228
F.
C. STEWARD AND
Η. Y. MOHAN RAM
converging planes. A striking example is in the case of the secretory hair
cells of Mentha (Figs. 4(a)-(d) drawn from photographs of Howe, 1951
in a study with F.C.S.). These hairs have a basal cell or stalk, and a disk
composed of eight cells. These last form exactly in the way predicted by
D'Arcy Thompson (1952, see Fig. 239) for a disc of 8 cells formed from
a single cell (cf. Fig. 4(e), drawn from M. piperita).
Thus polarity shows the manner in which growth occurs in response
to asymmetry in the environment. Contact, gravity, light, gradients of
oxygen or carbon dioxide or pH may each operate in particular situations.
But each such variable which triggers off a response in the cells must
make its impact through some growth-regulatory mechanism. The clues
here seem to be that the incidence of division, the directions and intersections of new cleavage planes, may be affected or directed by the
degree to which walls remain fluid and plastic on the one hand or solid,
elastic or rigid on the other. Thus growth substances that control the
balance between cell division and cell enlargement, through a regulatory
role upon the fibrillar arrangement of the cell wall, could be conducive
to these ends.
C. Longevity, Senescence and Death of Cells
Above there has been a discussion of various ways in which living,
but quiescent, mature cells can be brought back again to a state of active
growth. In the metabolic properties of the transformed cells one sees the
resultants of certain chemical regulatory controls and, in their morphogenetic responses, one sees that this goes hand in hand with the altered
and special metabolic behaviour.
Cells which seem to possess or retain the unlimited ability to survive
and grow seem to be those which, like lower forms, are relatively
undifferentiated, as for example the cambium of such plants as Sequoia,
or tissue cultures composed of unorganized cells or of tumour cells.
The converse also applies, namely that differentiation and morphogenetic specialization carry with them the fate of ultimate senescence
and death. In this context the words of Minot (1913) may be recalled,
'Senescence is brought about by increase and differentiation of the
protoplasm.
5 In different circumstances there are vast differences in the
pace or onset of these events. Many cells, e.g. certain root hairs or pollen
grains (e.g. of barley and rice) are ephemeral, while other cells may be
long-lived. In birch trees, pith cells may remain alive as long as 42
years, after which heart wood formation may kill them (Fritzche, 1910),
MacDougal (1926), and MacDougal and Long (1927) have given some
astounding longevity figures for pith cells of cacti. Carnegia gigantea, of
the deserts of Arizona, has pith parenchyma cells which retain the
F.
C. STEWARD AND
Η. Y. MOHAN RAM
converging planes. A striking example is in the case of the secretory hair
cells of Mentha (Figs. 4(a)-(d) drawn from photographs of Howe, 1951
in a study with F.C.S.). These hairs have a basal cell or stalk, and a disk
composed of eight cells. These last form exactly in the way predicted by
D'Arcy Thompson (1952, see Fig. 239) for a disc of 8 cells formed from
a single cell (cf. Fig. 4(e), drawn from M. piperita).
Thus polarity shows the manner in which growth occurs in response
to asymmetry in the environment. Contact, gravity, light, gradients of
oxygen or carbon dioxide or pH may each operate in particular situations.
But each such variable which triggers off a response in the cells must
make its impact through some growth-regulatory mechanism. The clues
here seem to be that the incidence of division, the directions and intersections of new cleavage planes, may be affected or directed by the
degree to which walls remain fluid and plastic on the one hand or solid,
elastic or rigid on the other. Thus growth substances that control the
balance between cell division and cell enlargement, through a regulatory
role upon the fibrillar arrangement of the cell wall, could be conducive
to these ends.
C. Longevity, Senescence and Death of Cells
Above there has been a discussion of various ways in which living,
but quiescent, mature cells can be brought back again to a state of active
growth. In the metabolic properties of the transformed cells one sees the
resultants of certain chemical regulatory controls and, in their morphogenetic responses, one sees that this goes hand in hand with the altered
and special metabolic behaviour.
Cells which seem to possess or retain the unlimited ability to survive
and grow seem to be those which, like lower forms, are relatively
undifferentiated, as for example the cambium of such plants as Sequoia,
or tissue cultures composed of unorganized cells or of tumour cells.
The converse also applies, namely that differentiation and morphogenetic specialization carry with them the fate of ultimate senescence
and death. In this context the words of Minot (1913) may be recalled,
'Senescence is brought about by increase and differentiation of the
protoplasm.
5 In different circumstances there are vast differences in the
pace or onset of these events. Many cells, e.g. certain root hairs or pollen
grains (e.g. of barley and rice) are ephemeral, while other cells may be
long-lived. In birch trees, pith cells may remain alive as long as 42
years, after which heart wood formation may kill them (Fritzche, 1910),
MacDougal (1926), and MacDougal and Long (1927) have given some
astounding longevity figures for pith cells of cacti. Carnegia gigantea, of
the deserts of Arizona, has pith parenchyma cells which retain the