LOCALIZATION IN THE DEVELOPING FUCUS EGG
325
that the field somehow controlled the affected cell's development rather
than rotating the cell.
7 Griffithsia cells are asymmetric enough to have
made this easily observed. In the cases of both Fucus serratus (Bentrup,
1967) and of certain pollen tubes (Wulff, 1935), the already initiated
outgrowths are observed to grow in a curved path toward the positive
pole; since this does not involve rotation, it is unlikely that the apparently related effect upon their origin does.
Whatever the mechanism, it seems worth noting that in most of these
cases a sufficient field induces practically all of the outgrowths to form
toward the positive pole.
IV. Summary
1. The developing Fucus egg is considered as a prototype of the
phenomenon of localization. The zygote is apolar except, perhaps, for
an easily overcome bias toward initiating the rhizoid at the point of
sperm entry. A wide variety of other environmental vectors may subsequently leave traces which are then integrated and amplified so as to
fix irreversibly its axis after half a day. A process of tip growth soon
ensues which generates an outgrowth. This is cut off as a rhizoid cell
differentiated from the thallus cell.
2. There are reasons, derived in part from the phenomena of polarized
plasmolysis and of polarotropism, to believe that these traces lie in or
near the plasma membrane.
3. There are several leads toward understanding the amplification
process:
a. It is inferred from the correlation of the axes of nearby cell pairs
that a short-lived, diffusible, locally effective stimulator of rhizoid
initiation is concentrated in the medium outside of the incipient rhizoid.
This substance may be IAA.
b. Current (considered as a flow of positive ions) moves through the
egg from its rhizoidal to its thallus pole and returns through the medium.
It starts to flow at about the time of axis fixation. Since the imposition
of comparable currents may determine the egg's polarity, it is suggested that the self-generated current is a cause as well as a consequence
of localization. The current seems to be driven by episodes of depolarization of the membrane at the growth point. It is large enough to be able
8
Suspended Naegleria cells in the process of pseudopod formation are rapidly
rotatable (by fields of 6 volts/cm) so as to direct the outgrowth toward the positive
pole. Apparently the pseudopod has a higher density of negative charges (Forrester
et al, 1967).
325
that the field somehow controlled the affected cell's development rather
than rotating the cell.
7 Griffithsia cells are asymmetric enough to have
made this easily observed. In the cases of both Fucus serratus (Bentrup,
1967) and of certain pollen tubes (Wulff, 1935), the already initiated
outgrowths are observed to grow in a curved path toward the positive
pole; since this does not involve rotation, it is unlikely that the apparently related effect upon their origin does.
Whatever the mechanism, it seems worth noting that in most of these
cases a sufficient field induces practically all of the outgrowths to form
toward the positive pole.
IV. Summary
1. The developing Fucus egg is considered as a prototype of the
phenomenon of localization. The zygote is apolar except, perhaps, for
an easily overcome bias toward initiating the rhizoid at the point of
sperm entry. A wide variety of other environmental vectors may subsequently leave traces which are then integrated and amplified so as to
fix irreversibly its axis after half a day. A process of tip growth soon
ensues which generates an outgrowth. This is cut off as a rhizoid cell
differentiated from the thallus cell.
2. There are reasons, derived in part from the phenomena of polarized
plasmolysis and of polarotropism, to believe that these traces lie in or
near the plasma membrane.
3. There are several leads toward understanding the amplification
process:
a. It is inferred from the correlation of the axes of nearby cell pairs
that a short-lived, diffusible, locally effective stimulator of rhizoid
initiation is concentrated in the medium outside of the incipient rhizoid.
This substance may be IAA.
b. Current (considered as a flow of positive ions) moves through the
egg from its rhizoidal to its thallus pole and returns through the medium.
It starts to flow at about the time of axis fixation. Since the imposition
of comparable currents may determine the egg's polarity, it is suggested that the self-generated current is a cause as well as a consequence
of localization. The current seems to be driven by episodes of depolarization of the membrane at the growth point. It is large enough to be able
8
Suspended Naegleria cells in the process of pseudopod formation are rapidly
rotatable (by fields of 6 volts/cm) so as to direct the outgrowth toward the positive
pole. Apparently the pseudopod has a higher density of negative charges (Forrester
et al, 1967).