THE ACRASINA
153
Apart from a variety of unspecialized metabolites, another factor that
could be responsible is the one, other than the attractor, that promotes
cantres (Ennis and Sussman, 1958a). It is also just possibly relevant that
the highly developed P. violaceum founder stops moving before starting
a centre.
As a cloud ages, its cells form small clumps, which slowly increase
in size till one or more of them start to act, temporarily or permanently,
as obvious centres of attraction and aggregation (Shaffer, 1959; Samuel,
1961; Figs. 11 and 12). In general, clumping might be a reaction solely
to diffusion gradients of an attractor; or solely to a change in properties, which at least initially would be classed as greater adhesiveness,
increasing the average time cells remained together when they chanced
to come in contact; or to both, as appears to be the case within clouds.
Moreover, these factors could interact, diffused acrasin increasing adhesiveness, and in some species sustained large-scale acrasin secretion
perhaps being possible only within a clump. The clearest evidence that
cells might become slightly adhesive in the absence of attractor, maybe
by responding to the diffusible factor promoting centres or by maturing
spontaneously, was provided by a mucoroides mutant isolated by
Pfützner-Eckert (1950), which would immediately flow radially towards
an implanted wild centre, but by itself could get no further than forming small clumps and unoriented chains. A stage of similar appearance
in wild mucoroides was named the preplasmodium by Arndt (1937).
Contact following would slowly have imposed a uniform orientation on
cells within a clump, if they were sufficiently adhesive and remained
motile. Unrestrained by a gradient of attractor, any cell that began to
leave a clump would have tended to draw the others out into a chain.
'Aggregation' in the simplest Acrasina and in some 'solitary' amoebae
is no more than clumping (Ray and Hayes, 1954; Raper, 1960), but it is
not known whether any diffusible factors are involved. A most interesting case is Guttulina, a genus showing an intermediate complexity of
development. In Kessler's recently isolated strain, which he has kindly
allowed me to examine, very small clumps appear uniformly over a large
area cleared of bacteria, yet sometimes for several days the only change
is a slightly increased average clump size, despite such rapid flux that
individual cells join or leave each clump every minute. It is possible to
see how an equilibrium size may exist even if no attractor or repellent
is produced. If adhesiveness remains constant and too slight for chain
building, large clumps will tend to grow at the expense of smaller ones,
both because they represent a larger target for collisions, and because
they possess proportionately fewer peripheral cells and these are the
only ones that can escape. Growth will stop when there has been a
sufficient fall in the number of free cells and sufficient increase in the
153
Apart from a variety of unspecialized metabolites, another factor that
could be responsible is the one, other than the attractor, that promotes
cantres (Ennis and Sussman, 1958a). It is also just possibly relevant that
the highly developed P. violaceum founder stops moving before starting
a centre.
As a cloud ages, its cells form small clumps, which slowly increase
in size till one or more of them start to act, temporarily or permanently,
as obvious centres of attraction and aggregation (Shaffer, 1959; Samuel,
1961; Figs. 11 and 12). In general, clumping might be a reaction solely
to diffusion gradients of an attractor; or solely to a change in properties, which at least initially would be classed as greater adhesiveness,
increasing the average time cells remained together when they chanced
to come in contact; or to both, as appears to be the case within clouds.
Moreover, these factors could interact, diffused acrasin increasing adhesiveness, and in some species sustained large-scale acrasin secretion
perhaps being possible only within a clump. The clearest evidence that
cells might become slightly adhesive in the absence of attractor, maybe
by responding to the diffusible factor promoting centres or by maturing
spontaneously, was provided by a mucoroides mutant isolated by
Pfützner-Eckert (1950), which would immediately flow radially towards
an implanted wild centre, but by itself could get no further than forming small clumps and unoriented chains. A stage of similar appearance
in wild mucoroides was named the preplasmodium by Arndt (1937).
Contact following would slowly have imposed a uniform orientation on
cells within a clump, if they were sufficiently adhesive and remained
motile. Unrestrained by a gradient of attractor, any cell that began to
leave a clump would have tended to draw the others out into a chain.
'Aggregation' in the simplest Acrasina and in some 'solitary' amoebae
is no more than clumping (Ray and Hayes, 1954; Raper, 1960), but it is
not known whether any diffusible factors are involved. A most interesting case is Guttulina, a genus showing an intermediate complexity of
development. In Kessler's recently isolated strain, which he has kindly
allowed me to examine, very small clumps appear uniformly over a large
area cleared of bacteria, yet sometimes for several days the only change
is a slightly increased average clump size, despite such rapid flux that
individual cells join or leave each clump every minute. It is possible to
see how an equilibrium size may exist even if no attractor or repellent
is produced. If adhesiveness remains constant and too slight for chain
building, large clumps will tend to grow at the expense of smaller ones,
both because they represent a larger target for collisions, and because
they possess proportionately fewer peripheral cells and these are the
only ones that can escape. Growth will stop when there has been a
sufficient fall in the number of free cells and sufficient increase in the