58
H. W A R I S A N D P. K A L L I O
When cells in a late mitotic stage and particularly in the early stage
of septum formation are transferred from the normal culture temperature to 0° C, cytokinesis ceases. When, after a period of about 1 h, the cells
are warmed to room temperature, a high proportion develop abnormally.
The septum, which in normal cases is cleaved when it is complete,
is opened in these cases when it is still ring-shaped and a middle
section instead of two new semicells is produced (Kallio, 1949, 1951).
The primary morphological units of the semicells are seen in this middle
section although often in abnormal position relative to one another.
There are two isthmuses in this type of complex cell and each contains
one nucleus. These complex cells are called binucleate cells of type I
(TI)(Fig.5).
Cells of this type have been produced by cold treatment of the
following species: M. thomasiana, M. thomasiana var. notata, M.
rotata, M. rotata var. evoluta, M. angulosa, M. fimbriata, M. cruxmelitensis, M. truncata, M. radiata, M. sol, M. americana (Pleurotaenium
trabecula, P. ehrenbergii, P . miniatum, P. kayei, Euastrum didelta, E.
crassum). The method does not work with Micrasterias torreyi, which
dies on exposure to cold.
In heat shock treatments the cells are transferred from the culture
temperature to temperatures of 36-47° C, usually for 10-30 min.
Septum formation is often disturbed by this treatment as by cold
treatment. Heat shock is, however, more detrimental to the cells and
middle section formation (i.e. growth) is often prevented, the end result
being a double cell with only a lengthened isthmus. In some cases the
heat causes the nucleus to move away from its normal site in the isthmus,
and an enucleate and a binucleate cell are subsequently formed.
Many species of Micrasterias are 'short-day plants'. Photoperiodicity
is necessary for the optimal development of the clone. These conditions
are not compensated by exposure to any form of continuous illumination.
Continuous illumination exerts, however, very different effects on
different species. Particularly sensitive species are M. fimbriata var.
caudata, M. torreyi and M. rotata var. evoluta.
The characteristic results of exposure to uninterrupted illumination
in M. rotata var. evoluta are an increased average cell size, and disturbed
synchronization of the division of the chloroplast and other components
of the cell so that cells with a deficient chloroplast are formed. The
chloroplast is yellowish in colour. The nucleus is often displaced and the
next division leads to enucleate and binucleate cells. Often also the
chloroplast is displaced and prevents septum formation, whereupon
double cells are formed.
The cell is transferred in prophase-metaphase to a glass tube drawn
to a narrow flat end and centrifuged so that the direction of the centri-
H. W A R I S A N D P. K A L L I O
When cells in a late mitotic stage and particularly in the early stage
of septum formation are transferred from the normal culture temperature to 0° C, cytokinesis ceases. When, after a period of about 1 h, the cells
are warmed to room temperature, a high proportion develop abnormally.
The septum, which in normal cases is cleaved when it is complete,
is opened in these cases when it is still ring-shaped and a middle
section instead of two new semicells is produced (Kallio, 1949, 1951).
The primary morphological units of the semicells are seen in this middle
section although often in abnormal position relative to one another.
There are two isthmuses in this type of complex cell and each contains
one nucleus. These complex cells are called binucleate cells of type I
(TI)(Fig.5).
Cells of this type have been produced by cold treatment of the
following species: M. thomasiana, M. thomasiana var. notata, M.
rotata, M. rotata var. evoluta, M. angulosa, M. fimbriata, M. cruxmelitensis, M. truncata, M. radiata, M. sol, M. americana (Pleurotaenium
trabecula, P. ehrenbergii, P . miniatum, P. kayei, Euastrum didelta, E.
crassum). The method does not work with Micrasterias torreyi, which
dies on exposure to cold.
In heat shock treatments the cells are transferred from the culture
temperature to temperatures of 36-47° C, usually for 10-30 min.
Septum formation is often disturbed by this treatment as by cold
treatment. Heat shock is, however, more detrimental to the cells and
middle section formation (i.e. growth) is often prevented, the end result
being a double cell with only a lengthened isthmus. In some cases the
heat causes the nucleus to move away from its normal site in the isthmus,
and an enucleate and a binucleate cell are subsequently formed.
Many species of Micrasterias are 'short-day plants'. Photoperiodicity
is necessary for the optimal development of the clone. These conditions
are not compensated by exposure to any form of continuous illumination.
Continuous illumination exerts, however, very different effects on
different species. Particularly sensitive species are M. fimbriata var.
caudata, M. torreyi and M. rotata var. evoluta.
The characteristic results of exposure to uninterrupted illumination
in M. rotata var. evoluta are an increased average cell size, and disturbed
synchronization of the division of the chloroplast and other components
of the cell so that cells with a deficient chloroplast are formed. The
chloroplast is yellowish in colour. The nucleus is often displaced and the
next division leads to enucleate and binucleate cells. Often also the
chloroplast is displaced and prevents septum formation, whereupon
double cells are formed.
The cell is transferred in prophase-metaphase to a glass tube drawn
to a narrow flat end and centrifuged so that the direction of the centri-