68
JOHN G. TORREY
ferentiated into xylem. The first divisions were disposed at random at a
given distance below the applied auxin and, subsequently, were arranged
cambial-like in radial rows. More recently (Wetmore and Rier, 1963)
it has been possible, by manipulation of the auxin and sugar concentrations, to control the position of the vascular tissue in the callus tissue, as
well as the type of mature vascular tissue formed. At a constant low
sugar concentration, increasing the auxin concentration placed on top of
the callus tissue piece produced an increasing diameter of the concentric
ring of nests forming xylem. At a constant auxin concentration, raising the sucrose concentration led to a change from xylem differentiation
(2% sucrose) to almost entirely phloem differentiation (4% sucrose),
with intermediate sucrose concentration leading to both xylem and
phloem formation from the localized nests of meristematic cells. In the
absence of added auxin, no vascular tissues were formed. In a somewhat
similar system, Clutter (1960) reported feeding IAA to cultured undifferentiated tobacco pith sections via a micropipette inserted into the tissue.
She observed localized regions of cell division and the subsequent differentiation of xylem tracheids. These experiments can be viewed as paralleling those on pea and radish roots mentioned earlier, in which auxin
and sugar and kinin provided via the basal end of isolated roots in
culture induced vascular cambium initiation and subsequent differentiation of xylem and phloem. In discussing these experiments there are, once
again, difficulties in ascribing to auxin or to kinin a direct role in vascular
tissue differentiation since in each case cell division was stimulated first
and then was followed by the differentiation of specific cell types. Recently Bergmann (1964) reported the stimulating effect of kinetin on
xylem differentiation in callus tissues of Nicotiana
tabacum. He found
evidence suggesting the importance of unequal divisions in this differentiation.
One is hard pressed to find a tissue system in which vascular tissue
differentiation occurs quite independent of immediately associated cell
divisions. Even in the differentiation of mature primary vascular elements from procambial elements, the final division of xylem or phloem
mother cells is frequently followed rapidly by the changes leading to a
differentiated cell type. One is really looking for a tissue made up of
undifferentiated cells which, without the intervention of rapid cell divisions, can be induced to differentiate directly into xylem or phloem
elements possessing characteristic cellular features.
One such system has been studied in some detail, that of the regeneration of vascular strands in Coleus. Jacobs (1952, 1954, 1956) presented
considerable evidence that the differentiation of xylem strands in wounded
stems of Coleus was limited by auxin moving into the stem from the
JOHN G. TORREY
ferentiated into xylem. The first divisions were disposed at random at a
given distance below the applied auxin and, subsequently, were arranged
cambial-like in radial rows. More recently (Wetmore and Rier, 1963)
it has been possible, by manipulation of the auxin and sugar concentrations, to control the position of the vascular tissue in the callus tissue, as
well as the type of mature vascular tissue formed. At a constant low
sugar concentration, increasing the auxin concentration placed on top of
the callus tissue piece produced an increasing diameter of the concentric
ring of nests forming xylem. At a constant auxin concentration, raising the sucrose concentration led to a change from xylem differentiation
(2% sucrose) to almost entirely phloem differentiation (4% sucrose),
with intermediate sucrose concentration leading to both xylem and
phloem formation from the localized nests of meristematic cells. In the
absence of added auxin, no vascular tissues were formed. In a somewhat
similar system, Clutter (1960) reported feeding IAA to cultured undifferentiated tobacco pith sections via a micropipette inserted into the tissue.
She observed localized regions of cell division and the subsequent differentiation of xylem tracheids. These experiments can be viewed as paralleling those on pea and radish roots mentioned earlier, in which auxin
and sugar and kinin provided via the basal end of isolated roots in
culture induced vascular cambium initiation and subsequent differentiation of xylem and phloem. In discussing these experiments there are, once
again, difficulties in ascribing to auxin or to kinin a direct role in vascular
tissue differentiation since in each case cell division was stimulated first
and then was followed by the differentiation of specific cell types. Recently Bergmann (1964) reported the stimulating effect of kinetin on
xylem differentiation in callus tissues of Nicotiana
tabacum. He found
evidence suggesting the importance of unequal divisions in this differentiation.
One is hard pressed to find a tissue system in which vascular tissue
differentiation occurs quite independent of immediately associated cell
divisions. Even in the differentiation of mature primary vascular elements from procambial elements, the final division of xylem or phloem
mother cells is frequently followed rapidly by the changes leading to a
differentiated cell type. One is really looking for a tissue made up of
undifferentiated cells which, without the intervention of rapid cell divisions, can be induced to differentiate directly into xylem or phloem
elements possessing characteristic cellular features.
One such system has been studied in some detail, that of the regeneration of vascular strands in Coleus. Jacobs (1952, 1954, 1956) presented
considerable evidence that the differentiation of xylem strands in wounded
stems of Coleus was limited by auxin moving into the stem from the