THE ACRASINA
321
posture, both important in the morphogenesis of fruiting bodies with multiple stalks. All
a grex's cells, though previously said to be unoriented or transversely oriented, are
oriented towards its tip. The transverse ones are merely flattened from front to back, and
not by pressure, but by the adhesion of a greater area of their ends. The grex tip may be
essential for migration only because it supplies some factor, possibly the slime sheath,
disciplining cell movement. The tip cells do not make ten times as much acrasin as the
others : grex cells are guided by contact following. However, a Polysphondylium pallidum
grex does make Dictyostelium acrasin only at its tip. Grex grafting shows that most strains
can lead only cells of the same strain, but that some strains can lead cells of many strains;
this suggests that a cell's front and back ends have different specificity. The relations
between the velocity and the size and shape of a grex have not been adequately explained.
Existing mechanical models imply that the grex should elongate till its cells are in single
file. In fact, it may have an equilibrium length, possibly maintained by metabolic interdependence of its ends; the jointly created, internal, chemical milieu would fix the
velocity. Because both ends migrate at the same speed, the anterior cells cannot be held
to be travelling faster than the rear ones; they do so only when surrounded by them.
Probably the cells position themselves along the axis according to their ability to displace
other cells from contact with one another ; this is an aspect of adhesiveness. It is unlikely
that stalk-cell vacuolation or cellulose deposition is ever the main lifting force in culmination; the mass is raised by the amoeboid movement of both the prespore and
prestalk cells. The stalk rudiment is not pushed down through passive prespore cells, nor
does it have to reach the substratum before the grex can start to rise ; it descends because
the cells climb it, and the reduction in height of D. discoideum in early culmination is due
to the prespores moving faster than the prestalk cells. Although the prestalk cells move
into the interior to form stalk, this invagination results from their being overtaken while
still advancing in the same direction. The grex's geometry shows that their speed must
have been greatly increased immediately before this. When an Acrasis aggregate turns
into a single chain of spores, the cells are probably guided perpendicularly to the axis. A
stagnant water layer inhibits all stages of development to a varying extent, but it is uncertain whether slight desiccation is essential for culmination. The critical stimulus for
D. polycephalum culmination is probably not desiccation but removal of a volatile inhibitor.
References
Blauuw, A. H. (1918). Meded. LandbHoogesch., Wageningen 15, 91.
Bonner, J. T. (1944). Amer. J. Bot. 31, 175.
Bonner, J. T. (1947). J. exp. Zool. 106, 1.
Bonner, J. T. (1950). Biol. Bull., Wood's Hole 99, 143.
Bonner, J. T. (1952). Amer. Nat. 86, 79.
Bonner, J. T. (1957). Quart. Rev. Biol. 32, 232.
Bonner, J. T. (1959). 'The Cellular Slime Molds', 150 pp. Princeton University
Press, New Jersey.
Bonner, J. T. and Dodd, M. R. (1962). Develop. Biol. 5, 344.
Bonner, J. T. and Eldredge, D. Jr., (1945). Growth 9, 287.
Bonner, J. T. and Shaw, M. J. (1957). J. cell. comp. Physiol. 50, 145.
Bonner, J. T. and Slifkin, M. K. (1949). Amer. J. Bot. 36, 727.
Bonner, J. T., Clark, W. W., Neely, C. L., Jr. and Slifkin, M. K. (1950). J. cell.
comp. Physiol. 36, 149.
Bonner, J. T., Koontz, P. G. and Paton, D. (1953). Mycologia 45, 235.
Bonner, J. T., Chiquoine, A. D. and Kolderie, M. Q. (1955). J. exp. Zool. 130,133.
Brefeld, Ο. (1884). 'Untersuchungen aus dem Gesammtgebiet der Mykologie', Pt.
6, pp. 1-34. Felix, Leipzig.
Buder, J. (1920). Ber. dtsch. bot. Ges. 38, 10.
