spring vegetation until the stem reaches 39% of its annual length increment. Subsequently, formation of the pith is carried out mainly by cell expansion until the end of seasonal stem elongation. The annual shoot morphogenesis from spring pith cell proliferation to completion of the stem extension takes about 5–6 cycles of pith cell doubling. In the transverse walls, growing pith cells accumulate more abundant birefringent microfibrils of crystalline cellulose, than in the longitudinal ones. Microfibrils, along with determining cell mechanical properties, might be involved in pith apoplast formation in transverse, radial and tangential cell walls. Structural anisotropy is also observed in pith parenchyma from the beginning of vegetation until maximum bud swelling. Since the tangential and radial cell diameters exceed the longitudinal one, the easier transversal symplastic transport of metabolites is possible due to fewer cell membranes and cell walls per unit length. In the later stages of stem development, upon formation of narrow primary pits on the longitudinal cell walls of pith cells, the symplastic radial transport of metabolites from vascular bundles of stele to the stem centre is maintained. Structure factors of pith anisotropy possibly control synchronization in the mitotic apparatus orientation and formation of transverse cell plates in cell cytokinesis.  REFERENCES 1. Skupchenko V.B., Ladanova N.V. 1984. Razvitiye khvoi Picea obovata (Pinaceae) [The development of Picea obovata (Pinaceae) needles]. Botanicheskii zhurnal. 69(2): 203–206. (In Russian) 2. Skupchenko V.B. 1985. 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