72
H . W A R I S A N D P . K A L L I O
haploid in form. The development of the latter is hardly solely dependent on the effect of the unirradiated nucleus. Two fully normal
haploid semicells are never formed under the control of a single haploid
nucleus (cf. Section VII). The early development of a double cell after
irradiation of one end closely resembles the development of a double
cell of type I I . After some time, however, the development closely
resembles that of a unirradiated binucleate cell of type I.
When both ends of a binucleate type I cell are exposed successively
to a lethal dose of irradiation in an early stage of cell division, the
following result is typical (Fig. 9). If the interval between the two exposures is less than 1 h, all three cells formed die. If the interval is
longer, both single cells resemble the typical ultraviolet-irradiated cell
in having enucleate features and usually die on the 2nd day. The new
double cell has two new semicells which are hypohaploid in shape. This
cell survives and divides normally after a normal or somewhat longer
period. This can be explained only by assuming that recovery from the
standard lethal dose of ultraviolet irradiation requires about 1 h.
The double cell of type I I is very appropriate for the study of the
effect of radiation on the plasm. When the enucleate end is irradiated
by a lethal dose, only a weak effect is seen. The lobes in the enucleate
cell remain somewhat shorter on average than in unirradiated enucleate
cells. No characteristic effects of radiation are seen in the semicells of
the new double cell. Irradiation of the nucleate end leads to the same
result as irradiation of the whole cell. All four semicells formed are
three-lobed (Fig. 8B.)
This emphasizes the specific effect of irradiation on the nucleus.
Basic differences are noted when the effects of radiation in Acetabularia and Micrasterias are compared. Ultraviolet irradiation has a
marked effect on enucleate Acetabularia cells (Six, 1956, 1958). Because
the enucleate fragments of Acetabularia are capable of regeneration
which is gene-controlled, substances produced by genes must exist in
the plasm of enucleate Acetabularia. These nucleic acids (Richter,
1959; Brächet, 1962) control the cytoplasmic synthesis in the enucleate
cells and are sensitive to ultraviolet irradiation. In Micrasterias only
very small amounts of nucleic acids are stored in the plasm. The nuclear
effect is more immediate. The irradiation has hardly any effect on
enucleate cells although the semicell is sometimes smaller in size. I t is,
however, possible that the typical five-lobed form of the M. thomasiana
enucleate cell reveals some after-effect of the nucleus. This effect may
be eliminated by ultraviolet irradiation, whereupon the three-lobed
form is obtained (Kallio, 1963).
Sublethal doses of ultraviolet irradiation lead to different forms
intermediate between enucleate and normal cells. These results agree with
H . W A R I S A N D P . K A L L I O
haploid in form. The development of the latter is hardly solely dependent on the effect of the unirradiated nucleus. Two fully normal
haploid semicells are never formed under the control of a single haploid
nucleus (cf. Section VII). The early development of a double cell after
irradiation of one end closely resembles the development of a double
cell of type I I . After some time, however, the development closely
resembles that of a unirradiated binucleate cell of type I.
When both ends of a binucleate type I cell are exposed successively
to a lethal dose of irradiation in an early stage of cell division, the
following result is typical (Fig. 9). If the interval between the two exposures is less than 1 h, all three cells formed die. If the interval is
longer, both single cells resemble the typical ultraviolet-irradiated cell
in having enucleate features and usually die on the 2nd day. The new
double cell has two new semicells which are hypohaploid in shape. This
cell survives and divides normally after a normal or somewhat longer
period. This can be explained only by assuming that recovery from the
standard lethal dose of ultraviolet irradiation requires about 1 h.
The double cell of type I I is very appropriate for the study of the
effect of radiation on the plasm. When the enucleate end is irradiated
by a lethal dose, only a weak effect is seen. The lobes in the enucleate
cell remain somewhat shorter on average than in unirradiated enucleate
cells. No characteristic effects of radiation are seen in the semicells of
the new double cell. Irradiation of the nucleate end leads to the same
result as irradiation of the whole cell. All four semicells formed are
three-lobed (Fig. 8B.)
This emphasizes the specific effect of irradiation on the nucleus.
Basic differences are noted when the effects of radiation in Acetabularia and Micrasterias are compared. Ultraviolet irradiation has a
marked effect on enucleate Acetabularia cells (Six, 1956, 1958). Because
the enucleate fragments of Acetabularia are capable of regeneration
which is gene-controlled, substances produced by genes must exist in
the plasm of enucleate Acetabularia. These nucleic acids (Richter,
1959; Brächet, 1962) control the cytoplasmic synthesis in the enucleate
cells and are sensitive to ultraviolet irradiation. In Micrasterias only
very small amounts of nucleic acids are stored in the plasm. The nuclear
effect is more immediate. The irradiation has hardly any effect on
enucleate cells although the semicell is sometimes smaller in size. I t is,
however, possible that the typical five-lobed form of the M. thomasiana
enucleate cell reveals some after-effect of the nucleus. This effect may
be eliminated by ultraviolet irradiation, whereupon the three-lobed
form is obtained (Kallio, 1963).
Sublethal doses of ultraviolet irradiation lead to different forms
intermediate between enucleate and normal cells. These results agree with