LOCALIZATION IN THE DEVELOPING FUCUS EGG
323
effective (still lower concentrations were not tried). Moreover, both the
current and growth responses to 10~
9 M auxin are substantially smaller
than those to 10~
7 M auxin, thus conforming, as far as the data go, to
the expected parallelism.
Incidentally, it should be noted that the evidence of the Tasmanian
group certainly supports their view that auxin's primary action is upon
the ion transport properties of the plasma membrane; a view which also
seems to be supported by the well-known rapid effects of auxin addition
upon streaming rates (Sweeney, 1944).
2. Hyphae
The Slaymans have reported that the intracellular potential of growing
Neurospora
hyphae, as measured via intracellular salt bridges, rises by
60 mV in moving from the tip to a point 1 mm behind it (Slayman and
Slayman, 1962).
Now, in principle, one cannot tell whether such a potential difference
arises from a current or from a gradient of fixed charged density, for
there is, in theory, no difficulty in generating potential differences of this
size via fixed charge gradients (Overbeek, 1956, p. 61). Moreover, the
large potential differences found between the inner and outer parts of
the cytoplasm in fish eggs (Hori, 1958), anesthetized amebas (Bruce
and Christiansen, 1965), and frog eggs (Morrill and Watson, 1966) are all
likely to be examples of this possibility, since currents cannot, of course,
move exclusively outward from the cell interior and because no membranes seem to have lain between the points probed.
Nevertheless, an elongating hypha is obviously similar to an elongating
rhizoid; moreover, the current which could correspond to this gradient
would move in the same direction as that in Fucus, i.e., into the growing
tip from the medium. Hence it is tempting to wonder if the potential
gradient noted in Neurospora might not, in fact, arise from a current
loop. The intrahyphal resistivity (as recalculated from Fig. 10 of Slayman, 1965) is about 300 ohm-cm, so the observed 600-mV/cm field would
indicate a density of 2000 jaamp/cm
2 if produced by currents. This is
about 100 times the current density inferred for the Fucus rhizoid!
However, Neurospora hyphae respire about 30 times faster than Fucus
eggs under the measurement conditions [17 cc 0 2 per cubic centimeter
cells per hour for Neurospora
(Slayman, 1965) vs. 0.6 cc 0 2 for Fucus
(Jaffe, 1955) ]. So Neurospora
should have the power to drive such
intense currents. Moreover, these cells were elongating about a 1000
323
effective (still lower concentrations were not tried). Moreover, both the
current and growth responses to 10~
9 M auxin are substantially smaller
than those to 10~
7 M auxin, thus conforming, as far as the data go, to
the expected parallelism.
Incidentally, it should be noted that the evidence of the Tasmanian
group certainly supports their view that auxin's primary action is upon
the ion transport properties of the plasma membrane; a view which also
seems to be supported by the well-known rapid effects of auxin addition
upon streaming rates (Sweeney, 1944).
2. Hyphae
The Slaymans have reported that the intracellular potential of growing
Neurospora
hyphae, as measured via intracellular salt bridges, rises by
60 mV in moving from the tip to a point 1 mm behind it (Slayman and
Slayman, 1962).
Now, in principle, one cannot tell whether such a potential difference
arises from a current or from a gradient of fixed charged density, for
there is, in theory, no difficulty in generating potential differences of this
size via fixed charge gradients (Overbeek, 1956, p. 61). Moreover, the
large potential differences found between the inner and outer parts of
the cytoplasm in fish eggs (Hori, 1958), anesthetized amebas (Bruce
and Christiansen, 1965), and frog eggs (Morrill and Watson, 1966) are all
likely to be examples of this possibility, since currents cannot, of course,
move exclusively outward from the cell interior and because no membranes seem to have lain between the points probed.
Nevertheless, an elongating hypha is obviously similar to an elongating
rhizoid; moreover, the current which could correspond to this gradient
would move in the same direction as that in Fucus, i.e., into the growing
tip from the medium. Hence it is tempting to wonder if the potential
gradient noted in Neurospora might not, in fact, arise from a current
loop. The intrahyphal resistivity (as recalculated from Fig. 10 of Slayman, 1965) is about 300 ohm-cm, so the observed 600-mV/cm field would
indicate a density of 2000 jaamp/cm
2 if produced by currents. This is
about 100 times the current density inferred for the Fucus rhizoid!
However, Neurospora hyphae respire about 30 times faster than Fucus
eggs under the measurement conditions [17 cc 0 2 per cubic centimeter
cells per hour for Neurospora
(Slayman, 1965) vs. 0.6 cc 0 2 for Fucus
(Jaffe, 1955) ]. So Neurospora
should have the power to drive such
intense currents. Moreover, these cells were elongating about a 1000