Observations on the Origin and Development of Sphaeroblasts in the Apple

The mode of origin of root buds and root sprouts in the clonal tree Sassafras albidum (Lauraceae)

“…Sphaeroblasts are woody inclusions that form in the bark and grow by means of the activity of a spherical vascular cambium (Stoutemyer, 1937). Sphaeroblasts have been found in the stems of many trees (Stoutemyer, 1937;Wellensiek, 1952;Baldini and Mosse, 1956;Fink, 1982), but to our knowledge, this is the first report of sphaeroblasts in woody roots. ''Whorls'' of xylem cells have been observed in the bark of root cuttings of P. tremuloides and Medicago sativa, but these features were not studied in enough detail to determine whether they were surrounded by cambia (Murray, 1957;Schier, 1973).…”

“…Sphaeroblasts have been frequently reported to serve as sites of organogenesis both in vitro and in vivo (Baldini and Mosse, 1956;Steward, Mapes, and Mears, 1958;Goldacre, Unt, and Kefford, 1962;Mehta, 1965;Chen and Galston, 1967;Halperin, 1969;Prinsen et al, 1992). In Pisum epicotyls, for example, the cambial cell layers of the sphaeroblasts that develop following infection by A. rhizogenes serve as sites for the mode of origin of hairy roots (Prinsen et al, 1992).…”

“…In Pisum epicotyls, for example, the cambial cell layers of the sphaeroblasts that develop following infection by A. rhizogenes serve as sites for the mode of origin of hairy roots (Prinsen et al, 1992). In Pyrus malus, sphaeroblasts may produce adventitious shoots that are of horticultural importance as a source of easy-to-root stem cuttings (Stoutemyer, 1937;Baldini and Mosse, 1956).…”

“…The mode of origin of sphaeroblasts has been relatively well studied in apple (P. malus). In 1937, Stoutemyer proposed that the sphaeroblasts of apple are derived from adventive cambia that form around groups of pericyclic fibers in the bark, but in his micrographs these cambia resemble cork cambia rather than vascular cambia and subsequent, more detailed studies by Baldini and Mosse (1956) found no role for pericyclic fibers in sphaeroblast ontogeny. To induce sphaeroblast formation, Baldini and Mosse debudded and decapitated several coppice sprouts (Derman, 1948) and in response to this treatment, the cortex and phloem parenchyma of the sprouts began to divide.…”

“…Reparative buds may in fact be prevented from sprouting in situ by a lack of connections with the vasculature of their parent roots. Although sphaeroblasts of Laburnum can sprout upon being isolated and planted in peat (Baldini and Mosse, 1956), experiments to determine whether the sphaeroblasts of S. albidum roots are able to regenerate new plants upon being removed from the bark were not performed. Thus, while the additional root buds of S. albidum have a clear role in clonal spread, reparative buds originating in the proliferated pericycle and their associated sphaeroblasts have no known function at this time.…”