62
JOHN G.
TORREY
those already discussed with respect to development in the angiosperms.
Because of the relative simplicity of certain of their vegetative structures, especially of the gametophyte generation, the ferns, mosses, and
liverworts offer rich sources of experimental material for studies of
morphogenesis.
VI. The Initiation and Differentiation of Vascular Tissues
At its earliest stages, the initiation of embryonic development involves
the setting apart of a cell or a few cells from associated cells in a tissue
whose fate thereafter is determined to be different from the others. Such
a cell, through repeated rapid divisions, will form a meristematic mass
leading to a meristem, an embryo, or a whole organism. At this earliest
stage, the process is not distinguishable from processes of cell differentiation in general. A cell begins along a path which makes it distinct from
other cells. In the case of meristem initiation, the fate is one of continued cell division in a particular pattern, which leads to a new multicellular structure.
One can view vascular tissue differentiation in the same way. Under
appropriate stimulus a cell is set apart from adjacent cells; it may undergo one or many divisions before the progeny differentiate into xylem or
phloem elements. In the case of vascular tissue differentiation, the
sequence of events is a relatively short one and frequently ends with the
loss of the cell nucleus. In the case of tracheary elements of the xylem,
cell death results, whereas in the case of sieve tube elements of the
phloem, an arrested enucleate state develops. One can say that xylem
elements are derived from meristemoids and that this process is not
fundamentally different from that involved in the initiation of embryos
from single cells in tissue culture, or the initiation of an organ primordium. The main distinction is perhaps that vascular tissue differentiation comes to a halt much earlier—that the course of differentiation is a
terminal one. Taken from this point of view, it is interesting to look at
the problem of the physiological control of vascular tissue differentiation
as a part of the general problem of the initiation of organization in
plants. Once again one is confronted with a concern for interacting
hormonal control mechanisms and their interpretation.
It is useful to review briefly the evidence supporting the general hypothesis that vascular tissue formation in higher plants is controlled by
the interplay of hormonal factors within the plant. The experimental
work is neither extensive nor direct, yet there is enough accumulated
evidence to consider the hypothesis seriously and to formulate new experimental approaches which would test the ideas more directly.
The distinction has already been made (Torrey, 1963) between hor-
JOHN G.
TORREY
those already discussed with respect to development in the angiosperms.
Because of the relative simplicity of certain of their vegetative structures, especially of the gametophyte generation, the ferns, mosses, and
liverworts offer rich sources of experimental material for studies of
morphogenesis.
VI. The Initiation and Differentiation of Vascular Tissues
At its earliest stages, the initiation of embryonic development involves
the setting apart of a cell or a few cells from associated cells in a tissue
whose fate thereafter is determined to be different from the others. Such
a cell, through repeated rapid divisions, will form a meristematic mass
leading to a meristem, an embryo, or a whole organism. At this earliest
stage, the process is not distinguishable from processes of cell differentiation in general. A cell begins along a path which makes it distinct from
other cells. In the case of meristem initiation, the fate is one of continued cell division in a particular pattern, which leads to a new multicellular structure.
One can view vascular tissue differentiation in the same way. Under
appropriate stimulus a cell is set apart from adjacent cells; it may undergo one or many divisions before the progeny differentiate into xylem or
phloem elements. In the case of vascular tissue differentiation, the
sequence of events is a relatively short one and frequently ends with the
loss of the cell nucleus. In the case of tracheary elements of the xylem,
cell death results, whereas in the case of sieve tube elements of the
phloem, an arrested enucleate state develops. One can say that xylem
elements are derived from meristemoids and that this process is not
fundamentally different from that involved in the initiation of embryos
from single cells in tissue culture, or the initiation of an organ primordium. The main distinction is perhaps that vascular tissue differentiation comes to a halt much earlier—that the course of differentiation is a
terminal one. Taken from this point of view, it is interesting to look at
the problem of the physiological control of vascular tissue differentiation
as a part of the general problem of the initiation of organization in
plants. Once again one is confronted with a concern for interacting
hormonal control mechanisms and their interpretation.
It is useful to review briefly the evidence supporting the general hypothesis that vascular tissue formation in higher plants is controlled by
the interplay of hormonal factors within the plant. The experimental
work is neither extensive nor direct, yet there is enough accumulated
evidence to consider the hypothesis seriously and to formulate new experimental approaches which would test the ideas more directly.
The distinction has already been made (Torrey, 1963) between hor-