THE ACRASINA
313
the aggregate then establishes an axis perpendicular to the substratum,
and elongates into a cylinder with a tapered tip. Without further gain of
mass (except sometimes for a slight influx of cells into its base) this grex
transforms itself into a fruiting body of characteristic and often complex
shape. Throughout development there is a steady increase in the precision
with which shape is defined.
The fruiting bodies of some species vary in size by a factor of 10,000
or more, yet the relative masses of stalk and spores, or prestalk cells and
prespores, remain virtually constant, or show only slight allometry.
This proportionality has been studied intensively (Bonner and Slifkin,
1949; Bonner, 1952, 1957; Bonner and Dodd, 1962), and Bonner (1957)
has proposed a theory to account for it. As we unfortunately have not
the space to discuss this absorbing problem with the detail it demands,
we shall pass on to examples of proportionality in slime-mould development that have not received so much attention.
In the larger species of Dictyostelium
a whole aggregation centre
commonly turns itself into a single grex. The centre first produces a
papilla on its upper surface, and one presumes that the diameter of the
centre within certain limits determines that of the papilla; then the
papilla leads out all the remaining cells into a cylinder of roughly its
own diameter and thus in turn determines the length of the grex. In
D. polycephalum
(Raper, 1956), there are often far more cells in an
aggregation centre than are able to enter a single grex, and then several
papillae of about 50 μ diameter—presumably about the maximum
possible—appear on its upper surface. (Comparable behaviour can in
fact occur in D. discoideum with very large aggregates.) Here too each
papilla leads out all the cells underlying it into a cylinder of corresponding diameter, but by the time all the cells have been used up, some of the
grex may have attained a length of a centimetre or more. In Acytostelium
(Raper and Quinlan, 1958), again small multiple papillae form on the
surface of a single centre while aggregation is in progress, but here the
growing grex do not anything like exhaust the available pool of cells
before they sever their connections with it : there seems to be a fairly
small maximum length as well as a maximum diameter, or put another
way, a fairly definite relation between length and diameter. Initial grex
proportions may then be modified as other factors come into play.
In D. discoideum,
the stalkless lying grex have been variously
described as having the form of slugs (perhaps ballistic rather than
gastropod), sausages, and cartridges. Typically they are cylinders with
the tip approximating to a half ellipsoid of revolution (Francis, 1962), and
their shape when they are crawling steadily in a straight line can be
essentially defined by the ratios of length to diameter, and of the axes
of the ellipse. In some strains, these ratios are quite variable; but
313
the aggregate then establishes an axis perpendicular to the substratum,
and elongates into a cylinder with a tapered tip. Without further gain of
mass (except sometimes for a slight influx of cells into its base) this grex
transforms itself into a fruiting body of characteristic and often complex
shape. Throughout development there is a steady increase in the precision
with which shape is defined.
The fruiting bodies of some species vary in size by a factor of 10,000
or more, yet the relative masses of stalk and spores, or prestalk cells and
prespores, remain virtually constant, or show only slight allometry.
This proportionality has been studied intensively (Bonner and Slifkin,
1949; Bonner, 1952, 1957; Bonner and Dodd, 1962), and Bonner (1957)
has proposed a theory to account for it. As we unfortunately have not
the space to discuss this absorbing problem with the detail it demands,
we shall pass on to examples of proportionality in slime-mould development that have not received so much attention.
In the larger species of Dictyostelium
a whole aggregation centre
commonly turns itself into a single grex. The centre first produces a
papilla on its upper surface, and one presumes that the diameter of the
centre within certain limits determines that of the papilla; then the
papilla leads out all the remaining cells into a cylinder of roughly its
own diameter and thus in turn determines the length of the grex. In
D. polycephalum
(Raper, 1956), there are often far more cells in an
aggregation centre than are able to enter a single grex, and then several
papillae of about 50 μ diameter—presumably about the maximum
possible—appear on its upper surface. (Comparable behaviour can in
fact occur in D. discoideum with very large aggregates.) Here too each
papilla leads out all the cells underlying it into a cylinder of corresponding diameter, but by the time all the cells have been used up, some of the
grex may have attained a length of a centimetre or more. In Acytostelium
(Raper and Quinlan, 1958), again small multiple papillae form on the
surface of a single centre while aggregation is in progress, but here the
growing grex do not anything like exhaust the available pool of cells
before they sever their connections with it : there seems to be a fairly
small maximum length as well as a maximum diameter, or put another
way, a fairly definite relation between length and diameter. Initial grex
proportions may then be modified as other factors come into play.
In D. discoideum,
the stalkless lying grex have been variously
described as having the form of slugs (perhaps ballistic rather than
gastropod), sausages, and cartridges. Typically they are cylinders with
the tip approximating to a half ellipsoid of revolution (Francis, 1962), and
their shape when they are crawling steadily in a straight line can be
essentially defined by the ratios of length to diameter, and of the axes
of the ellipse. In some strains, these ratios are quite variable; but