THE ACRASINA
319
precisely controlled, relative humidities (Bonner and Shaw, 1957; 5%
according to Cohen, 1953); whereas polycephalum will not fruit below
96% relative humidity, but will do so at 100% (Whittingham and
Raper, 1957). An alternative explanation of the polycephalum results is
that the porous lids and the acid decreased the concentration of a
volatile inhibitor produced by the cells themselves. Conceivably this
could have been ammonia, which in other species is known both to be
excreted and to inhibit fruiting (Gregg et al., 1954; Cohen, 1953), and
which would also have been removed by calcium chloride, the desiccant
used by Potts that allowed D. mucoroides to fruit in a sealed vessel.
Gerisch (1960) found that D. discoideum fruited normally in a small
sealed chamber in a water bath in a constant-temperature room; but
although he concluded that evaporation from an aggregate could be
safely excluded as an essential developmental stimulus, it must be
remembered that in air some evaporation will always be possible because
of the cells' own heat production.
In some ways it is easier to think of development as proceeding
spontaneously, and the cells' water relations as providing not a stimulus
but an inhibition. As the Acrasina flourish in nature in damp situations
but not under water, it is not surprising that an aquatic environment is
generally unfavourable to all stages of development. Thus, under a layer
of water: individual cells, whether on agar or glass, crawl about less
regularly; bacteria-free cells will form a homogeneous layer, and
subsequently radial aggregations rather than irregular clumps, only at
much lower densities than on normal culture plates ; the development of
founders and of definite centres is delayed and inhibited with varying
degrees of severity (Vol. 2, pp. 143-5); aggregation streams are more
diffuse ; and a grex of P. violaceum is prevented from making stalk and
therefore resembles a D. discoideum slug, whereas a discoideum slug
itself is reduced to a mere sphere (Shaffer, 1962). These effects may be
variously due to unfavourable oxygen or carbon dioxide relations or to
accumulation or escape of other metabolites. Thus, Runyon (1942)
described the progressive delaying and disturbance of aggregation
under increasing depths of water, ending in total suppression at a depth
of more than 1 mm, although Bonner (1947) found it to be unaffected by
up to 10 cm of overlying water. Other observations (Shaffer, 1962) show
that, at least in P. violaceum, aggregation is progressively inhibited as
the water deepens, and there are preliminary indications that stirring
the water lessens the inhibition. Certainly cells kept in suspension will
grow very satisfactorily and later form spherical aggregates (Gerisch,
1959). The complete suppression of spore and stalk formation in all
species in water, whether this is moving or not, may be due to a waterinsoluble inhibitor, or the loss of a morphogenetic substance. Or perhaps
319
precisely controlled, relative humidities (Bonner and Shaw, 1957; 5%
according to Cohen, 1953); whereas polycephalum will not fruit below
96% relative humidity, but will do so at 100% (Whittingham and
Raper, 1957). An alternative explanation of the polycephalum results is
that the porous lids and the acid decreased the concentration of a
volatile inhibitor produced by the cells themselves. Conceivably this
could have been ammonia, which in other species is known both to be
excreted and to inhibit fruiting (Gregg et al., 1954; Cohen, 1953), and
which would also have been removed by calcium chloride, the desiccant
used by Potts that allowed D. mucoroides to fruit in a sealed vessel.
Gerisch (1960) found that D. discoideum fruited normally in a small
sealed chamber in a water bath in a constant-temperature room; but
although he concluded that evaporation from an aggregate could be
safely excluded as an essential developmental stimulus, it must be
remembered that in air some evaporation will always be possible because
of the cells' own heat production.
In some ways it is easier to think of development as proceeding
spontaneously, and the cells' water relations as providing not a stimulus
but an inhibition. As the Acrasina flourish in nature in damp situations
but not under water, it is not surprising that an aquatic environment is
generally unfavourable to all stages of development. Thus, under a layer
of water: individual cells, whether on agar or glass, crawl about less
regularly; bacteria-free cells will form a homogeneous layer, and
subsequently radial aggregations rather than irregular clumps, only at
much lower densities than on normal culture plates ; the development of
founders and of definite centres is delayed and inhibited with varying
degrees of severity (Vol. 2, pp. 143-5); aggregation streams are more
diffuse ; and a grex of P. violaceum is prevented from making stalk and
therefore resembles a D. discoideum slug, whereas a discoideum slug
itself is reduced to a mere sphere (Shaffer, 1962). These effects may be
variously due to unfavourable oxygen or carbon dioxide relations or to
accumulation or escape of other metabolites. Thus, Runyon (1942)
described the progressive delaying and disturbance of aggregation
under increasing depths of water, ending in total suppression at a depth
of more than 1 mm, although Bonner (1947) found it to be unaffected by
up to 10 cm of overlying water. Other observations (Shaffer, 1962) show
that, at least in P. violaceum, aggregation is progressively inhibited as
the water deepens, and there are preliminary indications that stirring
the water lessens the inhibition. Certainly cells kept in suspension will
grow very satisfactorily and later form spherical aggregates (Gerisch,
1959). The complete suppression of spore and stalk formation in all
species in water, whether this is moving or not, may be due to a waterinsoluble inhibitor, or the loss of a morphogenetic substance. Or perhaps