Stem morphology and anatomy in Amaranthus L. (Amaranthaceae)-taxonomic significance

Journal of the Torrey Botanical Society, Jul-Sep 2001 by Costea, Mihai, DeMason, Darleen A

The phyllotaxis varies at different stages of shoot ontogeny of an individual plant and within the genus. The phyllotaxis of the seedling and therefore of the basal, juvenile leaves (at the first 4 nodes) is approximately 1/2 (distichous). During shoot ontogeny the apical meristem becomes larger and the phyllotaxy is transformed into more complex patterns. However, the phyllotaxis of leaves from nodes 5 to 21 (23) is relatively constant for each species. It varies within the genus, being predominantly 2/5 for species of subgenus Amaranthus and 1/3 for species of the subgenus Albersia. Later in shoot ontogeny the phyllotaxis can increase to 3/8 in the species of the subgenus Amaranthus (especially in Amaranthus caudatus, A. cruentus and A. hypochondriacus) and to 2/5 in some species of the subgenus Albersia (A. albus, A. blitoides and A. graecizans).

Axial Anastomotic Bundles. Illustrated diagrammatically the axial bundles describe a more or less regular reticulum (Fig. 3a-b; Fig. 4a-b). This pattern is produced by the major axial bundles from the inner rings) that we call anastomotic (A), following the terminology of Gravis and Constantinesco (1907). Nodal anatomy in the amaranths is unilacunar. A parenchymatous interfascicular region (i.e. leaf gap or lacuna) is associated with the leaf traces between a pair of major axial anastomotic bundles giving rise to and flanking them along their course through the stem. Finally, this pair of major axial bundles fuses above the node at which the leaf vascular supply diverges into the leaf closing the interfascicular region. The minor axial bundles in the outer ring are branches of the major axial anastomotic bundles and they are positioned between the external branch bundles. Each major axial bundle is associated with 2 to 4 minor axial bundles (not represented in the longitudinal diagrams but illustrated in the transverse sections.) After they ascend one or 2 internodes they rejoin the major axial anastomotic bundles at the nodes. All the axial bundles in the stem join at more or less regular points along their course through the stem axis as is typical of a closed system.

Leaf Traces. The organization of leaf traces in amaranths is very peculiar. The leaf vascular supply (examined along its entire course) consists of a central bundle-the median bundle (M), which is flanked laterally and symmetrically by 2 intermediate (i), 2 lateral (L) and a number (1-3) of marginal bundles of various orders (m, m' and m"). The number of bundles within the leaf vascular supply varies both during shoot ontogeny and also between the species (or groups of species). During shoot ontogeny, the number of bundles associated with juvenile leaves (base of the stem) is lower, it increases gradually reaching a maximum, and finally, decreases again toward the end of the plant's life span. An exception to this rule among the species examined is A. crispus, which displays a constant 3-bundle leaf vascular supply throughout shoot ontogeny. The range of variation in leaf trace bundle arrangements is stable during the life of the plant if the plants are grown in the same ecological conditions.