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

Unquestionably, these species can be more easily identified using classical characters. The above key is presented only to show that differences between some species exist and that these characters are reliable enough for identification.

PRIMARY VASCULAR SYSTEM. The presence of "medullary bundles", which we have called major axial anastomotic bundles, is regarded as a striking and anomalous feature that has been reported in more than 30 angiospermous families (Weiss) 1883; Lignier 1887; Col 1904; Wilson 1924; Dastur 1925; Maheshwari 1929, 1930; Joshi 1931a,b, 1933; Metcalfe and Chalk 1950, 1983; Pant and Mehra 1961; Davis 1961; Kirchoff and Fahn 1984; Raj and Nagar 1980, 1989). In Amaranthus there are two types of primary vascular bundles, the inner ring of major bundles, which gives rise to leaf traces and the inner branch trace bundles and the outer ring of minor bundles, which gives rise to the outer branch traces. The inner, so called medullary bundles of previous authors is actually comparable to the axial bundles of all seed plants. The existence of the outer, minor bundles, which are produced by and eventually fuse again with the major bundles, are "anomalous" compared to other seed plants. The number and arrangement of continuing bundles and leaf trace bundles in the stem are variable even along the same plant. In addition, Joshi (1931b) noted differences in the arrangement of the medullary bundles in plants of the same species from different localities. After comparing several Amaranthaceae and Chenopodiaceae, Wilson (1924) stated that Amaranthus "represents the highest point of development of the medullary condition in these two families" because all the major axial bundles in amaranths are "medullary." This pattern and arrangement of bundles is thought to have originated independently in the various groups of plants in which they are found. In amaranths, this particular disposition of bundles is purely functional being the most logic way to distribute-in a topological sense-the many primary bundles.

Information on the vascular system in the Caryophyllales is still uneven. Some families, such as the Chenopodiaceae (Wilson 1924; Joshi 1934; Fahn and Arzee 1959; Bisalputra 1961, 1962; Fahn and Broido 1963; Zamski and Azenkot 1981; Fahn and Zimmerman 1982) and Cactaceae (Gibson 1976; Gibson and Nobel 1986) have received a lot of attention. But information about the other families is much spottier (Gravis and Constantinesco 1907; Wilson 1924; Dastur 1925; Maheshwari 1929, 1930; Joshi 1931a, b, 1934; Inouye 1956; Balfour and Philipson 1962; Philipson and Balfour 1963; Stevenson and Popham 1973; Kirchoff and Fahn 1984; Gibson 1994). In view of this it is difficult to clarify the phylogenetic relationships and evolutionary trends in primary vascular tissue characteristics in the order. According to Gibson (1994), the primitive condition in the betalaine-containing families is an open primary vascular system in which the leaf vascular supply consists of 3 or more leaf trace bundles arising from two axial bundles and unilacular nodal anatomy. Closed systems, those with more leaf traces or systems in which the leaf vascular supply originates from a single bundle are considered potential synapomorphies. Therefore, the characteristics of Amaranthus, closed system, complex anastomotic axial bundles, leaf vascular system with more than 3 bundles arising from 2 axial bundles, is rather derived. Within the genus Amaranthus, the condition of having a leaf vascular supply of 3 bundles can be considered to be plesiomorphic (as in A. crispus), while having more bundles can be considered to be derived.