ORGANIZED DEVELOPMENT IN PLANTS
75
polarized or at least unidirectional flow of auxin into the tissue system
and the absence of cell divisions. When polarized application of auxin is
provided under similar conditions, cell divisions seem to be initiated and
xylem formation ensues.
The analogy between the initiation of primary or secondary vascular
tissues and the initiation of other newly differentiated structures in the
intact plant or in isolated tissues in culture is apparent at two levels. The
similarity of hormonal control mechanisms is one evident basis for comparison ; the other basis for comparison is the initial similarity in
anatomical details—i.e., the initiation of new cell divisions and subsequent differentiation. Anatomical comparisons are most easily made in
callus tissues in culture where early formation of nests of vascular elements is not easily distinguished from early divisions leading ultimately
to bud formation. Whether these comparisons are, in fact, valid requires
further experimental study.
VII. The Initiation of Root, Bud, and Leaf Primordia
In the intact higher vascular plant, the initiation of leaf primordia is
a continuous or discontinuous process proceeding on the periphery of the
apical meristem of the shoot apex in precise and predictable order.
Leaves arise as relatively superficial protuberances involving cell divisions in a well-defined surface cell layer of the apex and one or more
subsurface cell layers. The order of appearance of the primordia determines the position and arrangement of leaves of the fully developed
shoot. Lateral shoots are initiated by cell divisions in the surface and
immediately subsurface cell layers which lie at the base of the developing young leaf primordium on the adaxial side between the leaf and the
stem. The young lateral or axillary bud primordia, once initiated as nests
of meristematic cells, may be arrested in subsequent development and
remain as quiescent structures for the life of the plant, inhibited by
hormonal correlative interactions. Removal of the shoot tip, which means
removal of the major site of auxin synthesis, may effect release of the
axillary bud inhibition so that development of the already initiated bud
ensues. The complexities of such bud inhibition have interested plant
physiologists for centuries. Recent reviews (Gregory and Veale, 1957;
Thimann, 1960) summarize the present views about the development of
such structures.
Under a variety of natural circumstances, buds may arise at sites
other than in the axils of leaves. Adventitious buds arise from wound
callus or other regenerating sites in an injured plant. In such cases,
buds may be initiated from undifferentiated parenchyma, but, in many
cases, wounding or injury may simply activate bud primordia already in-
75
polarized or at least unidirectional flow of auxin into the tissue system
and the absence of cell divisions. When polarized application of auxin is
provided under similar conditions, cell divisions seem to be initiated and
xylem formation ensues.
The analogy between the initiation of primary or secondary vascular
tissues and the initiation of other newly differentiated structures in the
intact plant or in isolated tissues in culture is apparent at two levels. The
similarity of hormonal control mechanisms is one evident basis for comparison ; the other basis for comparison is the initial similarity in
anatomical details—i.e., the initiation of new cell divisions and subsequent differentiation. Anatomical comparisons are most easily made in
callus tissues in culture where early formation of nests of vascular elements is not easily distinguished from early divisions leading ultimately
to bud formation. Whether these comparisons are, in fact, valid requires
further experimental study.
VII. The Initiation of Root, Bud, and Leaf Primordia
In the intact higher vascular plant, the initiation of leaf primordia is
a continuous or discontinuous process proceeding on the periphery of the
apical meristem of the shoot apex in precise and predictable order.
Leaves arise as relatively superficial protuberances involving cell divisions in a well-defined surface cell layer of the apex and one or more
subsurface cell layers. The order of appearance of the primordia determines the position and arrangement of leaves of the fully developed
shoot. Lateral shoots are initiated by cell divisions in the surface and
immediately subsurface cell layers which lie at the base of the developing young leaf primordium on the adaxial side between the leaf and the
stem. The young lateral or axillary bud primordia, once initiated as nests
of meristematic cells, may be arrested in subsequent development and
remain as quiescent structures for the life of the plant, inhibited by
hormonal correlative interactions. Removal of the shoot tip, which means
removal of the major site of auxin synthesis, may effect release of the
axillary bud inhibition so that development of the already initiated bud
ensues. The complexities of such bud inhibition have interested plant
physiologists for centuries. Recent reviews (Gregory and Veale, 1957;
Thimann, 1960) summarize the present views about the development of
such structures.
Under a variety of natural circumstances, buds may arise at sites
other than in the axils of leaves. Adventitious buds arise from wound
callus or other regenerating sites in an injured plant. In such cases,
buds may be initiated from undifferentiated parenchyma, but, in many
cases, wounding or injury may simply activate bud primordia already in-