Ν
321
posture, both important in the morphogenesis of fruiting bodies with multiple stalks. All
a grex's cells, though previously said to be unoriented or transversely oriented, are
oriented towards its tip. The transverse ones are merely flattened from front to back, and
not by pressure, but by the adhesion of a greater area of their ends. The grex tip may be
essential for migration only because it supplies some factor, possibly the slime sheath,
disciplining cell movement. The tip cells do not make ten times as much acrasin as the
others : grex cells are guided by contact following. However, a Polysphondylium pallidum
grex does make Dictyostelium acrasin only at its tip. Grex grafting shows that most strains
can lead only cells of the same strain, but that some strains can lead cells of many strains;
this suggests that a cell's front and back ends have different specificity. The relations
between the velocity and the size and shape of a grex have not been adequately explained.
Existing mechanical models imply that the grex should elongate till its cells are in single
file. In fact, it may have an equilibrium length, possibly maintained by metabolic interdependence of its ends; the jointly created, internal, chemical milieu would fix the
velocity. Because both ends migrate at the same speed, the anterior cells cannot be held
to be travelling faster than the rear ones; they do so only when surrounded by them.
Probably the cells position themselves along the axis according to their ability to displace
other cells from contact with one another ; this is an aspect of adhesiveness. It is unlikely
that stalk-cell vacuolation or cellulose deposition is ever the main lifting force in culmination; the mass is raised by the amoeboid movement of both the prespore and
prestalk cells. The stalk rudiment is not pushed down through passive prespore cells, nor
does it have to reach the substratum before the grex can start to rise ; it descends because
the cells climb it, and the reduction in height of D. discoideum in early culmination is due
to the prespores moving faster than the prestalk cells. Although the prestalk cells move
into the interior to form stalk, this invagination results from their being overtaken while
still advancing in the same direction. The grex's geometry shows that their speed must
have been greatly increased immediately before this. When an Acrasis aggregate turns
into a single chain of spores, the cells are probably guided perpendicularly to the axis. A
stagnant water layer inhibits all stages of development to a varying extent, but it is uncertain whether slight desiccation is essential for culmination. The critical stimulus for
D. polycephalum culmination is probably not desiccation but removal of a volatile inhibitor.
References
Blauuw, A. H. (1918). Meded. LandbHoogesch., Wageningen 15, 91.
Bonner, J. T. (1944). Amer. J. Bot. 31, 175.
Bonner, J. T. (1947). J. exp. Zool. 106, 1.
Bonner, J. T. (1950). Biol. Bull., Wood's Hole 99, 143.
Bonner, J. T. (1952). Amer. Nat. 86, 79.
Bonner, J. T. (1957). Quart. Rev. Biol. 32, 232.
Bonner, J. T. (1959). 'The Cellular Slime Molds', 150 pp. Princeton University
Press, New Jersey.
Bonner, J. T. and Dodd, M. R. (1962). Develop. Biol. 5, 344.
Bonner, J. T. and Eldredge, D. Jr., (1945). Growth 9, 287.
Bonner, J. T. and Shaw, M. J. (1957). J. cell. comp. Physiol. 50, 145.
Bonner, J. T. and Slifkin, M. K. (1949). Amer. J. Bot. 36, 727.
Bonner, J. T., Clark, W. W., Neely, C. L., Jr. and Slifkin, M. K. (1950). J. cell.
comp. Physiol. 36, 149.
Bonner, J. T., Koontz, P. G. and Paton, D. (1953). Mycologia 45, 235.
Bonner, J. T., Chiquoine, A. D. and Kolderie, M. Q. (1955). J. exp. Zool. 130,133.
Brefeld, Ο. (1884). 'Untersuchungen aus dem Gesammtgebiet der Mykologie', Pt.
6, pp. 1-34. Felix, Leipzig.
Buder, J. (1920). Ber. dtsch. bot. Ges. 38, 10.
Ν