322
LIONEL F. JAFFE
In almost all of this literature, no more than one of these criteria is met
and no indications of developmental currents can be salvaged; however,
in three areas the data suffice to yield such indications.
1. Roots
The clearest results are those obtained by Scott's group in Tasmania
who have mapped the fields in the fluid near growing roots. These reveal
external currents with local densities of up to 1 /xamp/cm
2 (Scott and
Martin, 1962). There is, moreover, some evidence (described below)
suggesting that these currents are driven by a depolarization of membranes in the elongating region, that they are needed for elongation, and
thus may be analogous to those entering the rhizoid of the Fucus embryo.
a. The external current pattern depends greatly upon the ionic composition of the medium; nevertheless, current (i.e., positive ions) always
tends to enter the elongating zone and leave regions of fixed length (Scott
and Martin, 1962). Like the plasma membranes of almost all other cells,
those of the root's peripheral cells keep them internally negative (Jenkinson, 1962; Scott et al., 1968). Put together, these two facts indicate that
the currents are driven by a relative depolarization of membranes in the
elongating zone.
b. The chief basis for inferring that current is needed for elongation
in roots is a comparison of the changes in current and in elongation rate
upon changes in external auxin concentration. The current measurements
were made upon bean roots, whereas most of the pertinent growth studies
were made upon wheat and corn roots. For this and other reasons, the
comparison must be rough, but it is nonetheless very suggestive:
Addition and removal of 10~
7 M auxin are followed within 1 minute
by several millivolt hyperpolarization and depolarization of the peripheral cell's membranes (Jenkinson, 1962), and, hence, because of point
a, by a marked decrease and increase of current strength, respectively.
Addition and removal of any concentration of auxin from lO
- 1 1 to 10~
5 M
is followed in about 10 minutes by a marked decrease and increase of
elongation rate, respectively (Hejnowicz, 1961; List, 1966). Hence it
follows that appropriate changes in external auxin concentration produce
increases in current strength followed
by increases in elongation rate;
and similarly, produce decreases followed by decreases. Oscillatory
changes in auxin concentration yield large oscillatory changes in current
strength (Jenkinson and Scott, 1961); these results seem consistent with
the above inferred changes in current in response to single additions or
removals of auxin and add the information that 10~
9 M auxin is likewise
LIONEL F. JAFFE
In almost all of this literature, no more than one of these criteria is met
and no indications of developmental currents can be salvaged; however,
in three areas the data suffice to yield such indications.
1. Roots
The clearest results are those obtained by Scott's group in Tasmania
who have mapped the fields in the fluid near growing roots. These reveal
external currents with local densities of up to 1 /xamp/cm
2 (Scott and
Martin, 1962). There is, moreover, some evidence (described below)
suggesting that these currents are driven by a depolarization of membranes in the elongating region, that they are needed for elongation, and
thus may be analogous to those entering the rhizoid of the Fucus embryo.
a. The external current pattern depends greatly upon the ionic composition of the medium; nevertheless, current (i.e., positive ions) always
tends to enter the elongating zone and leave regions of fixed length (Scott
and Martin, 1962). Like the plasma membranes of almost all other cells,
those of the root's peripheral cells keep them internally negative (Jenkinson, 1962; Scott et al., 1968). Put together, these two facts indicate that
the currents are driven by a relative depolarization of membranes in the
elongating zone.
b. The chief basis for inferring that current is needed for elongation
in roots is a comparison of the changes in current and in elongation rate
upon changes in external auxin concentration. The current measurements
were made upon bean roots, whereas most of the pertinent growth studies
were made upon wheat and corn roots. For this and other reasons, the
comparison must be rough, but it is nonetheless very suggestive:
Addition and removal of 10~
7 M auxin are followed within 1 minute
by several millivolt hyperpolarization and depolarization of the peripheral cell's membranes (Jenkinson, 1962), and, hence, because of point
a, by a marked decrease and increase of current strength, respectively.
Addition and removal of any concentration of auxin from lO
- 1 1 to 10~
5 M
is followed in about 10 minutes by a marked decrease and increase of
elongation rate, respectively (Hejnowicz, 1961; List, 1966). Hence it
follows that appropriate changes in external auxin concentration produce
increases in current strength followed
by increases in elongation rate;
and similarly, produce decreases followed by decreases. Oscillatory
changes in auxin concentration yield large oscillatory changes in current
strength (Jenkinson and Scott, 1961); these results seem consistent with
the above inferred changes in current in response to single additions or
removals of auxin and add the information that 10~
9 M auxin is likewise