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

Each species or group of species is characterized by a maximum number of bundles that can be found in the leaf vascular supply at a certain level in the stem. This number can be: 3 bundles (LML), 5 bundles (LiMiL), 7 bundles (mLiMiLm), 9 bundles (m'mLiMiLmm') or 11 bundles (m"m'mLiMiLmm'm"). In transverse section, the traces composed of 7, 9, and 11 bundles display a characteristic zigzag pattern (Fig. 810). In the species with leaf traces that posses 3 or 5 bundles, this zigzag pattern is lacking (Fig. 5-7). The vascular supply to each cotyledon has a different organization, although uniform for all the amaranths examined. It consists of only 2 bundles (Fig 4a, Sa-c).

Leaf traces are produced by adjacent pairs of major axial anastomotic bundles and the number of bundles in the leaf vascular supply increases progressively and gradually up the stem. The median bundles (M) have the longest course up the axis and diverge first from major axial anastomotic bundles. Gradually, the intermediate (i) and the lateral (L) bundles diverge in pairs from the major axial bundles, and eventually the marginal bundles (m, m', m") do the same. These leaf trace bundles traverse the number of internodes equal to the number of orthostichies of the phyllotactic fraction (i.e. 5 internodes in the species with 2/5 phyllotaxy and 3 internodes in the species with 1/3 phyllotaxy, etc). The number of vascular bundles at the base of the petiole is not always equal to the number of bundles that diverge into it because the vascular bundles fuse and separate repeatedly within the petiole.

Branch Traces. All species of the genus Amaranthus possess lateral branches and the vascular system of these branches is connected to the vascular system of the main axis. These branches have a similar arrangement of vascular bundles as the main axis (although they lack anomalous secondary growth at their bases). Bundle arrangement in the main axis of the branches is as described by Gravis and Constantinesco (1907) in A. caudatus and is the same for all amaranth species. The only differences noticed between the species are in the number of the branch bundles. Since the lateral axes have more or less the same pattern of arrangement of sympodia as the main axis, the number of branch bundles is related to the number of bundles within the leaf vascular supply characteristic for that species.

In a transverse section through a node at the insertion point of a branch, there are branch bundles, which are connected to the vascular supply of the main axis. Depending on their position they are either external branch bundles (Be) or internal branch bundles (Bi) (Fig. 6a). External branch bundles are derived from the outer, minor axial bundles of the main stem and internal branch bundles are derived from the inner, major axial anastomotic bundles of the main stem. Approximately 1 mm below the node in question, the external branch bundles (Be) are parallel to one another and the internal branch bundles (Bi) are positioned centrally and are closer to the stem axis (Fig. 6b). In the region where the leaf traces diverge from the major axial bundles, the external branch bundles (Be) are positioned between the bundles of the leaf trace (F9) and the inner branch bundles (Bi), which are diverging from the major axial anastomotic bundles (A) (Fig. 6c). At the middle of the internode below the branch, the external branch bundles (Be) are positioned in a distinct ring of peripheral bundles along with small axial bundles and the internal branch bundles (Bi) have not yet diverged from the major axial anastomotic bundles (Fig 6d). Therefore the course of the external branch bundles (Be) is longer than that of the internal branch bundles. Typically, they traverse the same number of internodes between the position at which they diverge from the minor axial bundles until they enter the branch base (A) as the number of leaf orthostichies on the shoot. However, this is not a general rule since many external branch bundles traverse only 1-2 internodes.