V.
DETERMINING
FACTORS IN
CELL GROWTH
221
angles to the length, whereas in the tip cells asymmetric divisions may
be seen by a markedly curved wall (Fig. 4(^)). Such observations are
abundant in embryological literature dealing with the growth of
filamentous embryos. The causes which lead to asymmetry in a given
cell division are often obscure, but their consequences may be seen as
in the formation of stomata in Sedum shown by D'Arcy Thompson
(1952, cf. Fig. 277, p. 628).
It might have been expected that free cells, symmetrically exposed
to all known external stimuli, would grow in accordance with these
rules. This is not so in the case of carrot cells (Steward, Mapes and
Smith, 1958; Steward, 1958; Steward, Mapes and Mears, 1958). The
array of growth forms achieved by carrot cells freely suspended in a
liquid medium indicates that the most uniform external conditions fail
to induce uniform or regular behaviour in simple accordance with Sachs'
or Errera's Law. By contrast, when growth does conform to those
principles, as in many cases of normal development, it often seems to be
the result of growth proceeding in an environment which imposes
external limitations. When, in fact, the environment does not impose
such restrictions, the cells are free to indulge in a range of growth forms,
in response to intrinsic factors, that are suppressed in normal development. Even some embryos, freed from the restriction of their normal
environment, may grow in a disorderly way and produce tumour-like
growths or callus (cf. Sect. IV E). A major conclusion is that even in
normal development the contribution made to growth of divisions in
highly vacuolated cells may often have been overlooked.
In very recent work the behaviour of the nucleus in these freely
suspended cells has been investigated (Mitra, Mapes and Steward, 1960).
Interestingly enough, the preformed explant seems to exert a sufficient
influence on the cells which proliferate around it so that they remain
diploid, like the diploid cells of the initial explant. But, once they get
detached from the explant, the free cells exhibit a wide range of cytological conditions or events. High degrees of polyploidy, even haploidy
and abnormalities like the formation of di- and tri-centric bridges may
be observed. Such diverse cytological conditions have also been encountered in cells of pea root callus by Torrey (1959).
Thus, when a preformed resting carrot cell embarks upon renewed
active growth and cell division, the stimulus is not a simple one, and
many apparently diverse processes are set in motion. These affect the
intimate processes of metabolism, and especially of protein synthesis,
but they extend to a range of morphological events that distinguish the
behaviour of the free cell from the same type of cell as it grows within
the plant body and is there subjected to limitations by virtue of its
position. When freed cells respond in the manner described, they release,
DETERMINING
FACTORS IN
CELL GROWTH
221
angles to the length, whereas in the tip cells asymmetric divisions may
be seen by a markedly curved wall (Fig. 4(^)). Such observations are
abundant in embryological literature dealing with the growth of
filamentous embryos. The causes which lead to asymmetry in a given
cell division are often obscure, but their consequences may be seen as
in the formation of stomata in Sedum shown by D'Arcy Thompson
(1952, cf. Fig. 277, p. 628).
It might have been expected that free cells, symmetrically exposed
to all known external stimuli, would grow in accordance with these
rules. This is not so in the case of carrot cells (Steward, Mapes and
Smith, 1958; Steward, 1958; Steward, Mapes and Mears, 1958). The
array of growth forms achieved by carrot cells freely suspended in a
liquid medium indicates that the most uniform external conditions fail
to induce uniform or regular behaviour in simple accordance with Sachs'
or Errera's Law. By contrast, when growth does conform to those
principles, as in many cases of normal development, it often seems to be
the result of growth proceeding in an environment which imposes
external limitations. When, in fact, the environment does not impose
such restrictions, the cells are free to indulge in a range of growth forms,
in response to intrinsic factors, that are suppressed in normal development. Even some embryos, freed from the restriction of their normal
environment, may grow in a disorderly way and produce tumour-like
growths or callus (cf. Sect. IV E). A major conclusion is that even in
normal development the contribution made to growth of divisions in
highly vacuolated cells may often have been overlooked.
In very recent work the behaviour of the nucleus in these freely
suspended cells has been investigated (Mitra, Mapes and Steward, 1960).
Interestingly enough, the preformed explant seems to exert a sufficient
influence on the cells which proliferate around it so that they remain
diploid, like the diploid cells of the initial explant. But, once they get
detached from the explant, the free cells exhibit a wide range of cytological conditions or events. High degrees of polyploidy, even haploidy
and abnormalities like the formation of di- and tri-centric bridges may
be observed. Such diverse cytological conditions have also been encountered in cells of pea root callus by Torrey (1959).
Thus, when a preformed resting carrot cell embarks upon renewed
active growth and cell division, the stimulus is not a simple one, and
many apparently diverse processes are set in motion. These affect the
intimate processes of metabolism, and especially of protein synthesis,
but they extend to a range of morphological events that distinguish the
behaviour of the free cell from the same type of cell as it grows within
the plant body and is there subjected to limitations by virtue of its
position. When freed cells respond in the manner described, they release,