STRUCTURE AND DEVELOPMENT OF SECONDARY THICKENING MERISTEM IN MIRABILIS JALAPA (NYCTAGINACEAE)

The structure and development of vascular cambium and its derivatives were studied in normal and experimentally injured stems of Mirabilis jalapa L. (Nyctaginaceae). In normal stems the cambium was semi-storied and composed exclusively of fusiform cambial cells with no rays. Several collateral vascular bundles were joined by interfascicular cambium and formed a complete ring of activity. After a defi nite period a small segment of cambium that produced conducting elements of xylem and phloem ceased to divide. A new segment of cambium was developed from the parenchyma cells outside the phloem of previous cambium. This newly formed cambial segment replaced the nonfunctional segment by joining with other functional segments to form a continuous ring. Each successive segment of cambium followed a similar pattern of development. Functionally the cambium was bidirectional, producing both xylem and phloem on opposite sides, but the rate of cell division towards phloem was very slow, thus making the cambium appear functionally unidirectional. Functional sieve elements were observed in all the phloem islands; nonfunctional sieve elements showed heavy accumulation of callose. Experimentally we tried to induce the formation of rays by injuring the cambium, but instead of forming wound meristem it formed cork cambium around the wounds. The meristematic segments present between the xylem and phloem of medullary bundles also remained active even at the senescent stage.