THE ACRASINA
177
enveloped the grex. Certainly the collapsed slime sheaths left behind by
D. discoideum slugs are strongly adhesive; and if these happen to cross
one another or are crumpled up artificially, I have found it almost impossible to unravel them again. But adhesion by itself does not explain
why the polycephalum grex diverge. Raper (1956a) speculated that they
might start to make a repellent when the spores differentiated. Instead,
we may suppose that they make it all the time, just like those of other
species. And we may attribute the peculiarities of polycephalum development to the fact that the grex that form the individual fruiting bodies are
produced simultaneously by the longitudinal fission of an established
grex and are therefore in close contact right from the start. In contrast,
in D. lacteum and minutum, Acytostelium, and probably also R. R.
Sussman and Sussman's (1953) bushy mutants of purpureum and discoideum, all of which regularly yield a crop of separate fruiting bodies
from the centre of a single aggregation of sufficient size, the grex are
organized independently, and not necessarily at the same time, from
different parts of it; and we should expect those that did develop
simultaneously, being free to lean away from one another, to do so,
though this has not been studied. When D. polycephalum itself produces
a crop of a dozen or more grex directly from a single aggregation, these
areseparate (Raper, 1956b).
In an early stage of polycephalum culmination, neighbouring stalks
within the bundle of parallel cylindrical grex are separated by the difference in the diameters of stalk and grex. What then alters this spacing
when the stalks emerge from the bottom of the bundle? If we ascribe
the major role to the slime sheaths, it is not enough that each should
adhere to the next one: it must also contract to the diameter of the stalk
as it is vacated by the grex. This usually does happen in the culmination
of other species, apparently because the sheath is sufficiently fluid to
cohere to the grex's rear as it tapers. But it need not happen: if the
stalk spirals, it may sometimes be seen stretching from one turn to the
next (Raper and Fennell, 1952). And in fact the sheath's fluidity means
that it may even be expanded in diameter by a suitable force, as happens
continuously at the front of every grex because of its taper and also over
much of the surface whenever a grex rapidly shortens. Certainly in D.
polycephalum the rigidity of the stalks will oppose their being compacted
by cohesion, and indeed in many cases the stalks do not in fact come
together until some distance above the ground (this paper, Fig. 24;
Raper, 1956a). At first sight this looks uncomfortably like evidence for
attraction between the grex at this stage, but it may well be that the
stalks have merely retained their original spacing because of being
thicker at this level and anchored at their bases.
As the tips of the grex are tapered, they are not in contact, and lateral
177
enveloped the grex. Certainly the collapsed slime sheaths left behind by
D. discoideum slugs are strongly adhesive; and if these happen to cross
one another or are crumpled up artificially, I have found it almost impossible to unravel them again. But adhesion by itself does not explain
why the polycephalum grex diverge. Raper (1956a) speculated that they
might start to make a repellent when the spores differentiated. Instead,
we may suppose that they make it all the time, just like those of other
species. And we may attribute the peculiarities of polycephalum development to the fact that the grex that form the individual fruiting bodies are
produced simultaneously by the longitudinal fission of an established
grex and are therefore in close contact right from the start. In contrast,
in D. lacteum and minutum, Acytostelium, and probably also R. R.
Sussman and Sussman's (1953) bushy mutants of purpureum and discoideum, all of which regularly yield a crop of separate fruiting bodies
from the centre of a single aggregation of sufficient size, the grex are
organized independently, and not necessarily at the same time, from
different parts of it; and we should expect those that did develop
simultaneously, being free to lean away from one another, to do so,
though this has not been studied. When D. polycephalum itself produces
a crop of a dozen or more grex directly from a single aggregation, these
areseparate (Raper, 1956b).
In an early stage of polycephalum culmination, neighbouring stalks
within the bundle of parallel cylindrical grex are separated by the difference in the diameters of stalk and grex. What then alters this spacing
when the stalks emerge from the bottom of the bundle? If we ascribe
the major role to the slime sheaths, it is not enough that each should
adhere to the next one: it must also contract to the diameter of the stalk
as it is vacated by the grex. This usually does happen in the culmination
of other species, apparently because the sheath is sufficiently fluid to
cohere to the grex's rear as it tapers. But it need not happen: if the
stalk spirals, it may sometimes be seen stretching from one turn to the
next (Raper and Fennell, 1952). And in fact the sheath's fluidity means
that it may even be expanded in diameter by a suitable force, as happens
continuously at the front of every grex because of its taper and also over
much of the surface whenever a grex rapidly shortens. Certainly in D.
polycephalum the rigidity of the stalks will oppose their being compacted
by cohesion, and indeed in many cases the stalks do not in fact come
together until some distance above the ground (this paper, Fig. 24;
Raper, 1956a). At first sight this looks uncomfortably like evidence for
attraction between the grex at this stage, but it may well be that the
stalks have merely retained their original spacing because of being
thicker at this level and anchored at their bases.
As the tips of the grex are tapered, they are not in contact, and lateral