Seasonal changes in the structure of the secondary phloem of the Grewia tiliaefolia, a deciduous tree from India
1985
Deshpande, B.P. | Rajendrababu, T.
Structure of the secondary phloem of Grewia tillaefolia Roxb. was studied in samples of bark collected at monthly intervals from forest populations of Gujarat in western India. The secondary phloem in this species is vertically storied and the axial elements occur as alternate tangential bands of fibres and sieve elements produced in succession. On average, two to four bands of fibres and corresponding bands of sieve elements are produced in a year. The sieve elements function for more than one season and different phloem increments are separated by terminal zones made up of very narrow sieve elements which mature just before and immediately after the period of dormancy. The tree becomes leafless about eight to ten weeks preceding the spring equinox. Cambial activity, phloem differentiation and phloem function are suspended during this period. Differentiation of phloem begins after bud break which occurs in April, and continues until January, but most of the phloem is produced between July and September when the rainy season is well advanced. The width of the conducting zone is maximal at the end of the period of growth when the tree is in full leaf. Inactivation of sieve elements, apparently by callose plugging the sieve plates, begins with leaf abscission. The sieve elements produced in the preceding season, just before dormancy is imposed resume function in the following growing season and the older elements die. Companion cells and axial parenchyma cells surrounding sieve elements appear to have s significant role during senescence of the conducting elements. The development and activity of the secondary phloem seem to be related to other developmental phenomena occurring within the tree.
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