60
H. WARIS AND P. KALLIO
fugal force is parallel to the main axis of the cell. The effect of the
centrifugal force is seen as a displacement of the chloroplast. The
chloroplast protrudes into and through the isthmus and the length of
the bulge determines the further development of the cell. After the
treatment the bulge begins to retract. If the bulge is so short that it as
well as the nucleus following it have time to retract before the septum
forms, normal cell division occurs. If, on the other hand, the septum
forms between the chloroplast bulge and the nucleus, a binucleate and
an enucleate cell are formed. This requires usually a bulge of the size
seen in Fig. 6 A.
When septum formation takes place while the chloroplast is still in
the isthmus, development only rarely leads to normal cytokinesis. A
middle section is usually formed in the same way as in the cold shock
treatments. In this type of double cell, however, the two nuclei are
situated side by side in the same isthmus. This type of binucleate cell is
called type II. The two main types of double cells as well as their
development are shown in Figs. 4 and 5.
The subsequent development of a type I cell proceeds as follows. The
cell is normally capable of dividing. Both nuclei divide simultaneously
and at first the division rhythm is the same as that of the normal haploid cell. After several divisions, however, the division is retarded and
symptoms of 'senescence' are seen, whereas the normal haploid cell
(clone) is potentially immortal. The binucleate cells may represent the
primary level for studying the problem of senescence. The phenomenon
is more clearly seen in the double cells of type II and in the heterocaryotic double cells described below. The two single cells formed in
every division are quite normal haploid cells. The further development
of the double cell of type II may follow two routes depending mostly
on the genotype of the species. However, the environmental conditions
also have some effect on this development. In the next mitosis the two
nuclei fuse and a diploid cell is formed in the nucleate isthmus. In M.
torreyi cytokinesis occurs also in the enucleate isthmus and four new
semicells are formed. Usually the new semicell of the single diploid cell
is larger than the parent semicell and shows (weak) diploid characteristics. Its sister semicell, attached to the nucleate isthmus of the new
double cell, has mostly typical haploid features. A typical enucleate
cell, with all the characteristics of enucleate cells produced by other
methods, is formed at the other isthmus. The new semicell at the
enucleate end of the double cell is hypohaploid in shape.
In M. thomasiana as well as in many other Micrasterias species only the
nucleate end of the double cell is capable of dividing after mitosis. A,
usually large, new semicell with clear diploid features is formed by the
new double cell, but the new semicell of the single cell is not so promi-
H. WARIS AND P. KALLIO
fugal force is parallel to the main axis of the cell. The effect of the
centrifugal force is seen as a displacement of the chloroplast. The
chloroplast protrudes into and through the isthmus and the length of
the bulge determines the further development of the cell. After the
treatment the bulge begins to retract. If the bulge is so short that it as
well as the nucleus following it have time to retract before the septum
forms, normal cell division occurs. If, on the other hand, the septum
forms between the chloroplast bulge and the nucleus, a binucleate and
an enucleate cell are formed. This requires usually a bulge of the size
seen in Fig. 6 A.
When septum formation takes place while the chloroplast is still in
the isthmus, development only rarely leads to normal cytokinesis. A
middle section is usually formed in the same way as in the cold shock
treatments. In this type of double cell, however, the two nuclei are
situated side by side in the same isthmus. This type of binucleate cell is
called type II. The two main types of double cells as well as their
development are shown in Figs. 4 and 5.
The subsequent development of a type I cell proceeds as follows. The
cell is normally capable of dividing. Both nuclei divide simultaneously
and at first the division rhythm is the same as that of the normal haploid cell. After several divisions, however, the division is retarded and
symptoms of 'senescence' are seen, whereas the normal haploid cell
(clone) is potentially immortal. The binucleate cells may represent the
primary level for studying the problem of senescence. The phenomenon
is more clearly seen in the double cells of type II and in the heterocaryotic double cells described below. The two single cells formed in
every division are quite normal haploid cells. The further development
of the double cell of type II may follow two routes depending mostly
on the genotype of the species. However, the environmental conditions
also have some effect on this development. In the next mitosis the two
nuclei fuse and a diploid cell is formed in the nucleate isthmus. In M.
torreyi cytokinesis occurs also in the enucleate isthmus and four new
semicells are formed. Usually the new semicell of the single diploid cell
is larger than the parent semicell and shows (weak) diploid characteristics. Its sister semicell, attached to the nucleate isthmus of the new
double cell, has mostly typical haploid features. A typical enucleate
cell, with all the characteristics of enucleate cells produced by other
methods, is formed at the other isthmus. The new semicell at the
enucleate end of the double cell is hypohaploid in shape.
In M. thomasiana as well as in many other Micrasterias species only the
nucleate end of the double cell is capable of dividing after mitosis. A,
usually large, new semicell with clear diploid features is formed by the
new double cell, but the new semicell of the single cell is not so promi-