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JOHN G. TORREY
itiated but suppressed in their development. In still other situations,
buds arise either superficially or endogenously from the tissues of roots.
Such variations on the general theme are described in interesting fashion
by Priestley and Swingle (1929).
Root initiation, like bud initiation, occurs in many different morphological situations. Typically, after the elongation of the embryonic
root or radicle, new meristematic sites are initiated deep in the inner
tissues of the root in the pericycle and frequently involve also the endodermis. The lateral root primordia are thus typically endogenous in origin
and not derived from surface layers as are leaves and buds. The initiation of primordia occurs in seedling roots in approximately acropetal
sequence, but in no precise order comparable to leaf initiation.
Initiation of roots can occur from almost all other structures of the
plant in addition to the root. Under favorable environmental conditions
adventitious roots are initiated by stem pieces, so that whole plants are
formed; this regenerative process serves as the basis for important vegetative reproduction and propagation. Under experimental conditions,
root primordia may be initiated on excised shoot parts, e.g., leaves,
petioles, floral parts such as petals, stamens, or other isolated parts or
fragments. In many such cases, buds also may be initiated, and whole
plants may develop. Root initiation has important practical ramifications
in horticultural practices, and much interest and research has been devoted to the practical and theoretical problems, especially since the
discovery of the root-initiating hormones of the auxin type in the
1930's. Recent reviews refer to some of this vast literature (Libbert,
1956-1957; Torrey, 1965a). In the context of the present discussion, it
is interesting to restrict our consideration of bud, leaf, and root initiation
to that occurring in isolated tissue pieces or callus in culture, even though
no sharp lines separate the processes observable in vitro from those seen
in injured, regenerating plants or plant parts or from those in the intact
plant. Gautheret (1959) in his comprehensive treatise on plant tissue
culture has considered these problems in some detail.
The classic model for organ initiation in plant tissues cultivated in
vitro is that provided by Skoog and Miller (1957). Although organ initiation had been observed in various callus tissues in vitro almost
from the time tissues were first cultured successfully (Gautheret, 1942,
1959; White, 1943), Skoog and Miller were able to culture aseptically
isolated pieces of stem tissue of tobacco, Nicotiana
tabacum, in a defined medium. By subtle manipulation of auxin (IAA) and kinin (6furfurylaminopurine) concentrations, they were able to induce (a)
further proliferation of undifferentiated parenchyma tissue, (b) the
initiation of roots at relatively high auxin concentration, or (c) the
JOHN G. TORREY
itiated but suppressed in their development. In still other situations,
buds arise either superficially or endogenously from the tissues of roots.
Such variations on the general theme are described in interesting fashion
by Priestley and Swingle (1929).
Root initiation, like bud initiation, occurs in many different morphological situations. Typically, after the elongation of the embryonic
root or radicle, new meristematic sites are initiated deep in the inner
tissues of the root in the pericycle and frequently involve also the endodermis. The lateral root primordia are thus typically endogenous in origin
and not derived from surface layers as are leaves and buds. The initiation of primordia occurs in seedling roots in approximately acropetal
sequence, but in no precise order comparable to leaf initiation.
Initiation of roots can occur from almost all other structures of the
plant in addition to the root. Under favorable environmental conditions
adventitious roots are initiated by stem pieces, so that whole plants are
formed; this regenerative process serves as the basis for important vegetative reproduction and propagation. Under experimental conditions,
root primordia may be initiated on excised shoot parts, e.g., leaves,
petioles, floral parts such as petals, stamens, or other isolated parts or
fragments. In many such cases, buds also may be initiated, and whole
plants may develop. Root initiation has important practical ramifications
in horticultural practices, and much interest and research has been devoted to the practical and theoretical problems, especially since the
discovery of the root-initiating hormones of the auxin type in the
1930's. Recent reviews refer to some of this vast literature (Libbert,
1956-1957; Torrey, 1965a). In the context of the present discussion, it
is interesting to restrict our consideration of bud, leaf, and root initiation
to that occurring in isolated tissue pieces or callus in culture, even though
no sharp lines separate the processes observable in vitro from those seen
in injured, regenerating plants or plant parts or from those in the intact
plant. Gautheret (1959) in his comprehensive treatise on plant tissue
culture has considered these problems in some detail.
The classic model for organ initiation in plant tissues cultivated in
vitro is that provided by Skoog and Miller (1957). Although organ initiation had been observed in various callus tissues in vitro almost
from the time tissues were first cultured successfully (Gautheret, 1942,
1959; White, 1943), Skoog and Miller were able to culture aseptically
isolated pieces of stem tissue of tobacco, Nicotiana
tabacum, in a defined medium. By subtle manipulation of auxin (IAA) and kinin (6furfurylaminopurine) concentrations, they were able to induce (a)
further proliferation of undifferentiated parenchyma tissue, (b) the
initiation of roots at relatively high auxin concentration, or (c) the