312
Β. Μ. SHAFFER
account for the influence of a grafted tip taken from a faster or slower
slug, and—though less data are available—for the reciprocal effect of
the host on the graft. Presumably in all such cases a compromise
environment would develop. But this would leave open the question of
whether some regions had a greater influence than others in determining
the general environment, and whether the transmission of any factors
was at all polarized. This model of the slug does not preclude temporary
variation in speed along the axis due to local factors, whether or not
such variation is transmitted. During culmination, for example, the tip
of the D. discoideum grex often rises in a series of jerks, while the back
part climbs smoothly (Bonner, 1944).
Existing measurements on D. discoideum
(Bonner and Eldredge,
1945; Bonner et al., 1953) raise a further problem, for they seem to
reveal a difference between migrating and culminating grex: during
culmination, velocity varied with linear dimensions over the whole
range studied, and the maximum observed was only 0-25 mm/h as
compared with 1-5 mm/h during migration—a discrepancy probably too
large to be explained by any difference in the size of the grex measured
or in the temperature at which the observations were made. The
difference can hardly be ascribed to the work that has to be done in
raising the mass off the ground, for with bodies of this size gravity is
unimportant compared with surface forces; and Francis (1962) has
indeed found that slugs are decelerated only 7% by a force of 40 g. We
may reasonably assume that some aspect of stalk production is the
factor responsible, especially as it is in the critical tip region that the
stalk is extended. The inactivation of a large proportion of the tip cells
by being trapped within the stalk sheath may certainly be expected to
slow down a grex, though perhaps not to the extent observed. Possibly
the manufacture of the stalk sheath is the limiting process. We might
then account for the relation between velocity and size in terms not of
motive energy and resistance but of the ratio of number of sheath makers
to sheath area—the former increasing with the cube of the linear
dimensions, the latter with the square of them. D. purpureum
grex,
when lying on the ground, may make stalk intermittently (Raper and
Thorn, 1941) ; it would be interesting to find out how this activity affects
velocity.
3. Shape
The shape of aggregation streams merely records their pattern of
growth. It depends on the initial cell distribution and the timing of
various cell activities, as already discussed. When the cells have arrived
at the centre of aggregation, they form a symmetrical body that, if we
allow for the substratum, has the simplest possible shape. All or part of
Β. Μ. SHAFFER
account for the influence of a grafted tip taken from a faster or slower
slug, and—though less data are available—for the reciprocal effect of
the host on the graft. Presumably in all such cases a compromise
environment would develop. But this would leave open the question of
whether some regions had a greater influence than others in determining
the general environment, and whether the transmission of any factors
was at all polarized. This model of the slug does not preclude temporary
variation in speed along the axis due to local factors, whether or not
such variation is transmitted. During culmination, for example, the tip
of the D. discoideum grex often rises in a series of jerks, while the back
part climbs smoothly (Bonner, 1944).
Existing measurements on D. discoideum
(Bonner and Eldredge,
1945; Bonner et al., 1953) raise a further problem, for they seem to
reveal a difference between migrating and culminating grex: during
culmination, velocity varied with linear dimensions over the whole
range studied, and the maximum observed was only 0-25 mm/h as
compared with 1-5 mm/h during migration—a discrepancy probably too
large to be explained by any difference in the size of the grex measured
or in the temperature at which the observations were made. The
difference can hardly be ascribed to the work that has to be done in
raising the mass off the ground, for with bodies of this size gravity is
unimportant compared with surface forces; and Francis (1962) has
indeed found that slugs are decelerated only 7% by a force of 40 g. We
may reasonably assume that some aspect of stalk production is the
factor responsible, especially as it is in the critical tip region that the
stalk is extended. The inactivation of a large proportion of the tip cells
by being trapped within the stalk sheath may certainly be expected to
slow down a grex, though perhaps not to the extent observed. Possibly
the manufacture of the stalk sheath is the limiting process. We might
then account for the relation between velocity and size in terms not of
motive energy and resistance but of the ratio of number of sheath makers
to sheath area—the former increasing with the cube of the linear
dimensions, the latter with the square of them. D. purpureum
grex,
when lying on the ground, may make stalk intermittently (Raper and
Thorn, 1941) ; it would be interesting to find out how this activity affects
velocity.
3. Shape
The shape of aggregation streams merely records their pattern of
growth. It depends on the initial cell distribution and the timing of
various cell activities, as already discussed. When the cells have arrived
at the centre of aggregation, they form a symmetrical body that, if we
allow for the substratum, has the simplest possible shape. All or part of