V. DETERMINING FACTORS IN CELL GROWTH
225
A well known example of polarity, interpretable through known
gradients in the environment, occurs in Fucus eggs. Fertilized in the
sea, the eggs commonly alight upon a surface as more or less spherical
globules. They grow only when in clusters, and the asymmetry in their
growth commences by stimuli which radiate from the centre of the mass
toward which the rhizoidal processes grow (Whitaker, 1940). In this
case the asymmetric stimulus seems to be in large part due to a gradient
of pH which can replace the effects due to the proximity of other
zygotes (Whitaker and Lowrance, 1940). The appearance of a rhizoid
marks the plane of the first division which is always at right angles to it.
By unilateral illumination, using white light, a rhizoid may be induced
to develop on the least illuminated side (see Wardlaw, 1955 for summary).
The early appearance of polarity in embryos is almost universal, and
this is established in the majority of angiosperms with the first division
of the zygote. This first division is predominantly at right angles to the
direction of the micropyle, irrespective of the orientation of the ovule,
excepting in members of the family Loranthaceae (Maheshwari, 1950).
Whether the laying down of the first partition wall is a result or the
cause of polarity is not clear, although the former seems to be the case
(Wardlaw, 1955). The presence of a large vacuole toward the micropylar end is universal in zygotes of flowering plants (Coulter and
Chamberlain, 1903; Maheshwari, 1950). After the first transverse
division, this vacuole is always present in the basal cell. Thus within the
single protoplast of the zygote both structural asymmetry (shown by
the respective positions of the vacuole and the nucleus) and biochemical
asymmetry (arising from the contents of the vacuole) exist. It is along
this biochemical gradient that the first wall forms in such a way as to
separate the large vacuole in the basal cell and the dense cytoplasm in
the terminal cell. Thence forward polarity is established. The 2-celled
pro-embryo may give rise to the 4-celled condition in different ways,
and the subsequent embryo development is dependent on the respective
contributions of each one of these four cells. Very early in the development of the pro-embryo, the embryonal and suspensor parts are well
marked. The cells of the suspensor usually divide by transverse walls
and push the embryo proper into the endosperm, while the divisions in
the embryonal part are oriented differently. This contrasted growth of
the filamentous suspensor and the globular embryo is thus an outstanding case of asymmetric growth.
The growth of the pro-embryo, say to the 8-celled stage, suggests
that the cleavage planes occur in a quasi fluid system, in which equal
masses adjust to fluid equilibrium. The filamentous growth of the
suspensor, on the other hand, with divisions at right angles to its length,
suggests that the transverse cleavage planes form in cells already
225
A well known example of polarity, interpretable through known
gradients in the environment, occurs in Fucus eggs. Fertilized in the
sea, the eggs commonly alight upon a surface as more or less spherical
globules. They grow only when in clusters, and the asymmetry in their
growth commences by stimuli which radiate from the centre of the mass
toward which the rhizoidal processes grow (Whitaker, 1940). In this
case the asymmetric stimulus seems to be in large part due to a gradient
of pH which can replace the effects due to the proximity of other
zygotes (Whitaker and Lowrance, 1940). The appearance of a rhizoid
marks the plane of the first division which is always at right angles to it.
By unilateral illumination, using white light, a rhizoid may be induced
to develop on the least illuminated side (see Wardlaw, 1955 for summary).
The early appearance of polarity in embryos is almost universal, and
this is established in the majority of angiosperms with the first division
of the zygote. This first division is predominantly at right angles to the
direction of the micropyle, irrespective of the orientation of the ovule,
excepting in members of the family Loranthaceae (Maheshwari, 1950).
Whether the laying down of the first partition wall is a result or the
cause of polarity is not clear, although the former seems to be the case
(Wardlaw, 1955). The presence of a large vacuole toward the micropylar end is universal in zygotes of flowering plants (Coulter and
Chamberlain, 1903; Maheshwari, 1950). After the first transverse
division, this vacuole is always present in the basal cell. Thus within the
single protoplast of the zygote both structural asymmetry (shown by
the respective positions of the vacuole and the nucleus) and biochemical
asymmetry (arising from the contents of the vacuole) exist. It is along
this biochemical gradient that the first wall forms in such a way as to
separate the large vacuole in the basal cell and the dense cytoplasm in
the terminal cell. Thence forward polarity is established. The 2-celled
pro-embryo may give rise to the 4-celled condition in different ways,
and the subsequent embryo development is dependent on the respective
contributions of each one of these four cells. Very early in the development of the pro-embryo, the embryonal and suspensor parts are well
marked. The cells of the suspensor usually divide by transverse walls
and push the embryo proper into the endosperm, while the divisions in
the embryonal part are oriented differently. This contrasted growth of
the filamentous suspensor and the globular embryo is thus an outstanding case of asymmetric growth.
The growth of the pro-embryo, say to the 8-celled stage, suggests
that the cleavage planes occur in a quasi fluid system, in which equal
masses adjust to fluid equilibrium. The filamentous growth of the
suspensor, on the other hand, with divisions at right angles to its length,
suggests that the transverse cleavage planes form in cells already