Structure and seasonal development of secondary phloem of Juglans regia

Abstract

In Persian walnut (Juglans regia L.), as in other deciduous dicotyledonous trees, a small (0.5 mm) amount of secondary phloem is functional for only one season and a large amount is nonfunctional. In 1968 in Central California the cycle of phloem development began in late February and ended sometime before mid-October. The phloem annual ring was composed of distinctive tangential bands, allowing easy distinction of seasonal growth increments. Each growth increment could be divided into early- and late-season tissue. Early-season phloem, composed principally of large sieve tubes, was separated from late-season phloem by a band of fibers. The late-season phloem was composed of a mixture of narrow sieve tubes, parenchyma cells, and, occasionally, an incomplete tangential band of fibers. The annual rings were not crushed except for the early-season phloem, which was somewhat compressed by growth of the woody cylinder. The bark of walnut becomes thicker with age because of this and the enlargement of parenchyma cells.