T H E
A C R A S I N A
171
FIG. 20. A culminating grex of P. pallidum laid flat on agar. The mass further back
along the stalk had been destined to form a whorl of branches. Four heaps of D. mucoroides
cells were deposited in a line parallel to it. Only the one opposite the extreme tip of the
grex was attracted to it; whereas the second group moved parallel to the grex, responding to the induced secretion of the first one.
Harper (1926, 1929) and later Raper (1935) observed, the diameter of
the stalk is varied by using not only different numbers of roughly isodiametric cells but also, when there is but a single column of cells, different cell shapes. They found the ratio of cell height to width ranged
from 6 to \, and in my experience it may go even as high as 10, a
thirtyfold range. Harper believed that cells individually adjusted their
diameters so as to produce a well-proportioned fruiting body. But considering the inevitable variation in behaviour as well as in size, it hardly
seems possible, whatever sensory clues they might use, that they could
in this way produce a stalk that tapered uniformly, say 1 μ per 100 μ rise,
for a considerable distance. Raper and Fennell's (1952) much more
plausible view is that the width of the stalk is determined by the width
of the stalk sheath, and that the cells simply adopt whatever shape is
necessary to fill the lumen. But it remains to be determined to what
extent this can apply to the very smallest fruiting bodies, which contain
no more than two or three stalk cells (Sussman, 1955a; Bonner, 1960).
Does the last one really manage not only to produce a tubular sheath in
the middle of itself, but also, like the village magician, to disappear
inside it?
B. The Whole
1. Orientation
Sometimes an ordinary, virtually stationary, aggregation centre turns
into a stream: it simply starts to move in an external acrasin gradient
over the agar surface, without any determinate, prior change in shape,
though it tends to flatten and elongate progressively during its journey
(Shaffer, 1957c). In contrast, when the cells organize themselves as a
grex, they form what is roughly speaking a cylinder perpendicular to
A C R A S I N A
171
FIG. 20. A culminating grex of P. pallidum laid flat on agar. The mass further back
along the stalk had been destined to form a whorl of branches. Four heaps of D. mucoroides
cells were deposited in a line parallel to it. Only the one opposite the extreme tip of the
grex was attracted to it; whereas the second group moved parallel to the grex, responding to the induced secretion of the first one.
Harper (1926, 1929) and later Raper (1935) observed, the diameter of
the stalk is varied by using not only different numbers of roughly isodiametric cells but also, when there is but a single column of cells, different cell shapes. They found the ratio of cell height to width ranged
from 6 to \, and in my experience it may go even as high as 10, a
thirtyfold range. Harper believed that cells individually adjusted their
diameters so as to produce a well-proportioned fruiting body. But considering the inevitable variation in behaviour as well as in size, it hardly
seems possible, whatever sensory clues they might use, that they could
in this way produce a stalk that tapered uniformly, say 1 μ per 100 μ rise,
for a considerable distance. Raper and Fennell's (1952) much more
plausible view is that the width of the stalk is determined by the width
of the stalk sheath, and that the cells simply adopt whatever shape is
necessary to fill the lumen. But it remains to be determined to what
extent this can apply to the very smallest fruiting bodies, which contain
no more than two or three stalk cells (Sussman, 1955a; Bonner, 1960).
Does the last one really manage not only to produce a tubular sheath in
the middle of itself, but also, like the village magician, to disappear
inside it?
B. The Whole
1. Orientation
Sometimes an ordinary, virtually stationary, aggregation centre turns
into a stream: it simply starts to move in an external acrasin gradient
over the agar surface, without any determinate, prior change in shape,
though it tends to flatten and elongate progressively during its journey
(Shaffer, 1957c). In contrast, when the cells organize themselves as a
grex, they form what is roughly speaking a cylinder perpendicular to