Ontogeny and Correlative Relationships of the Primary Thickening Meristem in Four-O'Clock Plants (Nyctaginaceae) Maintained Under Long and Short Photoperiods

Abstract

The developmental anatomy of Mirabilis jalapa was investigated during the first 90 days of growth. The primary thickening meristem (PTM) initially differentiates in the pericycle at the top of the cotyledonary node 18 days after germination, then basipetally in the pericycle through the hypocotyl. The PTM differentiates acropetally into the stem and in the pericycle of the primary root, commencing 22 days after germination. Endodermis is easily identifiable in hypocotyls as well as in primary roots because of Casparian thickenings in its cells. It has not been definitely identified in stems. There are three rings of primary vascular bundles in the stem. The PTM differentiates as segments of cambium in a layer of cells (probably in the pericycle) on an arc between vascular bundles of the outer bundle ring. Later, arcs of PTM differentiate externally to the phloem of each bundle. Each arc forms a connection between original segments of PTM lying on either side of each vascular bundle. Thus, the PTM becomes a continuous cylinder. The PTM differentiates in the pericycle outside vascular tissue in the hypocotyl and root. Differentiation of the PTM and the mode of secondary thickening is similar in plants exposed to short (8-hr) and to long (l8-hr) photoperiods, but some differences were observed. The PTM differentiates closer to the stem apex in all plants over 18 days of age growing vegetatively under long photoperiods. That is, the diffuse lateral meristem, in whose cells the PTM differentiates in young internodes, is shorter in nearly all investigated plants growing in long photoperiods. The hypocotyl and base of the primary root of 40-dayold plants in short photoperiods were more enlarged than those of the same age plants in long photoperiods; but, at the end of 64 days, the hypocotyl and primary root base were larger in plants growing under short photoperiods. Thirty-four days after seed germination, flower initiation occurs in plants exposed to short photoperiods. One hundred fifty days after seed germination, flowers differentiate on plants exposed to long photoperiods. 427

Vascular Tissues

“…Maheshwari (1930) referred to the median leaf-trace bundles as "large central bundles," but many reports have termed these medullary bundles (Dastur 1925;Joshi 1931aJoshi ,b, 1934Metcalfe and Chalk 1950;Pant and Mehra 1961;Kirchoff and Fahn 1984). Other treatments have simply referred to these large bundles as the "inner ring of vascular bundles" of the primary vascular system (Stevenson and Popham 1973) or "interior vascular bundles" (Mikesell and Popham 1976). Because these bundles do not arise from ground tissue, i.e., the pith-rib meristem, authors should be cautious not to call these medullary bundles, and pith should be defined as a narrow region interior to these bundles.…”

“…(Nyctaginaceae), and Mestoklema tuberosa (Aizoaceae), where normally fibrous xylem differentiates instead as parenchyma with unlignified primary walls (Artschwager 1926;Metcalfe and Chalk 1950;Mikesell and Popham 1976;Webster and Wilson 1980). Large unlignified tangential bands of parenchyma also characterize anomalous thickening in stems of many succulent and fleshy Aizoaceae (Bhambie et al 1977), as in Aptenia cordifolia (Fig.…”

“…Leaf venation has been used to evaluate relationships of particular species within the Cactaceae (Bailey 1960(Bailey , 1968 and Portulacaceae (Hershkovitz 1991) but not in other families. Numerous important studies have been published on the ontogeny of secondary vascular tissues in roots (Artschwager 1926;Millner 1934;Joshi 1937;Esau and Cheadle 1969;Stevenson and Popham 1973;Mikesell and Popham 1976;Mikesell 1979;Webster and Wilson 1980), but these have not been followed by broader systematic surveys and analyses. Descriptions of phloem cell types, other than features of sieve-element plastids (see Chap.…”

Origin of successive cambia on stem in three species of Menispermaceae

“…In fact, there are more detailed studies on the following families: Amaranthaceae, Aizoaceae, Chenopodiaceae, Leguminosae, Nyctaginaceae, Convolvulaceae and Phytollacaceae. In these families, the pattern of secondary thickening is only superfi cially similar to Menispermaceae, and the literature has proposed different origins for successive cambia: cortex (Pfeiffer 1926, Chalk & Chattaway 1937, Balfour 1965, Mikesell & Popham 1976, Mikesell 1979, Carlquist 1999, 2007, Rajput & Rao 2008, pericycle (Haberlandt 1914, Pfeiffer 1926, Chalk & Chattaway 1937, Joshi 1937, Mikesell & Popham 1976, Mikesell 1979, Fahn & Zimmermann 1982, phloem (Metcalfe & Chalk 1950, Wheat 1977, Nair & Ram 1990). …”

“…For example, most genera in the Centrospermae family having a lateral meristem are described variously by different authors: primary thickening meristem -PTM (Stevenson & Popham 1973, Mikesell & Popham 1976, Mikessel 1979, Yarrow & Popham 1981, Carlquist 1988, diffuse lateral meristem (Carlquist 1999), confi ned meristem (Carlquist 2001), lateral meristem (Carlquist 2004) and master cambium (Carlquist 2007).…”