178
B. M. S H A F F E R
adhesion further back cannot prevent them repelling one another. Soon
—far below the point at which the completed stalks diverge—they
begin to move apart, and are followed by all the cells behind them. The
thrust on the newly formed stalk obliquely to the original axis must now
tend to push the earlier lengths together, if these are not already touching. The whole structure looks like a bunch of elongated balloons directly
attached to the end of a stick (Fig. 22). But instead of the balloons
separating immediately, as we might expect, for a considerable time
they remain in contact at their bases while the stick lengthens. We must
conclude that the bend in each of the component stalks travels along it. The
first problem is flexibility, but here the stalk is only one cell thick, and
perhaps the cells at the bend have yet to make their rigid cellulose walls.
The second problem is what zips together the sections of the slime
sheaths formed above the point of divergence. At least three possibilities seem worth examining: the cell masses shorten and increase in
diameter; cell vacuolation at the bends tends to straighten them out;
there are adequate components of the thrusts on the stalks as the cells
climb them. It is interesting that when Pfützner-Eckert (1950) once
planted an erect hair in a D. mucoroides centre, the resultant fruiting
body for its lower two-thirds adhered to it closely and then diverged—
quasi polycephalum. I recently had the shock of seeing some D. discoideum 'slugs' crawling over a surface and making stalk just like mucoroides.
Even more heretically, while still on the ground, they produced abortive
spores: spherical cells, only 3-4 μ in diameter, and lacking refractive
cases. The substratum must have been responsible: it was collodion not
agar. Now, although a substratum that removed the repellent would
attract a grex, it seems doubtful whether this would overcome the ordinary erection guidance. Probably the collodion's effect was due to the
slime sheath's stronger affinity for it, as shown by it not floating off when
flooded with water. The fact that only minute grex behaved so atypically
supported this interpretation. Presumably, they had received an adequate stimulus to culminate and were doing so, but were simply unable
to raise their tips from the ground. What is unknown is whether, in any
species other than polycephalum, when a sufficiently small stalked grex
that has been crawling over a highly adhesive surface becomes erect at
its tip, the sharp bend formed in the stalk will travel forwards along it.
Acknowledgements
I am most grateful to Dr. M. G. M. Pryor for his critical reading of the
manuscript, and to Professor K. B. Raper and Mr. D. Francis for permission to use their illustrations. This review was written and the previously unpublished results obtained while holding a Fellowship at
B. M. S H A F F E R
adhesion further back cannot prevent them repelling one another. Soon
—far below the point at which the completed stalks diverge—they
begin to move apart, and are followed by all the cells behind them. The
thrust on the newly formed stalk obliquely to the original axis must now
tend to push the earlier lengths together, if these are not already touching. The whole structure looks like a bunch of elongated balloons directly
attached to the end of a stick (Fig. 22). But instead of the balloons
separating immediately, as we might expect, for a considerable time
they remain in contact at their bases while the stick lengthens. We must
conclude that the bend in each of the component stalks travels along it. The
first problem is flexibility, but here the stalk is only one cell thick, and
perhaps the cells at the bend have yet to make their rigid cellulose walls.
The second problem is what zips together the sections of the slime
sheaths formed above the point of divergence. At least three possibilities seem worth examining: the cell masses shorten and increase in
diameter; cell vacuolation at the bends tends to straighten them out;
there are adequate components of the thrusts on the stalks as the cells
climb them. It is interesting that when Pfützner-Eckert (1950) once
planted an erect hair in a D. mucoroides centre, the resultant fruiting
body for its lower two-thirds adhered to it closely and then diverged—
quasi polycephalum. I recently had the shock of seeing some D. discoideum 'slugs' crawling over a surface and making stalk just like mucoroides.
Even more heretically, while still on the ground, they produced abortive
spores: spherical cells, only 3-4 μ in diameter, and lacking refractive
cases. The substratum must have been responsible: it was collodion not
agar. Now, although a substratum that removed the repellent would
attract a grex, it seems doubtful whether this would overcome the ordinary erection guidance. Probably the collodion's effect was due to the
slime sheath's stronger affinity for it, as shown by it not floating off when
flooded with water. The fact that only minute grex behaved so atypically
supported this interpretation. Presumably, they had received an adequate stimulus to culminate and were doing so, but were simply unable
to raise their tips from the ground. What is unknown is whether, in any
species other than polycephalum, when a sufficiently small stalked grex
that has been crawling over a highly adhesive surface becomes erect at
its tip, the sharp bend formed in the stalk will travel forwards along it.
Acknowledgements
I am most grateful to Dr. M. G. M. Pryor for his critical reading of the
manuscript, and to Professor K. B. Raper and Mr. D. Francis for permission to use their illustrations. This review was written and the previously unpublished results obtained while holding a Fellowship at