THE ACRASINA
317
(1959) has linked these facts by supposing that the addition of new cells
to the top of the stalk pushes it downwards until its base receives solid
support from the ground, and that only then can it begin to grow
upwards. However, whether pressure is generated by movements of the
prestalk cells or by vacuolation of stalk cells, it seems more likely that
it would raise the tip rather than ram the stalk down through passive
prespores, especially as the slime sheath surrounding the rear of the
grex—supposedly the region where it is strongest—has to be expanded
in diameter. Could it be that the stalk cells thrust their way down
through the prespores by their own amoeboid movements—rather like
an internal slug? It is not known for sure whether the cells within the
stalk are still capable of movement at this stage : Raper and Fennell
(1952) reported that these were not visibly differentiated, though
according to Bonner (1944) and Krivanek (1956) they are already
vacuolated. Even if the stalk cells could move, there would be the serious
difficulty of explaining why those initially within the stalk sheath should
all suddenly start to crawl backwards. (It would be easier to account for
stalk cells crawling towards the base of the grex if they had originated
outside the stalk sheath, because they could simply have made a U-turn
at the apex, without reversing their polarity, as they entered the upper
end of the stalk.) In any case, there is still another difficulty : downwards
crawling could produce downward stalk growth, but how could it move
the stalk sheath in the same direction and so account for the descent of
the stalk as a whole?
The remaining explanation of the passage of the stalk through the
prespores is that these actively climb it. This must eventually make the
stalk descend, because the rear of the grex obviously cannot leave the
ground till the stalk is touching it, but by itself it does not account for
the change in shape. The view that the tip is mechanically unable to rise
till the stalk has been grounded is attractive, but untenable: the grex
has earlier achieved a considerably greater height without any stalk at
all. Perhaps more striking evidence is provided by P. violaceum (Shaffer,
1962) : a grex formed under water, and therefore inhibited from making
stalk, at once starts to make it if its tip breaks the surface; and then
without the stalk having been pushed right down through it to the
bottom of the dish, the entire grex follows the tip straight up out of the
water and builds a normal fruiting body that floats on the surface. In
fact, this happens without the stalk having been pushed down into the
submerged part of the grex at all, an observation that strongly supports
our conclusion that no force is generated able to bring this about.
Why then does the stalk descend in D. discoideum'
1
. The clue to what
is happening is that it is not only the stalk that moves but the entire tip
with the stalk inside it. Again, it would be difficult to explain this by
317
(1959) has linked these facts by supposing that the addition of new cells
to the top of the stalk pushes it downwards until its base receives solid
support from the ground, and that only then can it begin to grow
upwards. However, whether pressure is generated by movements of the
prestalk cells or by vacuolation of stalk cells, it seems more likely that
it would raise the tip rather than ram the stalk down through passive
prespores, especially as the slime sheath surrounding the rear of the
grex—supposedly the region where it is strongest—has to be expanded
in diameter. Could it be that the stalk cells thrust their way down
through the prespores by their own amoeboid movements—rather like
an internal slug? It is not known for sure whether the cells within the
stalk are still capable of movement at this stage : Raper and Fennell
(1952) reported that these were not visibly differentiated, though
according to Bonner (1944) and Krivanek (1956) they are already
vacuolated. Even if the stalk cells could move, there would be the serious
difficulty of explaining why those initially within the stalk sheath should
all suddenly start to crawl backwards. (It would be easier to account for
stalk cells crawling towards the base of the grex if they had originated
outside the stalk sheath, because they could simply have made a U-turn
at the apex, without reversing their polarity, as they entered the upper
end of the stalk.) In any case, there is still another difficulty : downwards
crawling could produce downward stalk growth, but how could it move
the stalk sheath in the same direction and so account for the descent of
the stalk as a whole?
The remaining explanation of the passage of the stalk through the
prespores is that these actively climb it. This must eventually make the
stalk descend, because the rear of the grex obviously cannot leave the
ground till the stalk is touching it, but by itself it does not account for
the change in shape. The view that the tip is mechanically unable to rise
till the stalk has been grounded is attractive, but untenable: the grex
has earlier achieved a considerably greater height without any stalk at
all. Perhaps more striking evidence is provided by P. violaceum (Shaffer,
1962) : a grex formed under water, and therefore inhibited from making
stalk, at once starts to make it if its tip breaks the surface; and then
without the stalk having been pushed right down through it to the
bottom of the dish, the entire grex follows the tip straight up out of the
water and builds a normal fruiting body that floats on the surface. In
fact, this happens without the stalk having been pushed down into the
submerged part of the grex at all, an observation that strongly supports
our conclusion that no force is generated able to bring this about.
Why then does the stalk descend in D. discoideum'
1
. The clue to what
is happening is that it is not only the stalk that moves but the entire tip
with the stalk inside it. Again, it would be difficult to explain this by