ANOMALOUS SECONDARY THICKENING IN PHYTOLACCA AMERICANA L. (PHYTOLACCACEAE)

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.