ORGANIZED DEVELOPMENT IN PLANTS
41
of undifferentiated parenchyma cells showing no organization can be
propagated indefinitely in callus tissue culture. It is when cellular events
are thus freed of the highest level of control, as occurs also in cancer or
the crown-gall disease, that normal plant form and organization disappear, even though structure at the cellular level appears to be unaffected. Yet, as we shall see, one can impose upon the in vitro cellular
system physiological controls which either resemble or duplicate the
controls actually operating in vivo; thus one allows the process of initiation of organization to begin anew. The controlled initiation of an organ
primordium from a randomly arranged mass of parenchyma cells in
culture under the influence of specific additions to the nutrient medium
is a dramatic and potentially powerful approach to understanding the
mechanisms controlling organization in the intact system. It is a tool
which permits an examination of the activities of cells as cells, but also
their activities as they are components of a multicellular system. The
evidence from organ and tissue culture studies seems clear that multicellular aggregates show capacities for specialized syntheses which
lead to correlative interactions between cellular populations, making distinctive organization possible. It is the purpose of this review to examine
the problem of the initiation of organization, especially as it can be
studied in cell, tissue, and organ cultures. The problem centers around
understanding the changes that ensue when cells become members of a
population and when unorganized cell populations develop into organized
multicellular structures of characteristic morphology.
In the discussion of the initiation of organized structures in cultures,
it is convenient to start with a particular premise which is supported in
part by the experimental data to be discussed later and which runs
through the discussions, explicitly or implicitly, as a unifying theme. The
assumption is made that the initiation of organized structure takes place
in a cell population in very much the same fundamental way for all
the variety of different types of structures which are formed. Typically,
a single cell in a mass of cells is activated in some way and undergoes
a cell division or a series of divisions to form a small mass of meristematic tissue, referred to by some as a "nodule" or "nest," by others
as a "cambial-like" center. The author prefers the term used by
Bünning (1952)—"meristemoid," a meristem-like cell or group of cells.
In extending Bünning's use of the term, a meristemoid could refer to a
single cell or to a group of cells acting together as a meristematic
center. It is in this sense that the term is most useful. The argument
can be made that the differentiation of single isolated xylem elements in
a callus parenchyma involves meristemoids and occurs by processes not
significantly different in kind from those involved in differentiation of
41
of undifferentiated parenchyma cells showing no organization can be
propagated indefinitely in callus tissue culture. It is when cellular events
are thus freed of the highest level of control, as occurs also in cancer or
the crown-gall disease, that normal plant form and organization disappear, even though structure at the cellular level appears to be unaffected. Yet, as we shall see, one can impose upon the in vitro cellular
system physiological controls which either resemble or duplicate the
controls actually operating in vivo; thus one allows the process of initiation of organization to begin anew. The controlled initiation of an organ
primordium from a randomly arranged mass of parenchyma cells in
culture under the influence of specific additions to the nutrient medium
is a dramatic and potentially powerful approach to understanding the
mechanisms controlling organization in the intact system. It is a tool
which permits an examination of the activities of cells as cells, but also
their activities as they are components of a multicellular system. The
evidence from organ and tissue culture studies seems clear that multicellular aggregates show capacities for specialized syntheses which
lead to correlative interactions between cellular populations, making distinctive organization possible. It is the purpose of this review to examine
the problem of the initiation of organization, especially as it can be
studied in cell, tissue, and organ cultures. The problem centers around
understanding the changes that ensue when cells become members of a
population and when unorganized cell populations develop into organized
multicellular structures of characteristic morphology.
In the discussion of the initiation of organized structures in cultures,
it is convenient to start with a particular premise which is supported in
part by the experimental data to be discussed later and which runs
through the discussions, explicitly or implicitly, as a unifying theme. The
assumption is made that the initiation of organized structure takes place
in a cell population in very much the same fundamental way for all
the variety of different types of structures which are formed. Typically,
a single cell in a mass of cells is activated in some way and undergoes
a cell division or a series of divisions to form a small mass of meristematic tissue, referred to by some as a "nodule" or "nest," by others
as a "cambial-like" center. The author prefers the term used by
Bünning (1952)—"meristemoid," a meristem-like cell or group of cells.
In extending Bünning's use of the term, a meristemoid could refer to a
single cell or to a group of cells acting together as a meristematic
center. It is in this sense that the term is most useful. The argument
can be made that the differentiation of single isolated xylem elements in
a callus parenchyma involves meristemoids and occurs by processes not
significantly different in kind from those involved in differentiation of