MORPHOGENESIS IN
MICRASTERIAS
69
in the metaphase stage, leads to the formation of a new semicell that
closely resembles the enucleate cell. Exposure of the cell for 6 min to
light from a Philips TUV 6 W tube from a distance of 4 cm (the water
layer covering the cell being 2 mm thick) leads to this result.
When the cell is irradiated in the metaphase stage, the mitosis is
retarded and may last more than 1 h. However, the cell always continues to develop after the exposure to the standard lethal dose. The
irradiation usually has no effect on septum formation. The development
of the new semicell as well as its final form after irradiation closely
resemble the development and form of an enucleate cell. Both contain
'pectic lenses' (Waris, 1951), the flow of the cytoplasm is similar, a
permanent membrane is not formed, and the chloroplast expands very
little into the new semicell. The lifetime is, however, longer, often 2-3
days, whereas it is always 1 day in the enucleate cell. Three-lobed new
semicells are formed by these treatments from M. thomasiana, whereas
the enucleate semicell is mostly five-lobed. The same end result is
obtained as in the previous treatments if the irradiation takes place in
the small bulge stage. If the new semicells have already formed lobes,
the ultraviolet radiation stops the differentiation, but growth continues
and forms intermediate between enucleate and normal cells are obtained. The sensitivity to ultraviolet irradiation is weaker in late phases
of semicell development and cells irradiated with the standard lethal
dose often survive and divide after a normal or slightly prolonged
mitotic interval. The ability to survive seems to be inversely correlated
with the abnormality of the morphology of the new semicell.
When whole cells are exposed, it is not possible to distinguish the
effects of radiation on the plasm and on the nucleus. Partial irradiation
of the cell has shown that the short-term effect on the plasm of the
developing semicell is weak. If one wing of the semicell is exposed while
the other wing is protected from the direct effect of radiation by tin
foil, the former develops more slowly and its final form may be less
differentiated.
The complex cells are suitable objects for detailed studies of the
effects of ultraviolet irradiation (Kallio, 1963).
If a whole double cell of type I is irradiated with the lethal dose in
the sensitive stage, the result is the same as with normal cells. Four
semicells resembling enucleate semicells are formed and all three cells
that are formed die (Fig. 8 A).
If only one half of the cell and its nucleus are irradiated and the other
is protected, the development at the protected end is normal. Two
different semicells are formed at the exposed end (Fig. 8 C). The single
cell formed is typically enucleate with a three-lobed new semicell. Its
sister semicell in the double cell is haploid or, usually, somewhat hypoC2
A.M.4
MICRASTERIAS
69
in the metaphase stage, leads to the formation of a new semicell that
closely resembles the enucleate cell. Exposure of the cell for 6 min to
light from a Philips TUV 6 W tube from a distance of 4 cm (the water
layer covering the cell being 2 mm thick) leads to this result.
When the cell is irradiated in the metaphase stage, the mitosis is
retarded and may last more than 1 h. However, the cell always continues to develop after the exposure to the standard lethal dose. The
irradiation usually has no effect on septum formation. The development
of the new semicell as well as its final form after irradiation closely
resemble the development and form of an enucleate cell. Both contain
'pectic lenses' (Waris, 1951), the flow of the cytoplasm is similar, a
permanent membrane is not formed, and the chloroplast expands very
little into the new semicell. The lifetime is, however, longer, often 2-3
days, whereas it is always 1 day in the enucleate cell. Three-lobed new
semicells are formed by these treatments from M. thomasiana, whereas
the enucleate semicell is mostly five-lobed. The same end result is
obtained as in the previous treatments if the irradiation takes place in
the small bulge stage. If the new semicells have already formed lobes,
the ultraviolet radiation stops the differentiation, but growth continues
and forms intermediate between enucleate and normal cells are obtained. The sensitivity to ultraviolet irradiation is weaker in late phases
of semicell development and cells irradiated with the standard lethal
dose often survive and divide after a normal or slightly prolonged
mitotic interval. The ability to survive seems to be inversely correlated
with the abnormality of the morphology of the new semicell.
When whole cells are exposed, it is not possible to distinguish the
effects of radiation on the plasm and on the nucleus. Partial irradiation
of the cell has shown that the short-term effect on the plasm of the
developing semicell is weak. If one wing of the semicell is exposed while
the other wing is protected from the direct effect of radiation by tin
foil, the former develops more slowly and its final form may be less
differentiated.
The complex cells are suitable objects for detailed studies of the
effects of ultraviolet irradiation (Kallio, 1963).
If a whole double cell of type I is irradiated with the lethal dose in
the sensitive stage, the result is the same as with normal cells. Four
semicells resembling enucleate semicells are formed and all three cells
that are formed die (Fig. 8 A).
If only one half of the cell and its nucleus are irradiated and the other
is protected, the development at the protected end is normal. Two
different semicells are formed at the exposed end (Fig. 8 C). The single
cell formed is typically enucleate with a three-lobed new semicell. Its
sister semicell in the double cell is haploid or, usually, somewhat hypoC2
A.M.4