ANATOMY OF THE TRANSITION REGION OF Vigna radiata (L.) Wilczek. (MUNG BEAN).

Abstract

Seedlings of Vigna radiata (L.) Wilczek (Mung bean) were examined by light microscopy to study vascular development through the transition region. The beginning of vascular transition took place about one centimeter below the soil surface by the appearance of a pith between the large central vessels. At the higher level, pith parenchyma increased and the vessels are separated into four groups. The protoxylem maintains its peripheral position, but the metaxylem instead of differentiating toward the center, diverges laterally from the protoxylem. At successively higher levels, the hypocotyl axis widens and the distances between the protoxylem poles as well as the divergence of each xylem arms increased. The reorientation of primary xylem took place gradually upward. The length of the transition region from a point where the axis is exarch and protostele to a point where the xylem form approximately endarch condition was about two centimeter. Thus, most stages of vascular transition in mung bean seedling occur in the lower portion of the hypocotyl, and the reorientation of vascular tissue from exarch radial to the endarch collateral arrangement may be completed in the upper portion of the hypocotyl.The behavior of vascular cambium activity and the difference in maturation degree at the different levels of the hypocotyl indicating that the upper portion of mung bean hypocotyl is older than the lower one. Adventitious root primordia were recorded at the basal portion of hypocotyl and their development was associated with the activity of vascular cambium.