DEVELOPMENTAL CHANGES IN THE VASCULAR CAMBIUM OF POLYGONUM LAPATHIFOLIUM

Developmental changes in the vascular cambium of Polygonum lapathtifolium were determined primarily by an analysis of the secondary xylem. The cambium and xylem consist of fascicular and interfascicular regions in this herbaceous dicotyledon. Near the pith vessels are restricted to the fascicular regions of the xylem. During secondary growth vessels are formed in some radial files in the interfascicular regions. Anticlinal divisions are of two types, oblique and lateral. In interfascicular files consisting of fibers only, about two-thirds of the anticlinal divisions are oblique. The oblique partition averages 31% of the length of the dividing initials. In interfascicular files consisting of vessel elements and fibers, there are almost equal numbers of oblique and lateral divisions. The oblique partition averages 37% of the length of the dividing initials in these files. Lateral divisions account for approximately three-fifths of the anticlinal divisions in the fascicular regions, consisting of vessel elements and fibers. The partitions formed in oblique anticlinal divisions average 64% of the length of the dividing cells in the fascicular regions. The frequency of anticlinal division is much higher in files consisting of vessel elements and fibers than in those consisting of fibers only. There is no loss of fusiform initials, except by ray formation. Ray initiation occurs by simple subdivision of fusiform initials. The findings are discussed in relation to the developmental changes in the vascular cambium in plants of different habits. THE MOST intensive studies on the developmental changes in the vascular cambium associated with the increase in circumference of this meristem have been made on conifers (Bannan, 1950, 1957, 1960b; Whalley, 1950; Hejnowicz, 1961) and on some species of arborescent dicotyledons (Evert, 1961; Cheadle and Esau, 1964; Cumbie, 1967). Except for an investigation of a species of Hibiscus (Cumbie, 1963), there have been no intensive studies on the developmental changes in the vascular cambium in the herbaceous dicotyledons. This study describes the developmental changes in the vascular cambium of another herbaceous species, Polygonurn lapathifolium L. (Polygonaceae). In this species the fibrous elements in the secondary xylem, in addition to vessel elements and axial parenchyma strands, do not elongate to any appreciable extent during differentiation. Thus the cells in the secondary xylem replicate the basic cell pattern in the vascular cambium, and it was possible to follow the developmental changes in serial tangential sections of the xylem and cambial region. MVATERIALS AND METHODS-Developmental changes in the vascular cambium of P. lapathifolium were studied in three plants collected near Columbia, 1\/Io. Each plant for which a voucher specimen was prepared was approximately 1 m tall and 1 cm in diam near the base. At this level the radial thickness of the xylem ' Received for publication 19 April 1968. was 2.4-2.7 mm. This probably represents all or most of a season's growth because the plants were collected in late August and September. The secondary xylem in P. lapathifolium consists of fascicular and interfascicular regions. From serial tangential sections, 12 A in thickness, of the xylem and cambial region, developmental changes were followed in 163 radial files in the interfascicular regions and 40 radial files in the fascicular regions. In addition, the origin of 70 rays was studied. OBSERVATIONS-The primary vascular system in P. lapathifoliurn is in the form of discrete, widely spaced strands. The continuous vascular cambium and secondary xylem which develops later, therefore, consist of fascicular and interfascicular regions. Near the pith, vessels in the xylem are restricted to the fascicular regions (Fig. 1), but later vessels are produced in some radial files in the interfascicular regions. The fibrous elements in the xylem are thinwalled, living cells which occur in regular, radial rows (Fig. 1). Near the pith, the end walls of the fibers are transverse to abruptly tapered in tangential view (Fig. 2). A short distance out, however, the end walls are more tapered (Fig. 3). Axial xylem parenchyma is scanty, paratracheal and consists of strands of 2-7 cells. There are no rays in the first-formed secondary xylem (Fig. 2), but after the formation of 2.5 mm of xylem there are about 6.0 rays per mm2 of xylem (Fig. 3). All of the rays are uniseriate