Stem Phloem of a Middle Pennsylvanian Lepidodendron

Abstract

The distribution and histology of the primary phloem of a Middle Pennsylvanian Lepidodendron are described on the basis of 5 exceptionally well-preserved stems discovered in coal balls from southern Illinois, USA. In transverse sections of developmentally young stems lacking secondary xylem, the phloem is present as a discontinuous ring of strands separated by intervening regions of parenchyma that accompany departing leaf traces. The phloem is separated from the primary xylem by a primary parenchyma sheath. In stems with some secondary xylem, a similar distribution of tissue occurs. Strands of phloem parenchyma and cells interpreted as sieve cells are present in the phloem, the latter cells having large, horizontally elliptical sieve areas with up to 12 sieve pores per area. Possible remains of callose are present. In the developmentally oldest stem, meristematic activity in the primary parenchyma sheath leads to the production of secondary stelar parenchyma. No evidence of secondary phloem was found in any of the stems, indicating the unifacial nature of the vascular cambium. This further emphasizes the differences in secondary thickening between the arborescent lycopods and the seed plants in which a functionally bifacial vascular cambium arose by Middle Devonian in the progymnosperms and has been retained in most subsequently evolved gymnosperms and woody dicotyledonous angiosperms.