MORPHOGENESIS IN
MICRASTERIAS
47
chromosome number depends on fragmentation and/or fusion (King,
1953), for experimentally induced polyploidy and aneuploidy are
always accompanied by characteristic morphological changes (cf.
Section VIII). Taking into consideration that in the clones with varying
chromosome numbers mitoses have probably proceeded normally, the
chromosomes (except for the nucleolar organizers) seem likely to possess
polycentric centromeres (King, 1953), as Godward (1954) has shown for
some species of Spirogyra. From this the conclusion can be drawn that
the high chromosome number in various Micrasterias species is not
direct evidence of polyploidy.
The nucleolar conditions in desmids have been described by KopetzkyRechtperg (1932), Waris (1950a), King (1959) and Drawert and Mix
(1960). I t appears that there are differences in the number, size and
form of the nucleolar bodies which to a certain degree are characteristic
of the various species, although the nucleolar aspect is subject to environmental and developmental variation. Thus, under normal cultural
conditions, certain species of Micrasterias tend to possess a single
spherical nucleolus (M. radiata according to the present authors),
whereas others present several or many nucleolar particles (a 'multiple
nucleolus', e.g. M. thomasiana, M. rotata and M. angulosa). Drawert and
Mix (1960) doubt whether the value of a species characteristic can be
attributed to the nucleolar status, but in the cultures grown by the
present authors, although it is affected by the conditions obtaining, it is
certainly characteristic of the species. In the chromosome complements
of some of the Micrasterias species investigated two nucleolar organizers
have been seen, which are distinguished from the other chromosomes by
their greater length and by the fact that they protrude from the metaphase plate while the others lie exactly parallel to it (Waris, 1950a;
King, 1959). The different orientation is ascribed to a region devoid of
centromeres while in the other chromosomes the centromere is probably
polycentric (King, 1959). In the mitotic cycle the nucleolus undergoes
changes which make it possible to recognize prophase and to predict the
occurrence of metaphase in the living cell. This is of significance when
the cells are to be treated experimentally at definite nuclear stages.
The desmids multiply at the haploid stage by division into two
daughter cells, each of which receives a parent semi cell and forms a new
one which, under normal conditions, becomes a mirror image of the
former (Fig. 1). The degree of bipolar symmetry attained depends on the
environmental conditions, a change in which may cause considerable
differences in the shape of the two semicells, for the parent semicell is
transmitted as such to the daughter cell while the new semicell may
assume a deviant shape. For this reason the bipolar symmetry is frequently disturbed ; either the parent semicell is normal and the new one
MICRASTERIAS
47
chromosome number depends on fragmentation and/or fusion (King,
1953), for experimentally induced polyploidy and aneuploidy are
always accompanied by characteristic morphological changes (cf.
Section VIII). Taking into consideration that in the clones with varying
chromosome numbers mitoses have probably proceeded normally, the
chromosomes (except for the nucleolar organizers) seem likely to possess
polycentric centromeres (King, 1953), as Godward (1954) has shown for
some species of Spirogyra. From this the conclusion can be drawn that
the high chromosome number in various Micrasterias species is not
direct evidence of polyploidy.
The nucleolar conditions in desmids have been described by KopetzkyRechtperg (1932), Waris (1950a), King (1959) and Drawert and Mix
(1960). I t appears that there are differences in the number, size and
form of the nucleolar bodies which to a certain degree are characteristic
of the various species, although the nucleolar aspect is subject to environmental and developmental variation. Thus, under normal cultural
conditions, certain species of Micrasterias tend to possess a single
spherical nucleolus (M. radiata according to the present authors),
whereas others present several or many nucleolar particles (a 'multiple
nucleolus', e.g. M. thomasiana, M. rotata and M. angulosa). Drawert and
Mix (1960) doubt whether the value of a species characteristic can be
attributed to the nucleolar status, but in the cultures grown by the
present authors, although it is affected by the conditions obtaining, it is
certainly characteristic of the species. In the chromosome complements
of some of the Micrasterias species investigated two nucleolar organizers
have been seen, which are distinguished from the other chromosomes by
their greater length and by the fact that they protrude from the metaphase plate while the others lie exactly parallel to it (Waris, 1950a;
King, 1959). The different orientation is ascribed to a region devoid of
centromeres while in the other chromosomes the centromere is probably
polycentric (King, 1959). In the mitotic cycle the nucleolus undergoes
changes which make it possible to recognize prophase and to predict the
occurrence of metaphase in the living cell. This is of significance when
the cells are to be treated experimentally at definite nuclear stages.
The desmids multiply at the haploid stage by division into two
daughter cells, each of which receives a parent semi cell and forms a new
one which, under normal conditions, becomes a mirror image of the
former (Fig. 1). The degree of bipolar symmetry attained depends on the
environmental conditions, a change in which may cause considerable
differences in the shape of the two semicells, for the parent semicell is
transmitted as such to the daughter cell while the new semicell may
assume a deviant shape. For this reason the bipolar symmetry is frequently disturbed ; either the parent semicell is normal and the new one