46
H. W A R I S A N D P. K A L L I O
II. General Features of the Desmids
In dealing with the genus Micrasterias it seems appropriate to consider the morphological properties of the family Desmidiaceae in general
(cf. Krieger, 1937), for in this way a broader background will be obtained for the understanding of the phenomena elucidated by the
experiments.
Above all, it is the regular and highly differentiated form of the cells
that presents various aspects of morphogenetic interest. With the
exception of a few genetically asymmetrical species represented by two
Micrasterias species (M. incredibilis and M. sudanensis; cf. Grönblad
et al., 1958) and by Scottia mira (Grönblad and Kallio, 1954), the great
majority of desmids, when grown under optimal conditions, possess
symmetrical cells. The cells may show both bipolar and bilateral symmetry, or they may be 'poly symmetric' (Teiling, 1950). In many genera
the cell takes the form of a more or less flattened disk with two lobed
wings, but in certain cases it may show three or more wings protruding
in different directions from a longitudinal median region. In end view
the wings then appear as radii diverging from a central part, hence the
term Radiation' introduced by Teiling (1950). According to the number
of wings the cells are called biradiate, triradiate, quadriradiate and
polyradiate (five or more wings). The terms biradiate, triradiate and
quadriradiate had already been used by Heimans (1942) in the same
sense as Teiling, but earlier workers had not always applied the terms in
exactly the same sense. Telling's terminology can be extended to include
lower degrees of radiation, namely the uniradiate and aradiate forms of
Micrasterias which have arisen in the culture experiments made by the
present authors. Taking into consideration that cells with a lower degree
of radiation (except for the aradiate ones) can be experimentally induced
to form cells with a higher degree of radiation, it is evident that Telling's
terminology hits on a fundamental aspect of the morphogenesis of
desmids.
In many species the cells are fairly large and so is the nucleus, but the
chromosomes are very small. According to recorded counts, the haploid
chromosome number varies within wide limits, from 12 as a minimum
to 592 as a maximum (King, 1953). The last-mentioned number refers
to Netrium digitus and is one of the highest numbers known in the plant
kingdom (the number 30, mentioned by Krieger (1937), seems doubtful).
As to Micrasterias, the chromosome number has been estimated at about
34-37 in M. thomasiana var. notata, 61-64 in M. angulosa, and about 200
in M. rotata (Waris, 1950a). In some desmids, one and the same clone
may produce cells with different chromosome numbers without corresponding differences in shape. This indicates that the variation in the
H. W A R I S A N D P. K A L L I O
II. General Features of the Desmids
In dealing with the genus Micrasterias it seems appropriate to consider the morphological properties of the family Desmidiaceae in general
(cf. Krieger, 1937), for in this way a broader background will be obtained for the understanding of the phenomena elucidated by the
experiments.
Above all, it is the regular and highly differentiated form of the cells
that presents various aspects of morphogenetic interest. With the
exception of a few genetically asymmetrical species represented by two
Micrasterias species (M. incredibilis and M. sudanensis; cf. Grönblad
et al., 1958) and by Scottia mira (Grönblad and Kallio, 1954), the great
majority of desmids, when grown under optimal conditions, possess
symmetrical cells. The cells may show both bipolar and bilateral symmetry, or they may be 'poly symmetric' (Teiling, 1950). In many genera
the cell takes the form of a more or less flattened disk with two lobed
wings, but in certain cases it may show three or more wings protruding
in different directions from a longitudinal median region. In end view
the wings then appear as radii diverging from a central part, hence the
term Radiation' introduced by Teiling (1950). According to the number
of wings the cells are called biradiate, triradiate, quadriradiate and
polyradiate (five or more wings). The terms biradiate, triradiate and
quadriradiate had already been used by Heimans (1942) in the same
sense as Teiling, but earlier workers had not always applied the terms in
exactly the same sense. Telling's terminology can be extended to include
lower degrees of radiation, namely the uniradiate and aradiate forms of
Micrasterias which have arisen in the culture experiments made by the
present authors. Taking into consideration that cells with a lower degree
of radiation (except for the aradiate ones) can be experimentally induced
to form cells with a higher degree of radiation, it is evident that Telling's
terminology hits on a fundamental aspect of the morphogenesis of
desmids.
In many species the cells are fairly large and so is the nucleus, but the
chromosomes are very small. According to recorded counts, the haploid
chromosome number varies within wide limits, from 12 as a minimum
to 592 as a maximum (King, 1953). The last-mentioned number refers
to Netrium digitus and is one of the highest numbers known in the plant
kingdom (the number 30, mentioned by Krieger (1937), seems doubtful).
As to Micrasterias, the chromosome number has been estimated at about
34-37 in M. thomasiana var. notata, 61-64 in M. angulosa, and about 200
in M. rotata (Waris, 1950a). In some desmids, one and the same clone
may produce cells with different chromosome numbers without corresponding differences in shape. This indicates that the variation in the