ORGANIZED DEVELOPMENT IN PLANTS
63
monal stimuli that primarily influence cell division and those that influence or determine the later processes which lead to the different cell
types. In the initial blocking-out of tissue patterns, cell division may be
the predominant process. Strictly speaking, cell differentiation follpws cell
division and includes all the changes in the cell until its maturation and,
in some cases, until its death. It is quite possible that the cell division
itself may be an unequal or what might be called a "differential" division (Bünning, 1952) such as to produce two new cells that differ from
each other in their cytoplasmic contents and that are already by the end
of telophase destined to different fates. In discussing the experimental
evidence concerning vascular tissue formation it is useful to make this
distinction and to consider the two types of hormonal stimulations
separately in so far as this is possible. In most cases studied, the formation of vascular tissues is intimately associated with cell divisions. In
only a few cases is there evidence of xylem or phloem differentiation
which is not more or less directly preceded by cell divisions. These few
cases will be discussed last.
A. Secondary Vascular Tissue Formation
The most striking case of cell divisions immediately followed by the
differentiation of vascular tissues, i.e., xylem or phloem, is that of the
vascular cambium, whether in the root or shoot. Here, a cell divides, one
of the two cell products immediately begins radial enlargement, and in
a short time shows the distinctive cellular features that mark it as a
xylem or a phloem element. The cell products may vary in their final
destination, becoming tracheid, vessel elements, xylem parenchyma,
xylem fiber or, on the phloem side, sieve tube element, companion cell,
parenchyma cell, fiber, or other differentiated cell type.
The hormonal stimulation of the continuing cell divisions of the vascular cambium in the shoot has been extensively studied both before and
since the discovery of the indole auxins. There is strong evidence that in
deciduous trees hormones produced in the spring of each year by young
leaves and buds of the shoot initiate cambial divisions in the stem and
reactivate the development of secondary tissues of the xylem and
phloem. There is also the suggestion that the rapidly dividing cambial
zone itself may produce a stimulus to further cell divisions. Circumstantial evidence that auxins of the IAA type are responsible for this seasonal initiation of cell division in the cambium is quite extensive. The
literature has been reviewed in detail recently (Söding, 1961 ; Kozlowski,
1962; Wareing et al, 1964), and it is not necessary to pursue the evidence here. Suggestions concerning the importance of other internal
tissue constituents as limiting vascular cambial activity have included
63
monal stimuli that primarily influence cell division and those that influence or determine the later processes which lead to the different cell
types. In the initial blocking-out of tissue patterns, cell division may be
the predominant process. Strictly speaking, cell differentiation follpws cell
division and includes all the changes in the cell until its maturation and,
in some cases, until its death. It is quite possible that the cell division
itself may be an unequal or what might be called a "differential" division (Bünning, 1952) such as to produce two new cells that differ from
each other in their cytoplasmic contents and that are already by the end
of telophase destined to different fates. In discussing the experimental
evidence concerning vascular tissue formation it is useful to make this
distinction and to consider the two types of hormonal stimulations
separately in so far as this is possible. In most cases studied, the formation of vascular tissues is intimately associated with cell divisions. In
only a few cases is there evidence of xylem or phloem differentiation
which is not more or less directly preceded by cell divisions. These few
cases will be discussed last.
A. Secondary Vascular Tissue Formation
The most striking case of cell divisions immediately followed by the
differentiation of vascular tissues, i.e., xylem or phloem, is that of the
vascular cambium, whether in the root or shoot. Here, a cell divides, one
of the two cell products immediately begins radial enlargement, and in
a short time shows the distinctive cellular features that mark it as a
xylem or a phloem element. The cell products may vary in their final
destination, becoming tracheid, vessel elements, xylem parenchyma,
xylem fiber or, on the phloem side, sieve tube element, companion cell,
parenchyma cell, fiber, or other differentiated cell type.
The hormonal stimulation of the continuing cell divisions of the vascular cambium in the shoot has been extensively studied both before and
since the discovery of the indole auxins. There is strong evidence that in
deciduous trees hormones produced in the spring of each year by young
leaves and buds of the shoot initiate cambial divisions in the stem and
reactivate the development of secondary tissues of the xylem and
phloem. There is also the suggestion that the rapidly dividing cambial
zone itself may produce a stimulus to further cell divisions. Circumstantial evidence that auxins of the IAA type are responsible for this seasonal initiation of cell division in the cambium is quite extensive. The
literature has been reviewed in detail recently (Söding, 1961 ; Kozlowski,
1962; Wareing et al, 1964), and it is not necessary to pursue the evidence here. Suggestions concerning the importance of other internal
tissue constituents as limiting vascular cambial activity have included