Some salient points in the evolution of the secondary vascular cylinder of Amarantaceae and Chenopodiaceae

Abstract

A comparative study has been made of secondary thickening in the stem and root of species of Alternanthera, Achyranthes, Pupalia, Amarantus, Atriplex, Chenopodium, and Suaeda in order to bring out the evolution of the anomalous secondary vascular cylinder of the Amarantaceae and Chenopodiaceae. It has been found that anomalous secondary thickening may be present in the root or the base of the stem when it is absent from the larger part of the stem. In the seedlings, it appears earlier in the root than in the stem. It may be present in the region of a node when it is absent in the adjacent internodes. The parenchymatous conjunctive tissue is always more abundant in the root than in the stem and shows a progressive decrease from the base of the latter upwards. In some genera like Achyranthes and Atriplex, while the secondary cambiums in the root arise in the form of nearly complete rings or large arcs, in the stem they are renewed only in small segments toward the outside of the phloem strands formed by the preceding cambium. This shows that the presence of secondary thickening by a succession of secondary cambiums is an ancestral character in these families, that parenchymatous conjunctive tissue is more ancient than the fibrous, and that the renewal of secondary cambiums in complete rings or large arcs is a primitive character. The primitive form of the secondary vascular cylinder in these families may therefore be reconstructed as consisting of several zones of vascular bundles embedded in a parenchymatous ground tissue and formed from a similar number of secondary cambial rings. From this, evolution has taken place in two directions. In a few forms, reduction has led to the loss of anomalous secondary thickening either from the stem alone or from both the stem and the root. In other forms, the ground tissue gradually changed into the fibrous type, and the secondary cambiums began to be renewed in smaller and smaller segments. This has led to the complete modification of the conjunctive tissue into the fibrous type and ultimately the development of included phloem of the Strychnos-type. The secondary vascular cylinder of the root and stem of certain species of Amarantus is nearest to the ancestral condition. Alternanthera, Achyranthes, Pupalia, Atriplex, and Suaeda show the successive stages of this progressive evolution.