Secondary growth in the stem of some species of Alternanthera and Achyranthes aspera (Amaranthaceae)

Secondary growth in Achyranthes aspera, Alternanthera polygamous, A. pungen s, A. sessilis, and A. triandra was achieved by the development of a cambia l variant resulting in successive rings of xylem and phloem. Each new cambi um was developed at a distance about two to three cells external to the phl oem produced by the previous cambium. The development of phloem was not syn chronous in the species studied. Phloem development started either simultan eously with xylem or after the formation of a few xylem derivatives. In Ach yranthes, xylem production started first followed by the development of phl oem. Phloem mother cells differentiated into sieve tube elements, companion cells and axial parenchyma. Xylem was storied and exclusively composed of axial elements. Radial elements were absent in all the xylem rings of the s tem. Vessels were angular and mostly solitary, but radial and tangential mu ltiples were also observed occasionally. Xylem fibres were nonseptate and n ucleated. Development of phloem and the rayless nature of the xylem is disc ussed.