318
Β. Μ. SHAFFER
action of any sort at the upper end of the stalk. Clearly the tip's descent
is merely part of the general change in shape, which essentially is a
differential shortening of the axis. If we assume that all motile cells in
the grex remain oriented towards its anterior end throughout its life,
this shortening can be ascribed simply to a change in relative velocities,
the anterior, or prestalk, cells moving more slowly than the posterior
ones, or prespores. Indeed, the former are almost swallowed by the
prespore mass before the relationship is reversed, and the grex elongates
again. The slowing up of the anterior end may well be due to its
beginning to make stalk (Section V, B, 2) ; but if so, this must include
also its preparations for making it, for Bonner (1944) and Raper and
Fennell (1952) reported that at the end of migration, before the stalk
rudiment had appeared, not only did the tip become erect, but the slug
was already becoming more rounded (Fig. 25b). An exactly similar but
temporary rounding up can be induced by exposing a migrating slug to
dry conditions for a few minutes (Raper, 1940) ; this too can be ascribed
to a change in relative velocities.
C. Water as a Developmental Control
Potts (1902) made gallant attempts to explain an aggregate's later
development as a chain of responses to rather obvious environmental
stimuli. Though some of the links must have looked a bit weak even
then, we must rather shamefacedly admit that we cannot forge much
better ones to this day. Potts supposed that the cells had to lose water
to form stalk, and to lose more to sporulate : in his experience neither
happened in fully saturated air. He suggested that a
fluctuating
temperature promoted fruiting because the humidity decreased as the
culture warmed up. Certainly direct desiccation and also a rise in
temperature do hasten the onset of culmination (Raper, 1940; Bonner
and Shaw, 1957), and of aggregation too in Raper's culture conditions,
though not in Konijn's (1961). Bonner and Shaw suggested that all
known external stimuli of the final sporulation could be explained in
terms of a decrease in humidity, and in this they included osmotic
desiccation. If however water loss were a sufficient stimulus, fruiting
bodies should differentiate in hypertonic solutions.
It is not yet clear whether water loss is even a necessary stimulus.
Pure cultures of D. polycephalum in ordinary petri dishes form slugs
readily but fruiting bodies only rarely. Whittingham and Raper (1957)
obtained good fruiting by using porous lids or by drying the air to a
limited extent with sulphuric acid. They concluded that culmination
was triggered by a lower relative humidity than in other Acrasina.
Evidence to the contrary is that many of the other species have been
shown to form fairly typical fruiting bodies at very low, though less
Β. Μ. SHAFFER
action of any sort at the upper end of the stalk. Clearly the tip's descent
is merely part of the general change in shape, which essentially is a
differential shortening of the axis. If we assume that all motile cells in
the grex remain oriented towards its anterior end throughout its life,
this shortening can be ascribed simply to a change in relative velocities,
the anterior, or prestalk, cells moving more slowly than the posterior
ones, or prespores. Indeed, the former are almost swallowed by the
prespore mass before the relationship is reversed, and the grex elongates
again. The slowing up of the anterior end may well be due to its
beginning to make stalk (Section V, B, 2) ; but if so, this must include
also its preparations for making it, for Bonner (1944) and Raper and
Fennell (1952) reported that at the end of migration, before the stalk
rudiment had appeared, not only did the tip become erect, but the slug
was already becoming more rounded (Fig. 25b). An exactly similar but
temporary rounding up can be induced by exposing a migrating slug to
dry conditions for a few minutes (Raper, 1940) ; this too can be ascribed
to a change in relative velocities.
C. Water as a Developmental Control
Potts (1902) made gallant attempts to explain an aggregate's later
development as a chain of responses to rather obvious environmental
stimuli. Though some of the links must have looked a bit weak even
then, we must rather shamefacedly admit that we cannot forge much
better ones to this day. Potts supposed that the cells had to lose water
to form stalk, and to lose more to sporulate : in his experience neither
happened in fully saturated air. He suggested that a
fluctuating
temperature promoted fruiting because the humidity decreased as the
culture warmed up. Certainly direct desiccation and also a rise in
temperature do hasten the onset of culmination (Raper, 1940; Bonner
and Shaw, 1957), and of aggregation too in Raper's culture conditions,
though not in Konijn's (1961). Bonner and Shaw suggested that all
known external stimuli of the final sporulation could be explained in
terms of a decrease in humidity, and in this they included osmotic
desiccation. If however water loss were a sufficient stimulus, fruiting
bodies should differentiate in hypertonic solutions.
It is not yet clear whether water loss is even a necessary stimulus.
Pure cultures of D. polycephalum in ordinary petri dishes form slugs
readily but fruiting bodies only rarely. Whittingham and Raper (1957)
obtained good fruiting by using porous lids or by drying the air to a
limited extent with sulphuric acid. They concluded that culmination
was triggered by a lower relative humidity than in other Acrasina.
Evidence to the contrary is that many of the other species have been
shown to form fairly typical fruiting bodies at very low, though less