Anomalous Secondary Thickening in Phytolacca Americana L. (Phytolaccaceae)

Abstract

Differentiation of the primary thickening meristem (PTM) was investigated in seedlings and older plants of Phytolacca americana L. Initiation of the PTM occurs in pericycle or inner cortex at the hypocotyl‐primary root junction of young plants. Differentiation of the PTM in stems occurs acropetally in a cylinder of randomly dividing cells termed the diffuse lateral meristem (DLM). The PTM produces secondary tissue to the inside (internal conjunctive tissue) and to the outside (external conjunctive tissue). Patches of xylem and phloem differentiate, opposite each other, in recently produced internal and external conjunctive tissue, respectively. The resulting strands (desmogen strands) of xylem and phloem are secondary in origin, and are peripheral to primary vascular tissues. Phloem of desmogen strands usually differentiates first. Xylem of desmogen strands is composed of both tracheids and vessel elements; the latter sometimes becoming occluded with tyloses and unidentified substances. As root and hypocotyl increase in diameter, cylinders of PTMs differentiate successively and centrifugally in external conjunctive tissue. Even though the first PTM differentiates in pericycle or inner cortex and later PTMs differentiate in external conjunctive tissue, all are referred to as PTMs because of their similar activity. Multiple rings of desmogen strands can be observed in transections of lateral roots, primary roots and hypocotyls. Throughout the length of the stem, only one ring of desmogen strands is present. Fewer rings of desmogen strands are present in the top of the hypocotyl and cotylendonary node, as compared to the subjacent hypocotyl, due to anastomoses of centrifugally differentiating desmogen strands.

Origin of successive cambia on stem in three species of Menispermaceae

“…In fact, there are more detailed studies on the following families: Amaranthaceae, Aizoaceae, Chenopodiaceae, Leguminosae, Nyctaginaceae, Convolvulaceae and Phytollacaceae. In these families, the pattern of secondary thickening is only superfi cially similar to Menispermaceae, and the literature has proposed different origins for successive cambia: cortex (Pfeiffer 1926, Chalk & Chattaway 1937, Balfour 1965, Mikesell & Popham 1976, Mikesell 1979, Carlquist 1999, 2007, Rajput & Rao 2008, pericycle (Haberlandt 1914, Pfeiffer 1926, Chalk & Chattaway 1937, Joshi 1937, Mikesell & Popham 1976, Mikesell 1979, Fahn & Zimmermann 1982, phloem (Metcalfe & Chalk 1950, Wheat 1977, Nair & Ram 1990). …”

Wood anatomy of Agdestis (Caryophyllales): systematic position and nature of successive cambia

“…The analysis of Mikesell (1979) in Phytolacca shows that the role of terminology in studies of successive cambia is critical. Mikesell uses "primary thickening meristems" as a synonym for "successive (Fig.…”

“…) cambia" as used above, and envisions origin of these "primary thickening meristems" from "diffuse lateral meristems" (the latter = "lateral meristem" as used above), which Mikesell does not detail. Mikesell (1979) says that Rivina has "no primary thickening meristem," perhaps a way of saying that only a single cambium ·is present in stems and roots. Mikesell (1979) has the concept that the "primary thickening meristem" in interfascicular areas can give rise to "external conjunctive tissue" and "internal conjunctive tissue," concepts that could also be considered ray tissue produced by the successive cambia.…”

“…Mikesell (1979) says that Rivina has "no primary thickening meristem," perhaps a way of saying that only a single cambium ·is present in stems and roots. Mikesell (1979) has the concept that the "primary thickening meristem" in interfascicular areas can give rise to "external conjunctive tissue" and "internal conjunctive tissue," concepts that could also be considered ray tissue produced by the successive cambia. However, Mikesell states that, "At isolated locations along the primary thickening meristem in pokeberry, patches of xylem and phloem differentiate from newly produced internal and external conjunctive tissue, respectively."…”

“…More significantly, genera with successive cambia have been subject to varied interpretations with respect to ontogeny and action of a thickening meristem and of the cambia that produce secondary xylem and phloem (see CONCLUSIONS). With varied interpretations of these ontogenetic events, diverse terminology has also arisen (see Artschwager 1920Artschwager , 1926Balfour, 1965;Esau and Cheadle, 1969;Stevenson and Popham 1973;Wheat 1977;Mikesell 1979;Baird and Blackwell 1980;Gibson, 1994). The material of Agdestis was suitable for analysis of the successive cambia and their origin, although not definitively so.…”