222
F. C. STEWARD AND
Η. Y. MOHAN RAM
in greater or lesser degree, the totipotency of the preformed carrot cell
from the controls which restricted its full expression in the intact plant
body.
B. Polarity and Patterns of Cellular Growth
Division of a spherical cell in a completely symmetrical environment
might, therefore, have been predicted from Sach's law of equal masses,
of Errera's law of minimum surface, and of the principle that new planes
tend to intersect in the manner of weightless liquid films. Deviations
from such expectations may, in part, be due to asymmetry in the
environment, i.e. they are exogenous to the dividing cell and in part
they may be due to factors which inherently cause the growth to be
unequal, i.e. they are endogenous to the dividing cell. A few such
situations, now to be discussed, exemplify the interpretations which
may be based on newer knowledge of the chemical factors that regulate
growth.
Situations of 'free cell formation', in which out of a common mass of
protoplasm, a number of separate cells are simultaneously or all but
simultaneously differentiated (to quote D'Arcy Thompson, 1952) are to
be found in the growth of spores from pollen mother cells. In some
anthers, characterized by an amoeboid tapetum (as in many Araceae,
see Maheshwari, 1950) even the pollen mother cells become free from
one another and subsequently develop in the periplasmodium formed by
the dissolution of the walls of the tapetal cells. But even in the usual
case the microspores appear to develop 'free' within the mother cell.
Thus, the formation of tetrads of spores was treated by Sachs (1887,
cf. pp. 433 et seq.) and also by D'Arcy Thompson, (1952, cf. pp. 629
et seq.) as a system in which the cells form groups in various stable and
symmetrical configurations interpreted in accordance with laws which
apply to liquid films. But, when individual pollen grains alight upon a
receptive stigma, marked polarity in the subsequent growth of the
pollen tubes appears, and there can be little doubt that growth substances in the style play a special part.
Such polarized growth may be shown in non-cellular forms; it is not
a feature only of cells that cleave as they grow. It is common that a
multi-nucleate algal protoplast may divide by segregation—the segregative cell division of Borgesen (1913); (Fritsch, 1935)—into a large
number of spherical aplanospores. These may grow in size as spheres,
often within the old cell wall, until by mutual compression they form
typical morulloid masses (Figs. 3(6) and (c)). This has been well shown by
work on Valonia ocellata (Steward, 1939). The shape of the individual
aplanospores is dictated, as it were, by their need to fill the confining
F. C. STEWARD AND
Η. Y. MOHAN RAM
in greater or lesser degree, the totipotency of the preformed carrot cell
from the controls which restricted its full expression in the intact plant
body.
B. Polarity and Patterns of Cellular Growth
Division of a spherical cell in a completely symmetrical environment
might, therefore, have been predicted from Sach's law of equal masses,
of Errera's law of minimum surface, and of the principle that new planes
tend to intersect in the manner of weightless liquid films. Deviations
from such expectations may, in part, be due to asymmetry in the
environment, i.e. they are exogenous to the dividing cell and in part
they may be due to factors which inherently cause the growth to be
unequal, i.e. they are endogenous to the dividing cell. A few such
situations, now to be discussed, exemplify the interpretations which
may be based on newer knowledge of the chemical factors that regulate
growth.
Situations of 'free cell formation', in which out of a common mass of
protoplasm, a number of separate cells are simultaneously or all but
simultaneously differentiated (to quote D'Arcy Thompson, 1952) are to
be found in the growth of spores from pollen mother cells. In some
anthers, characterized by an amoeboid tapetum (as in many Araceae,
see Maheshwari, 1950) even the pollen mother cells become free from
one another and subsequently develop in the periplasmodium formed by
the dissolution of the walls of the tapetal cells. But even in the usual
case the microspores appear to develop 'free' within the mother cell.
Thus, the formation of tetrads of spores was treated by Sachs (1887,
cf. pp. 433 et seq.) and also by D'Arcy Thompson, (1952, cf. pp. 629
et seq.) as a system in which the cells form groups in various stable and
symmetrical configurations interpreted in accordance with laws which
apply to liquid films. But, when individual pollen grains alight upon a
receptive stigma, marked polarity in the subsequent growth of the
pollen tubes appears, and there can be little doubt that growth substances in the style play a special part.
Such polarized growth may be shown in non-cellular forms; it is not
a feature only of cells that cleave as they grow. It is common that a
multi-nucleate algal protoplast may divide by segregation—the segregative cell division of Borgesen (1913); (Fritsch, 1935)—into a large
number of spherical aplanospores. These may grow in size as spheres,
often within the old cell wall, until by mutual compression they form
typical morulloid masses (Figs. 3(6) and (c)). This has been well shown by
work on Valonia ocellata (Steward, 1939). The shape of the individual
aplanospores is dictated, as it were, by their need to fill the confining