Seasonal Changes in the Structure of the Secondary Phloem of Grewia tiliaefolia, a Deciduous Tree from India

Structure of the secondary phloem of Grewia tillaefolia Roxb.was studied in samples of bark collected at monthly intervalsfrom forest populations of Gujarat in western India. The secondaryphloem in this species is vertically storied and the axial elementsoccur as alternate tangential bands of fibres and sieve elementsproduced in succession. On average, two to four bands of fibresand corresponding bands of sieve elements are produced in ayear. The sieve elements function for more than one season anddifferent phloem increments are separated by terminal zonesmade up of very narrow sieve elements which mature just beforeand immediately after the period of dormancy. The tree becomesleafless about eight to ten weeks preceding the spring equinox.Cambial activity, phloem differentiation and phloem functionare suspended during this period. Differentiation of phloembegins after bud break which occurs in April, and continuesuntil January, but most of the phloem is produced between Julyand September when the rainy season is well advanced. The widthof the conducting zone is maximal at the end of the period ofgrowth when the tree is in full leaf. Inactivation of sieveelements, apparently by callose plugging the sieve plates, beginswith leaf abscission. The sieve elements produced in the precedingseason, just before dormancy is imposed resume function in thefollowing growing season and the older elements die. Companioncells and axial parenchyma cells surrounding sieve elementsappear to have s significant role during senescence of the conductingelements. The development and activity of the secondary phloemseem to be related to other developmental phenomena occurringwithin the tree.