COMPARATIVE ANATOMY OF ENDOGENOUS BUD AND LATERAL ROOT FORMATION IN CONVOLVULUS ARVENSIS ROOTS CULTURED IN VITRO

The anatomy of endogenous bud and lateral root formation was studied in Convolvulus arvensis roots cultured in vitro. Although early stages in the initiation of buds and roots were found to be identical, continued evelopment of primordia into roots or buds was accompanied by differences in participating primary root tissues, in rates of development, and in orientation of cell divisions. Evidence for the existence of a primordium capable of developing into either a bud or root is discussed with reference to Convolvulus roots and other plant parts shown to have similar morphogenetic potentials. THE FORMATION of buds from roots has been discussed extensively by Beijerinck (1887), van Tieghem and Duliot (1888), and Priestley and Swingle (1929). Prior to the monograph of Beijerinck, numerous investigators had contributed to a recognition of the variety of plants which form buds from their roots. Irmisch (1857) compiled a list of these plants comprising 42 species, 38 of them dicots; this list later grew to 132 species, 124 of them dicots (Wittrock, 1884). Among this large number of plants which develop buds from their roots, Convolvulus arvensis, the field bindweed, was recognized as one of a much smaller group of plants, representing several families, in which buds develop from the pericycle in a location identical to that of lateral root formation. The anatomy of bud formation from Convolvulus roots was first studied by Irmisch (1857) and Vogl (1863). They recognized that bud formation was endogenous and occurred on the protoxylem radii, but because of the occurrence of extensive secondary growth in roots they were studying, they were unable to determine the exact location of origin. Beijerinck studied bud formation on the hypocotyl and root of Convolvulus eedlings. He found that buds arose from the pericycle at sites opposite the protoxylem poles and showed no evidence of endodermal participation during their development. His attempts to obtain a series of stages in the formation of lateral roots were unsuccessful because of the small number which were present. Beijerinck suggested that endogenous buds develop in place of lateral roots. Van Tieghem and Duliot described the formation of bud primordia by growth and division in 1 Received for publication June 28, 1965. This investigation was supported in part by grants from the National Institutes of Health, U. S. Public Health Service, GM-10493 to HTB and GM-08145 to JGT. 2Present address: Department of Biology, University of Oregon, Eugene. all directions of an arc of pericycle cells, which produced a hemispherical protuberance. They believed that as a bud primordium developed it sequentially digested the endodermis and then all the cortical layers in its path to the periphery of the root. Lateral root formation in Convolvulus siculus and Convolvulus tricolor also proceeded by tangential divisions of six to eight pericycle cells surrounding a protoxylem pole. As the pericycle cells continued to divide, the endodermis divided radially and then tangentially; consequently the endodermis remained single-layered at the flanks of the lateral root, whereas at the tip it became several layers thick. Developmental stages of lateral root formation in Convolvulus arvensis were not illustrated, although one drawing of a lateral root before emergence showed a single-layered endodermis which had undergone only radial divisions. These investigators outlined the major steps in the anatomy of bud formation from Convolvulus roots. As part of a study of the physiology of organ formation in cultured Convolvulus roots, a reinvestigation of the anatomy of endogenous bud and root formation was undertaken. Culturing these roots has enabled such a study to be made in the absence of secondary growth and has allowed stages of organ formation to be placed in rather precise sequence by their time of development in organ culture. MATERIALS AND METHODS-Convolvulus arvensis root segments were cultured in 10-cm petri plates on a modified Bonner pea root medium (Torrey, 1958) solidified with agar and adjusted to a postautoclave pH of 4.5. Prior to a study of the anatomy of bud and root formation the distribution of buds and roots in cleared root segments was determined. Intact roots or isolated segments in culture characteristically developed protuberances which could not he classified reliably as buds or roots. However,