80
JOHN G. TORREY
similar sequence could be observed in bud initiation in cell colonies of
Convolvulus by Earle and Torrey (1965b). At the earliest stage, a group
of meristematic cells with dense cytoplasm, large nuclei, and of small
cell size was produced by rapid cell division. Such nests of cells later
showed cell differentiation within the group, and the characteristics of an
apical meristem of a shoot became apparent. No connection need occur
between vascular tissues and these primordia; in fact, frequently buds
could be found in cell colonies with no evident mature xylem tissue
present. These may then develop secondarily.
In Convolvulus
roots, both roots and buds may be initiated endogenously, usually in association with a protoxylem strand of the root
(Bonnett, 1964; Bonnett and Torrey, 1966). Anatomical studies of the
sequences of initiation of these two types of structures suggest that an
undetermined primordium is first formed, which subsequently becomes
characteristically root or shoot (see Figs. 7, 16, and 17). (See pp. 70
and 72.)
Control of bud initiation in callus tissues has been studied most with
respect to specific chemical constituents of the nutrient medium. From
these studies has emerged the view of the importance of hormonal regulation of cell division/cell enlargement by auxin-kinin ratios. Modification of the basic effects of hormones can be achieved by other nutritional
components, notably the supply of organic nitrogen (Sandstedt and
Skoog, 1960), but also substances that interact with and affect auxin or
kinin levels (Lee, 1962). Hägen (1962) has shown that organic nitrogen
supply also profoundly affects morphogenesis in tobacco hybrid tumor
tissue.
Recently, Murashige (1964) reported the inhibition of root and bud
initiation in tobacco stem callus cultures by gibberellins. He found
that gibberellic acid and several related gibberellins, while actually
stimulating callus proliferation, inhibited the initiation of the nests of
cells which seem to be the earliest stage of organized development of
buds or roots. No inhibition of vascular tissue differentiation was observed.
In addition to direct chemical manipulation of bud initiation, there
are a number of reports of effects of the physical environment on bud
initiation in culture—effects which are largely unexplained at present,
but which must be fitted into our overall understanding at some later
time. Thus White's early observation (1939) of organ initiation in liquid
submerged culture but not in agar cultures of tobacco needs interpretation in terms of our present knowledge. The observations of Dougall and
Shimbayashi (1960) are of interest in this connection. They found that
tobacco callus tissue cultured on an agar medium produced buds as
JOHN G. TORREY
similar sequence could be observed in bud initiation in cell colonies of
Convolvulus by Earle and Torrey (1965b). At the earliest stage, a group
of meristematic cells with dense cytoplasm, large nuclei, and of small
cell size was produced by rapid cell division. Such nests of cells later
showed cell differentiation within the group, and the characteristics of an
apical meristem of a shoot became apparent. No connection need occur
between vascular tissues and these primordia; in fact, frequently buds
could be found in cell colonies with no evident mature xylem tissue
present. These may then develop secondarily.
In Convolvulus
roots, both roots and buds may be initiated endogenously, usually in association with a protoxylem strand of the root
(Bonnett, 1964; Bonnett and Torrey, 1966). Anatomical studies of the
sequences of initiation of these two types of structures suggest that an
undetermined primordium is first formed, which subsequently becomes
characteristically root or shoot (see Figs. 7, 16, and 17). (See pp. 70
and 72.)
Control of bud initiation in callus tissues has been studied most with
respect to specific chemical constituents of the nutrient medium. From
these studies has emerged the view of the importance of hormonal regulation of cell division/cell enlargement by auxin-kinin ratios. Modification of the basic effects of hormones can be achieved by other nutritional
components, notably the supply of organic nitrogen (Sandstedt and
Skoog, 1960), but also substances that interact with and affect auxin or
kinin levels (Lee, 1962). Hägen (1962) has shown that organic nitrogen
supply also profoundly affects morphogenesis in tobacco hybrid tumor
tissue.
Recently, Murashige (1964) reported the inhibition of root and bud
initiation in tobacco stem callus cultures by gibberellins. He found
that gibberellic acid and several related gibberellins, while actually
stimulating callus proliferation, inhibited the initiation of the nests of
cells which seem to be the earliest stage of organized development of
buds or roots. No inhibition of vascular tissue differentiation was observed.
In addition to direct chemical manipulation of bud initiation, there
are a number of reports of effects of the physical environment on bud
initiation in culture—effects which are largely unexplained at present,
but which must be fitted into our overall understanding at some later
time. Thus White's early observation (1939) of organ initiation in liquid
submerged culture but not in agar cultures of tobacco needs interpretation in terms of our present knowledge. The observations of Dougall and
Shimbayashi (1960) are of interest in this connection. They found that
tobacco callus tissue cultured on an agar medium produced buds as