ORGANIZED DEVELOPMENT IN PLANTS
65
of radish Raphanus sativus L. In addition to sucrose and IAA, vascular
cambium formation in excised radish roots required the provision via
the base of the root of a kinin (benzylaminopurine was especially effective). Myoinositol greatly promoted secondary thickening. In both pea
and radish, provision of hormones and sugar at appropriate concentrations to the excised root via the base led to the initiation of oriented cell
divisions whose progeny differentiated into characteristic vascular tissue
elements. Cell division was an essential first step in the initiation of secondary tissue formation.
In the cases considered thus far concerned with secondary vascular
tissue formation, one is led to attribute to auxin, kinin, or gibberellin the
hormonal stimulation of cell divisions and to relate the differentiation
of xylem or phloem elements to secondary causes which become active
after the cell divisions have occurred. Such a view does not exclude the
possibility that auxin may play a dual role in xylem formation—first
the stimulation of cell divisions and, second, some specific effect on
xylem-cell maturation. In the second stage which most directly involves
differentiation processes per se, the interaction of other factors may
become important.
B. Primary Vascular Tissue Formation
Direct evidence of specific hormonal stimulation or control of primary
vascular tissue formation either in the root or the shoot is lacking. The
concept of such hormonal control of tissue differentiation in the shoot
apex was stated in general terms by Wardlaw (1944). Evidence that the
apical meristem of the shoot and associated young leaf primordia determine the pattern of vascular tissues of the stem is clear from the variety
of surgical experiments made on the shoot apex by Wardlaw, Ball, and
others (see review by Esau, 1954). Yet in none of this work did a clear
picture emerge suggesting the site of synthesis or the site of action of a
hormone which might control the differentiation of tissues below the apex.
By using quite a different approach to the problem, Jacobs and Morrow
(1957) sought correlations between primary xylem differentiation in
young leaves of Coleus and auxin production by these young leaves.
They were able to establish quite precise relationships between the time
of maximum rate of auxin production by a young expanding leaf and
the time of most rapid increase in number of mature primary xylem elements formed from procambial elements in that leaf. They visualized
xylem maturation as occurring under hormonal control, limited by available auxin coming from the young leaves. This evidence, although quite
indirect, fits into the earlier ideas developed by Jacobs in xylem-regeneration studies. In such studies on primary vascular tissue differentia-
65
of radish Raphanus sativus L. In addition to sucrose and IAA, vascular
cambium formation in excised radish roots required the provision via
the base of the root of a kinin (benzylaminopurine was especially effective). Myoinositol greatly promoted secondary thickening. In both pea
and radish, provision of hormones and sugar at appropriate concentrations to the excised root via the base led to the initiation of oriented cell
divisions whose progeny differentiated into characteristic vascular tissue
elements. Cell division was an essential first step in the initiation of secondary tissue formation.
In the cases considered thus far concerned with secondary vascular
tissue formation, one is led to attribute to auxin, kinin, or gibberellin the
hormonal stimulation of cell divisions and to relate the differentiation
of xylem or phloem elements to secondary causes which become active
after the cell divisions have occurred. Such a view does not exclude the
possibility that auxin may play a dual role in xylem formation—first
the stimulation of cell divisions and, second, some specific effect on
xylem-cell maturation. In the second stage which most directly involves
differentiation processes per se, the interaction of other factors may
become important.
B. Primary Vascular Tissue Formation
Direct evidence of specific hormonal stimulation or control of primary
vascular tissue formation either in the root or the shoot is lacking. The
concept of such hormonal control of tissue differentiation in the shoot
apex was stated in general terms by Wardlaw (1944). Evidence that the
apical meristem of the shoot and associated young leaf primordia determine the pattern of vascular tissues of the stem is clear from the variety
of surgical experiments made on the shoot apex by Wardlaw, Ball, and
others (see review by Esau, 1954). Yet in none of this work did a clear
picture emerge suggesting the site of synthesis or the site of action of a
hormone which might control the differentiation of tissues below the apex.
By using quite a different approach to the problem, Jacobs and Morrow
(1957) sought correlations between primary xylem differentiation in
young leaves of Coleus and auxin production by these young leaves.
They were able to establish quite precise relationships between the time
of maximum rate of auxin production by a young expanding leaf and
the time of most rapid increase in number of mature primary xylem elements formed from procambial elements in that leaf. They visualized
xylem maturation as occurring under hormonal control, limited by available auxin coming from the young leaves. This evidence, although quite
indirect, fits into the earlier ideas developed by Jacobs in xylem-regeneration studies. In such studies on primary vascular tissue differentia-