62
H. WAMS AND P. KALLIO
nently diploid in its main characteristics. At the enucleate isthmus
either no processes suggesting cytokinesis are seen or a small middle
section is formed. Only very rarely is the septum formed and an
enucleate cell produced in M. thomasiana.
If a double cell of type I is exposed to centrifugation in the mitotic
phase, other types of complex cells are produced. Some of the different
ways in which binucleate cells develop after centrifuging and the formation of chloroplast bulges of different sizes are shown in the Figs. 6 and 7.
Of particular interest are the double heterocaryotic cells, i.e. double
cells with one haploid nucleus in one of the two isthmuses and one
diploid nucleus in the other. Both nuclei divide synchronously in this
cell type, although normal haploid and diploid cells always have mitotic
cycles of different length. These cells show that there must be a plasmatic control of the beginning of mitosis (Kallio, 1961). The new semicells attached to the same middle section show only minor differences in
shape and size, whereas the new semicells of the single cells formed
always exhibit large differences. Also the phenomenon of 'senescence' is
seen in this heterocaryotic cell type, as a prolonged interval between
mitoses and an abnormal cell morphology.
A cell type with a middle section and only one haploid nucleus in one
of the isthmuses is also produced in these centrifugation experiments
(Pig. 6 B). This cell type—of both M. thomasiana and M. torreyi—is
capable of dividing only at the nucleate isthmus whereas no development occurs at the enucleate isthmus. The mitotic cycle may be shorter
than in a normal haploid cell.
The number of middle sections may also be greater than one. Different
types of complex cells are sometimes produced in cultures exposed to
continuous light and also spontaneously in old cultures, but the types
are the same in principle as those produced by centrifuging. In these
long cells, the correlation of the nuclear effect and the distance of the
nucleus from the isthmus is seen particularly in the rate of formation of
the septum and in the forms of the new semi-cells.
When centrifugation does not cause a disturbance in the formation of
the septum, no or only weak effects on the morphology and development
of the cell are noted. If the direction of the centrifugal force is parallel to
the sinus, there is no effect of twice or three times the force that leads to
the formation of enucleate and binucleate cells when acting parallel to
the long axis of the cell. If a short bulge which normally leads to formation of an enucleate cell is centrifuged back into the other semicell, the
result is the same as after a single centrifugation.
The value of the complex cells of Micrasterias in the investigation of
morphogenesis and cell physiology is due particularly to the following
characteristics of these cells: (1) the lack of any after-effect of the pro-
H. WAMS AND P. KALLIO
nently diploid in its main characteristics. At the enucleate isthmus
either no processes suggesting cytokinesis are seen or a small middle
section is formed. Only very rarely is the septum formed and an
enucleate cell produced in M. thomasiana.
If a double cell of type I is exposed to centrifugation in the mitotic
phase, other types of complex cells are produced. Some of the different
ways in which binucleate cells develop after centrifuging and the formation of chloroplast bulges of different sizes are shown in the Figs. 6 and 7.
Of particular interest are the double heterocaryotic cells, i.e. double
cells with one haploid nucleus in one of the two isthmuses and one
diploid nucleus in the other. Both nuclei divide synchronously in this
cell type, although normal haploid and diploid cells always have mitotic
cycles of different length. These cells show that there must be a plasmatic control of the beginning of mitosis (Kallio, 1961). The new semicells attached to the same middle section show only minor differences in
shape and size, whereas the new semicells of the single cells formed
always exhibit large differences. Also the phenomenon of 'senescence' is
seen in this heterocaryotic cell type, as a prolonged interval between
mitoses and an abnormal cell morphology.
A cell type with a middle section and only one haploid nucleus in one
of the isthmuses is also produced in these centrifugation experiments
(Pig. 6 B). This cell type—of both M. thomasiana and M. torreyi—is
capable of dividing only at the nucleate isthmus whereas no development occurs at the enucleate isthmus. The mitotic cycle may be shorter
than in a normal haploid cell.
The number of middle sections may also be greater than one. Different
types of complex cells are sometimes produced in cultures exposed to
continuous light and also spontaneously in old cultures, but the types
are the same in principle as those produced by centrifuging. In these
long cells, the correlation of the nuclear effect and the distance of the
nucleus from the isthmus is seen particularly in the rate of formation of
the septum and in the forms of the new semi-cells.
When centrifugation does not cause a disturbance in the formation of
the septum, no or only weak effects on the morphology and development
of the cell are noted. If the direction of the centrifugal force is parallel to
the sinus, there is no effect of twice or three times the force that leads to
the formation of enucleate and binucleate cells when acting parallel to
the long axis of the cell. If a short bulge which normally leads to formation of an enucleate cell is centrifuged back into the other semicell, the
result is the same as after a single centrifugation.
The value of the complex cells of Micrasterias in the investigation of
morphogenesis and cell physiology is due particularly to the following
characteristics of these cells: (1) the lack of any after-effect of the pro-