ORGANIZED DEVELOPMENT IN PLANTS
69
leaves above the cut. Excision of the leaf above a wound and replacement on the cut petiole of an amount of auxin equivalent to that normally diffusing from the excised leaf led to xylem regeneration around
the wound equivalent to that when the leaf was left intact. The quantitative relationship demonstrated between diffusing auxin and xylem
formation seems well established. Jacobs (1952) pointed out that the
first xylem strands to regenerate were not the result of cambial activity
per se which appeared later and gave rise to "wound-xylem." In earlier
careful anatomical studies of this wounding response, Sinnott and Bloch
(1945) did show, however, that the initial regeneration response involved mitoses of pith cells along the locus of the future strand. In some
cases pith cells developed directly into xylem elements, retaining their
original rectangular shape. Here, at least cell division need not take
place for xylem differentiation, but one is still uncertain whether the
primary auxin response studied by Jacobs was a direct effect of auxin
on xylem differentiation or the stimulation of cell division which was,
in turn, followed by xylem differentiation. Excised stem segments of
Coleus cultured aseptically in nutrient medium show xylem differentiation. Fosket and Roberts (1964) found that externally supplied auxin
(2,4-D, IAA) or kinin (kinetin) or triiodobenzoic acid at relatively low
concentrations inhibited xylem formation. Only at very low auxin concentrations (0.01-0.05 mg/liter) was xylem formation stimulated. The
authors concluded that these substances offset the transport of endogenous auxin involved in xylem initiation.
With the availability of cell suspension cultures of higher plant tissues,
one could hope that the problem of vascular tissue differentiation might
be approached quite directly. If, in fact, uniform, viable, mature parenchyma cell populations, such as can be obtained from liquid tissue
cultures, were subjected directly to the proper environmental conditions,
including the proper hormonal environment, one should be able to induce
the differentiation of specific cell types such as tracheids or other xylem
elements. To date, this ideal has not been achieved. Steward et al.
(1958) made the interesting observation that carrot callus tissue cells
grown in a complex liquid medium containing coconut milk divide to
form rounded nodular clumps of cells; these clumps show internal differentiation in the formation of concentric layers of lignifying elements
derived from orderly rows of cells in division. According to these authors
the internal differentiation of a nest of vascular tissue depends upon the
establishment during the development of these cell aggregates of a
gradient, presumably of nutrient and/or hormonal factors absorbed from
the bathing medium. No direct involvement in this differentiation process
of specific hormones or cell division factors was demonstrated. Mahlberg
69
leaves above the cut. Excision of the leaf above a wound and replacement on the cut petiole of an amount of auxin equivalent to that normally diffusing from the excised leaf led to xylem regeneration around
the wound equivalent to that when the leaf was left intact. The quantitative relationship demonstrated between diffusing auxin and xylem
formation seems well established. Jacobs (1952) pointed out that the
first xylem strands to regenerate were not the result of cambial activity
per se which appeared later and gave rise to "wound-xylem." In earlier
careful anatomical studies of this wounding response, Sinnott and Bloch
(1945) did show, however, that the initial regeneration response involved mitoses of pith cells along the locus of the future strand. In some
cases pith cells developed directly into xylem elements, retaining their
original rectangular shape. Here, at least cell division need not take
place for xylem differentiation, but one is still uncertain whether the
primary auxin response studied by Jacobs was a direct effect of auxin
on xylem differentiation or the stimulation of cell division which was,
in turn, followed by xylem differentiation. Excised stem segments of
Coleus cultured aseptically in nutrient medium show xylem differentiation. Fosket and Roberts (1964) found that externally supplied auxin
(2,4-D, IAA) or kinin (kinetin) or triiodobenzoic acid at relatively low
concentrations inhibited xylem formation. Only at very low auxin concentrations (0.01-0.05 mg/liter) was xylem formation stimulated. The
authors concluded that these substances offset the transport of endogenous auxin involved in xylem initiation.
With the availability of cell suspension cultures of higher plant tissues,
one could hope that the problem of vascular tissue differentiation might
be approached quite directly. If, in fact, uniform, viable, mature parenchyma cell populations, such as can be obtained from liquid tissue
cultures, were subjected directly to the proper environmental conditions,
including the proper hormonal environment, one should be able to induce
the differentiation of specific cell types such as tracheids or other xylem
elements. To date, this ideal has not been achieved. Steward et al.
(1958) made the interesting observation that carrot callus tissue cells
grown in a complex liquid medium containing coconut milk divide to
form rounded nodular clumps of cells; these clumps show internal differentiation in the formation of concentric layers of lignifying elements
derived from orderly rows of cells in division. According to these authors
the internal differentiation of a nest of vascular tissue depends upon the
establishment during the development of these cell aggregates of a
gradient, presumably of nutrient and/or hormonal factors absorbed from
the bathing medium. No direct involvement in this differentiation process
of specific hormones or cell division factors was demonstrated. Mahlberg