THE ACRASINA
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way to the ground. Raper and Fennell reported that the slime sheath
was severed over the rear of the grex when this began to turn into spores.
From this stage onwards, the prestalk cells might be lifted passively
by the vacuolation pressure, if adhering only to the slime sheath. The
question could be settled by using markers to see whether the slime
sheath was no longer extended at the tip and stationary behind this but
was carried up with the grex. (If in fact it still is stationary, what can its
fate be? We know spores do not travel up inside it; but if they were
outside it, it is not obvious why surface tension should make them
follow the prestalk. Can the liquid in which the spores are suspended
digest it? This would account for its being 'severed', and in any case
seems to be a biological necessity.)
One factor that would certainly hinder the grex from being pushed up
the stalk is that the stalk sheath is widest at the top and tapers below
this like a wineglass (Fig. 17, Vol. 2, p. 165; Fig. 26). Raper and Fennell
attributed the stalk sheath's terminal expansion to the pressure inside it ;
but the same pressure cannot then also power the advance of the grex,
because this advance must be responsible for the subsequent narrowing
of this sheath. As the slime sheath is probably weaker at the tip than
elsewhere, enlargement of the stalk cells, by itself, would perhaps do no
more than make the tip of the grex swell.
In Acytostelium,
Raper and Quinlan (1958) found that all the grex
cells were 'transverse' before they turned into spores, and again this led
them to search elsewhere for a lifting force. Tentatively they supposed
that secretion of cellulose into an axial channel would produce sufficient
pressure to drive the coherent mass of cells upwards. But again there is
the problem of overcoming adhesion to the stalk and of why the postulated pressure should act in any particular direction. Actually, although
these authors thought the stalk was a tube, Gezelius (1959) was unable
to detect a lumen by electron microscopy, and close inspection shows
that the stalk tapers from its final diameter of 1 to 2 μ where it leaves the
base of the grex to about a tenth of this at the tip (Shaffer, 1962). Thus
the cellulose contributed by the vast majority of cells simply leads to an
increase in stalk diameter, and the filament at the very tip is so extremely
fine that its extension, even by orderly cellulose deposition, is unlikely
to be able to shift a mass that may contain many hundred cells. The
alternative view that the cells are motile and can thus actively climb the
stalk receives some support from Raper and Quinlan's observation that
if an Acytostelium grex happens to topple over onto the ground, its cells
may reorganize themselves into several daughter grex.
Bonner (1959) holds that all the cells in the Acytostelium grex and the
prestalk ones in Dictyostelium provide the motive power for culmination.
But he also takes the view—again the opposite of Raper's—that in
305
way to the ground. Raper and Fennell reported that the slime sheath
was severed over the rear of the grex when this began to turn into spores.
From this stage onwards, the prestalk cells might be lifted passively
by the vacuolation pressure, if adhering only to the slime sheath. The
question could be settled by using markers to see whether the slime
sheath was no longer extended at the tip and stationary behind this but
was carried up with the grex. (If in fact it still is stationary, what can its
fate be? We know spores do not travel up inside it; but if they were
outside it, it is not obvious why surface tension should make them
follow the prestalk. Can the liquid in which the spores are suspended
digest it? This would account for its being 'severed', and in any case
seems to be a biological necessity.)
One factor that would certainly hinder the grex from being pushed up
the stalk is that the stalk sheath is widest at the top and tapers below
this like a wineglass (Fig. 17, Vol. 2, p. 165; Fig. 26). Raper and Fennell
attributed the stalk sheath's terminal expansion to the pressure inside it ;
but the same pressure cannot then also power the advance of the grex,
because this advance must be responsible for the subsequent narrowing
of this sheath. As the slime sheath is probably weaker at the tip than
elsewhere, enlargement of the stalk cells, by itself, would perhaps do no
more than make the tip of the grex swell.
In Acytostelium,
Raper and Quinlan (1958) found that all the grex
cells were 'transverse' before they turned into spores, and again this led
them to search elsewhere for a lifting force. Tentatively they supposed
that secretion of cellulose into an axial channel would produce sufficient
pressure to drive the coherent mass of cells upwards. But again there is
the problem of overcoming adhesion to the stalk and of why the postulated pressure should act in any particular direction. Actually, although
these authors thought the stalk was a tube, Gezelius (1959) was unable
to detect a lumen by electron microscopy, and close inspection shows
that the stalk tapers from its final diameter of 1 to 2 μ where it leaves the
base of the grex to about a tenth of this at the tip (Shaffer, 1962). Thus
the cellulose contributed by the vast majority of cells simply leads to an
increase in stalk diameter, and the filament at the very tip is so extremely
fine that its extension, even by orderly cellulose deposition, is unlikely
to be able to shift a mass that may contain many hundred cells. The
alternative view that the cells are motile and can thus actively climb the
stalk receives some support from Raper and Quinlan's observation that
if an Acytostelium grex happens to topple over onto the ground, its cells
may reorganize themselves into several daughter grex.
Bonner (1959) holds that all the cells in the Acytostelium grex and the
prestalk ones in Dictyostelium provide the motive power for culmination.
But he also takes the view—again the opposite of Raper's—that in