66
JOHN G. TORREY
tion, the formation of procambial elements by cell division and differential cell elongation is intimately tied in time to subsequent maturation
phenomena, so that complete separation of these into dependent processes is almost impossible. In the shoot apex of rosette plants, Sachs
et al. (1959) showed that externally supplied gibberellins markedly stimulated cell divisions in the subapical region of the meristem and had
little effect on divisions in the apical meristem itself. The region stimulated into division by GA treatment in these plants included the vascular
tissue region associated with young expanding leaves.
In caulescent plants such as Chrysanthemum,
Sachs and Lang (1960)
showed that AMO-1618, a substance known to antagonize GA action,
inhibits stem elongation by inhibiting cell divisions in the subapical
meristem region ; this inhibition is released by applied GA. Leaf initiation
was unaffected by these treatments. No direct examination was made
in this system of the effects of hormones on vascular tissue formation in
the stem itself, although clearly these are the morphological regions of
interest in Jacobs' study in Coleus. It would be most interesting to study
in Coleus the development of the primary vascular system under the
influence of gibberellins and gibberellin inhibitors and to examine in
detail vascular differentiation in the plants studied by Sachs and Lang to
determine the possible involvement of both auxins and gibberellins in
hormonal control of vascular tissue formation.
In ferns, Steeves and Briggs (1960) have provided strong evidence for
the quite direct involvement of diffusible auxin in limiting the terminal
stages of primary xylem maturation in the rachis of leaves of Osmunda.
If, after the primary xylem tissues are all blocked out in the procambial
strands by cell division, one removes the terminal fronds, secondary wall
formation and subsequent lignification of the existing xylem elements are
arrested. Replacement of the excised terminal leaf with auxin in lanoline
paste almost completely restores normal maturation of these elements.
In the root only indirect evidence exists for hormonal control of primary
vascular tissue differentiation. Experiments involving removal or injury
of the root apex, external application of hormones, and similar studies
which provide indirect evidence for a possible role of auxin in vascular
tissue formation have been reviewed (Clowes, 1959; Torrey, 1965a).
In most of these studies, the response to auxin could be interpreted
equally well as an effect on cell division preceding xylem maturation or
an effect on xylem maturation. Torrey (1953) interpreted the accelerated
maturation of protoxylem in pea roots in response to auxin treatment as
a direct effect on maturation and not one on cell division. No assessment
of a possible initial stimulus to cell division was made.
Jensen (1955) found that, in roots of Phaseolus treated with IAA for
JOHN G. TORREY
tion, the formation of procambial elements by cell division and differential cell elongation is intimately tied in time to subsequent maturation
phenomena, so that complete separation of these into dependent processes is almost impossible. In the shoot apex of rosette plants, Sachs
et al. (1959) showed that externally supplied gibberellins markedly stimulated cell divisions in the subapical region of the meristem and had
little effect on divisions in the apical meristem itself. The region stimulated into division by GA treatment in these plants included the vascular
tissue region associated with young expanding leaves.
In caulescent plants such as Chrysanthemum,
Sachs and Lang (1960)
showed that AMO-1618, a substance known to antagonize GA action,
inhibits stem elongation by inhibiting cell divisions in the subapical
meristem region ; this inhibition is released by applied GA. Leaf initiation
was unaffected by these treatments. No direct examination was made
in this system of the effects of hormones on vascular tissue formation in
the stem itself, although clearly these are the morphological regions of
interest in Jacobs' study in Coleus. It would be most interesting to study
in Coleus the development of the primary vascular system under the
influence of gibberellins and gibberellin inhibitors and to examine in
detail vascular differentiation in the plants studied by Sachs and Lang to
determine the possible involvement of both auxins and gibberellins in
hormonal control of vascular tissue formation.
In ferns, Steeves and Briggs (1960) have provided strong evidence for
the quite direct involvement of diffusible auxin in limiting the terminal
stages of primary xylem maturation in the rachis of leaves of Osmunda.
If, after the primary xylem tissues are all blocked out in the procambial
strands by cell division, one removes the terminal fronds, secondary wall
formation and subsequent lignification of the existing xylem elements are
arrested. Replacement of the excised terminal leaf with auxin in lanoline
paste almost completely restores normal maturation of these elements.
In the root only indirect evidence exists for hormonal control of primary
vascular tissue differentiation. Experiments involving removal or injury
of the root apex, external application of hormones, and similar studies
which provide indirect evidence for a possible role of auxin in vascular
tissue formation have been reviewed (Clowes, 1959; Torrey, 1965a).
In most of these studies, the response to auxin could be interpreted
equally well as an effect on cell division preceding xylem maturation or
an effect on xylem maturation. Torrey (1953) interpreted the accelerated
maturation of protoxylem in pea roots in response to auxin treatment as
a direct effect on maturation and not one on cell division. No assessment
of a possible initial stimulus to cell division was made.
Jensen (1955) found that, in roots of Phaseolus treated with IAA for