174
B. M. SHAFFER
Potts (1902) and Harper, would seem to be more promising: the cells
might be slowed down on the drier side either directly or by a hardening
of the slime sheath. But Bonner and Shaw (1957) were unable to demonstrate hydrotropism experimentally. Moreover, fruiting is normal even
when the greatest care is taken to have the air fully saturated (Gerisch,
1960).
In many species there is a period of 'migration', in which the guidance
mechanism used during culmination is 'switched off', more or less effectively. That is why the adjective lying has been applied to it, to contrast
with standing. As the lying phase must clearly have arisen latest in
evolution, we may reasonably regard it as having been interpolated in
culmination, as all grex are initially erect. However, in species in which
it occurs, it is convenient to adhere to existing usage and to restrict the
term culmination to the stage following it.
Having lost one guidance mechanism, the lying grex has developed to
a remarkable degree of sensitivity its ability to orient towards light and
warmth—responses which are shown but weakly by the standing grex
and not at all in any earlier stages (Raper, 1940a, b, 1941b; Bonner et
al., 1950; Samuel, 1961). But it must be emphasized that a 'lying' grex
has to remain on the ground only in so far as it needs support, which in
D. polycephalum, despite the absence of stalk, is for no more than a
minute fraction of its length (Raper, 1956b).
Bonner's time-lapse film made in the early forties recorded the development of three slices of a D. discoideum slug that were left close together. Each made a fruiting body. The one in the middle was erect,
and the outer two leant slightly away from it. This effect has recently been
confirmed by two students, Rosenthal and Rorke, in Bonner's laboratory
(privately). In Polysphondylium violaceum, repulsion between grex is very
striking (Shaffer, 1961b). If a large number of cells are deposited in an
area about half a centimetre across, in darkness the resultant grex
are initially more or less erect; but soon the tips bend outwards while
in mid-air and predominantly without touching one another, so that
the crop of fruiting bodies fills a hemispherical air space, with the
primary stalks projecting along the radii. Grex that come in contact
with the agar again may continue to move radially outwards from the
centre of the group while remaining on the ground. This is not merely a
question of following a direction established while in the air, because the
grex will reorient if turned round. We may conclude that grex repel
one another, lying as well as standing, and therefore that they cannot
be using negative hydrotropism. Almost certainly they react to a
gradient of a gas or a volatile metabolite—their heat production is
unlikely to be involved, as D. discoideum slugs move towards higher
temperatures (Raper, 1940b). Interestingly enough, if one bears
B. M. SHAFFER
Potts (1902) and Harper, would seem to be more promising: the cells
might be slowed down on the drier side either directly or by a hardening
of the slime sheath. But Bonner and Shaw (1957) were unable to demonstrate hydrotropism experimentally. Moreover, fruiting is normal even
when the greatest care is taken to have the air fully saturated (Gerisch,
1960).
In many species there is a period of 'migration', in which the guidance
mechanism used during culmination is 'switched off', more or less effectively. That is why the adjective lying has been applied to it, to contrast
with standing. As the lying phase must clearly have arisen latest in
evolution, we may reasonably regard it as having been interpolated in
culmination, as all grex are initially erect. However, in species in which
it occurs, it is convenient to adhere to existing usage and to restrict the
term culmination to the stage following it.
Having lost one guidance mechanism, the lying grex has developed to
a remarkable degree of sensitivity its ability to orient towards light and
warmth—responses which are shown but weakly by the standing grex
and not at all in any earlier stages (Raper, 1940a, b, 1941b; Bonner et
al., 1950; Samuel, 1961). But it must be emphasized that a 'lying' grex
has to remain on the ground only in so far as it needs support, which in
D. polycephalum, despite the absence of stalk, is for no more than a
minute fraction of its length (Raper, 1956b).
Bonner's time-lapse film made in the early forties recorded the development of three slices of a D. discoideum slug that were left close together. Each made a fruiting body. The one in the middle was erect,
and the outer two leant slightly away from it. This effect has recently been
confirmed by two students, Rosenthal and Rorke, in Bonner's laboratory
(privately). In Polysphondylium violaceum, repulsion between grex is very
striking (Shaffer, 1961b). If a large number of cells are deposited in an
area about half a centimetre across, in darkness the resultant grex
are initially more or less erect; but soon the tips bend outwards while
in mid-air and predominantly without touching one another, so that
the crop of fruiting bodies fills a hemispherical air space, with the
primary stalks projecting along the radii. Grex that come in contact
with the agar again may continue to move radially outwards from the
centre of the group while remaining on the ground. This is not merely a
question of following a direction established while in the air, because the
grex will reorient if turned round. We may conclude that grex repel
one another, lying as well as standing, and therefore that they cannot
be using negative hydrotropism. Almost certainly they react to a
gradient of a gas or a volatile metabolite—their heat production is
unlikely to be involved, as D. discoideum slugs move towards higher
temperatures (Raper, 1940b). Interestingly enough, if one bears