The Ontogeny of the Primary Thickening Meristem of Atriplex Hortensis L. (Chenopodiaceae)

Abstract

Seedlings of Atriplex hortensis were studied to ascertain; 1) in which organ the primary thickening meristem (PTM) first differentiates; 2) the direction of differentiation of the PTM, and 3) the pattern of differentiation of conjunctive tissue. The PTM initially differentiates in pericycle of the primary root base 11 days after emergence of the primary root. It then differentiates in the transition region of the hypocotyl, mostly in cells of pericycle between pairs of vascular bundles. In the upper hypocotyl, PTM differentiates by day 20 in the inner layer of cortical parenchyma. In the epicotyl, PTM apparently differentiates in the inner layer of cortex, by day 24. Desmogic xylem differentiates from radial files of internal conjunctive tissue cells and desmogic phloem differentiates opposite desmogic xylem strands from newly formed cells of external conjunctive tissue. No interfascicular cambium differentiates in the root, hypocotyl, or epicotyl.

Vascular Tissues

“…New supernumerary cambia later developed external to conducting phloem of leaf-trace bundles. In Phytolacca, the meristem was initiated at the hypocotylprimary root junction as two arcs parallel to the sides of the diarch vascular core and caused early thickening of that region (Mikesell 1979), whereas in Atriplex anomalous thickening commenced opposite xylem poles (Yarrow and Popham 1981). In the rosette-forming Beta vulgaris, the first supernumerary cambium presumably was induced along traces of leaves 1-4, but sampling was not refined enough to record the earliest initiation events (Zamski and Azenkot 1981).…”

“…Use of the term cambium received another temporary setback when several authors elected to reclassify these unusual regions as primary thickening meristems (Stevenson and Popham 1973;Mikesell and Popham 1976;Mikesell 1979;Yarrow and Popham 1981), until then generally regarded as unidirectional meristems of monocotyledons (Esau 1965a;DeMason 1983). From the outset this approach appeared flawed because primary thickening meristem characteristically applies to a layer with centripetal cell divisions immediately subtending a shoot apical meristem.…”

“…Events in the early establishment of lateral meristems were described in considerable detail for Bougainvillea spectabilis (Stevenson and Popham 1973), Mirabilis jalapa (Mikesell and Popham 1976), Phytolacca americana (Mikesell 1979), and Atriplex hortensis (Yarrow and Popham, 1981).…”

Morphology and anatomy in annual taxa of Beta vulgaris s.l. (Chenopodiaceae)

“…In accordance with the investigations of other authors the first cambium ring is formed by the cambia of the first vascular bundles and between the vascular bundles by peripheral cells of the central cylinder, whereas the cambia of the later growth rings arise at the periphery of the last phloem (cf. Yarrow & Popham 1981, Fahn 1990, Bhambie et al 1990, Heklau 1990). This mode of secondary growth has also been described by several authors for other members of the Bhambie et al (1990) the first formed cambium did not cease its activity (cf.…”

Origin of successive cambia on stem in three species of Menispermaceae

“…For example, most genera in the Centrospermae family having a lateral meristem are described variously by different authors: primary thickening meristem -PTM (Stevenson & Popham 1973, Mikesell & Popham 1976, Mikessel 1979, Yarrow & Popham 1981, Carlquist 1988, diffuse lateral meristem (Carlquist 1999), confi ned meristem (Carlquist 2001), lateral meristem (Carlquist 2004) and master cambium (Carlquist 2007).…”