TRENDS IN SYSTEMATIC BOTANY
69
25, 30, 35, and 40); (b) a six series (12, 18, 36, 54, 72, and 90); (c) a
seven series (7, 14, 21, 28, 35, 42, 56, and 84); (d) an eleven series (11,
22, 33, and 66(?)); and (e) a thirteen series (13, 26, 39, and 65).
In the great majority of plants investigated, polyploidy is attained by
an addition or multiplication of the chromosome sets in the nucleus. In
the genus Luzula, however, in addition to the ordinary type of polyploidy, a special type is found in which the chromosome size regularly
decreases as the chromosome number increases through successive levels
of polyploidy. The higher numbers in the polyploid series of this type
are attained without any apparent increase in the amount of chromatin
material. It is believed that this is polyploidy through fragmentation
rather than through addition of chromosomes (Nordenskiöld, 1951). It
is a phenomenon undoubtedly related to the very unusual "diffuse centromere" found in some species of Luzula and widely studied cytologically.
No definition yet proposed serves to distinguish autoploids and amphiploids in all cases, for the two categories are not sharply distinct from
each other. A test commonly employed for checking whether a plant is
an autoploid or an amphiploid is to observe the behavior of its chromosomes at meiosis. The question is whether its basic sets of chromosomes
(genomes) are alike or different. If the chromosomes pair normally,
as in diploid species, presumably the chromosomes from one parent are
pairing inter se, as are those derived from the other parent, and the
plant is an amphiploid; but if the chromosomes tend to come together
in fours (tetravalents being formed), the supposition is that the four
genomes are so closely related that they could not have come from
distinct species, and the plant is an autoploid. Exceptions are found
to the application of this test, as emphasized by Nordenskiöld (1945).
Differences in the chromosomes may be either in genie content or in
structural arrangement even within a species, and these differences may
be independent of each other. It is possible for a polyploid to contain
two chromosome sets which differ appreciably in genie content, but
which are structurally identical, so that the plant is fertile (obviously an
autoploid as the term is generally used), but it could be morphologically
unlike its parents and segregate new types. Furthermore, if structural
changes of the chromosomes involve interchanges and inversions of
rather large segments, meiotic irregularities and even sterility can result.
But many of the structural differences between chromosomes may involve segments so small that they permit nearly regular meiosis to
continue. Plants having sets of chromosomes containing such cryptic
structural hybridity behave as autoploids and are so classified, although
in the strict sense they should be classed as amphiploids.
69
25, 30, 35, and 40); (b) a six series (12, 18, 36, 54, 72, and 90); (c) a
seven series (7, 14, 21, 28, 35, 42, 56, and 84); (d) an eleven series (11,
22, 33, and 66(?)); and (e) a thirteen series (13, 26, 39, and 65).
In the great majority of plants investigated, polyploidy is attained by
an addition or multiplication of the chromosome sets in the nucleus. In
the genus Luzula, however, in addition to the ordinary type of polyploidy, a special type is found in which the chromosome size regularly
decreases as the chromosome number increases through successive levels
of polyploidy. The higher numbers in the polyploid series of this type
are attained without any apparent increase in the amount of chromatin
material. It is believed that this is polyploidy through fragmentation
rather than through addition of chromosomes (Nordenskiöld, 1951). It
is a phenomenon undoubtedly related to the very unusual "diffuse centromere" found in some species of Luzula and widely studied cytologically.
No definition yet proposed serves to distinguish autoploids and amphiploids in all cases, for the two categories are not sharply distinct from
each other. A test commonly employed for checking whether a plant is
an autoploid or an amphiploid is to observe the behavior of its chromosomes at meiosis. The question is whether its basic sets of chromosomes
(genomes) are alike or different. If the chromosomes pair normally,
as in diploid species, presumably the chromosomes from one parent are
pairing inter se, as are those derived from the other parent, and the
plant is an amphiploid; but if the chromosomes tend to come together
in fours (tetravalents being formed), the supposition is that the four
genomes are so closely related that they could not have come from
distinct species, and the plant is an autoploid. Exceptions are found
to the application of this test, as emphasized by Nordenskiöld (1945).
Differences in the chromosomes may be either in genie content or in
structural arrangement even within a species, and these differences may
be independent of each other. It is possible for a polyploid to contain
two chromosome sets which differ appreciably in genie content, but
which are structurally identical, so that the plant is fertile (obviously an
autoploid as the term is generally used), but it could be morphologically
unlike its parents and segregate new types. Furthermore, if structural
changes of the chromosomes involve interchanges and inversions of
rather large segments, meiotic irregularities and even sterility can result.
But many of the structural differences between chromosomes may involve segments so small that they permit nearly regular meiosis to
continue. Plants having sets of chromosomes containing such cryptic
structural hybridity behave as autoploids and are so classified, although
in the strict sense they should be classed as amphiploids.