THE ACRASINA
175
in mind the question of repulsion when examining a published record
of the tracks of these slugs in a temperature gradient too small to
orient them (Bonner, et al., 1950), one sees that they initially show
nearly perfect radial divergence from their common origin (though
because of the nature of the preparation, this observation alone does not
prove that the slugs themselves were producing the factor responsible).
Furthermore, from Raper's (1940a, b) illustrations, one can see that
when a slug is splitting longitudinally, the twin tips move apart at a
wide angle. This is also the case with a P. violaceum grex, which continues to make stalk throughout division (Shaffer, 1961b). It is true
that if two slugs approach at a suitable angle, they may unite (Raper,
1940a, b); but collisions are not evidence that no action was taken to
avoid them.
If violaceum cells are deposited close to the edge of a dish, most of the
grex that form nearest the side wall repel themselves from it without
making contact with it, and consequently a corridor next to it remains
almost empty of fruiting bodies. We naturally enquire whether a grex
erects itself when culminating primarily by repelling itself from the
substratum. Now, even a single, large, lying grex, travelling over agar
and approaching the side wall of the plate, will commonly make a Uturn at any distance up to several millimetres away from it. Presumably
this is because of the accumulation of its own repellent between itself
and the impermeable wall. As such a grex may have its tip either flat
on the agar, or raised off it a distance only a small fraction of that at
which it can react to glass obstacles, it can neither be strongly attracted
to agar nor strongly repelled from it, despite being highly sensitive to
the repellent. Yet it readily culminates on agar. Therefore, although a
standing grex undoubtedly guides itself by repulsion to some extent,
we must reluctantly conclude that this is probably not the basic mechanism by which the stalk is initially built out perpendicularly to the substratum, because it is hard to think of any reasonable set of hypotheses
that would account for a grex's changing behaviour towards agar as
compared with other materials.
It is obviously simplest to suppose that the repellent is the same as
affects individual cells during the other stages of the life cycle. The only
indication that this is not the case is that when P. violaceum is inoculated at one point on a nutrient-agar plate spread with bacteria, the
grex still bend outwards despite being surrounded by zones of all stages
between feeding and aggregation; and they may even land on these stages
and continue to move radially outward through them. This point requires further study. It is known that aerial fungal hyphae may repel
those of another strain and must therefore make specific repellents
(Banbury, 1955). Whether repellents in the Acrasina are specific or not
175
in mind the question of repulsion when examining a published record
of the tracks of these slugs in a temperature gradient too small to
orient them (Bonner, et al., 1950), one sees that they initially show
nearly perfect radial divergence from their common origin (though
because of the nature of the preparation, this observation alone does not
prove that the slugs themselves were producing the factor responsible).
Furthermore, from Raper's (1940a, b) illustrations, one can see that
when a slug is splitting longitudinally, the twin tips move apart at a
wide angle. This is also the case with a P. violaceum grex, which continues to make stalk throughout division (Shaffer, 1961b). It is true
that if two slugs approach at a suitable angle, they may unite (Raper,
1940a, b); but collisions are not evidence that no action was taken to
avoid them.
If violaceum cells are deposited close to the edge of a dish, most of the
grex that form nearest the side wall repel themselves from it without
making contact with it, and consequently a corridor next to it remains
almost empty of fruiting bodies. We naturally enquire whether a grex
erects itself when culminating primarily by repelling itself from the
substratum. Now, even a single, large, lying grex, travelling over agar
and approaching the side wall of the plate, will commonly make a Uturn at any distance up to several millimetres away from it. Presumably
this is because of the accumulation of its own repellent between itself
and the impermeable wall. As such a grex may have its tip either flat
on the agar, or raised off it a distance only a small fraction of that at
which it can react to glass obstacles, it can neither be strongly attracted
to agar nor strongly repelled from it, despite being highly sensitive to
the repellent. Yet it readily culminates on agar. Therefore, although a
standing grex undoubtedly guides itself by repulsion to some extent,
we must reluctantly conclude that this is probably not the basic mechanism by which the stalk is initially built out perpendicularly to the substratum, because it is hard to think of any reasonable set of hypotheses
that would account for a grex's changing behaviour towards agar as
compared with other materials.
It is obviously simplest to suppose that the repellent is the same as
affects individual cells during the other stages of the life cycle. The only
indication that this is not the case is that when P. violaceum is inoculated at one point on a nutrient-agar plate spread with bacteria, the
grex still bend outwards despite being surrounded by zones of all stages
between feeding and aggregation; and they may even land on these stages
and continue to move radially outward through them. This point requires further study. It is known that aerial fungal hyphae may repel
those of another strain and must therefore make specific repellents
(Banbury, 1955). Whether repellents in the Acrasina are specific or not