TRENDS IN SYSTEMATIC BOTANY
95
by similarity in chromosome morphology, that the relatively primitive
species are marked by the similarity in both the size and morphology of
their chromosomes, and that these primitive types tend to have median
or near-median constrictions, but that the more advanced species have
chromosomes increasingly unequal in size and more frequently with
subterminal constrictions (Babcock et al. 1937). Levan (1935) found a
similar situation in Allium.
Speciation in the Vascular Cryptogams.
Turning to an ancient group
of plants, the cytological story in the pteridophytes is most illuminating.
A monumental work on this subject has been produced by Mantón
(1950). In contrast to the low chromosome numbers that Babcock
found in Crepis, which is certainly a relatively modern genus, Mantón
shows that the prevailing generic chromosome numbers among the
leptosporangiate ferns of temperate latitudes fall, with few exceptions,
between 29 and 42, and that the 47 species of these ferns in the British
Isles exhibit a high degree of polyploidy. Hybridization within the latter
is also shown to be one of the causes of the polyploidy, and opportunities
for it have increased during the climatic changes attendant upon the
series of glacial and interglacial periods. The question then arose as to
whether or not the ferns of Britain could be a representative sample of
the ferns of the world. Mantón (1953) provided an answer to this by
making a cytological study of about 160 items from Ceylon, representing
over half the known fern flora of that island. She found that the same
base numbers prevail there as in Britain, and that "each and all of the
various detectable expressions of cytological evolution which were encountered in Europe have been found again in Ceylon on a larger scale."
An interesting conclusion drawn by Mantón from this comparison of the
two floras was that evolution must be proceeding faster in the tropics than
in temperate latitudes.
The very high polyploidy in the Psilotales seems to Mantón to suggest
that these are the end-products of very ancient polyploid series. The
genus Ophioglossum, one of the most primitive genera of true ferns, exhibits perhaps the highest known chromosome numbers in plants, η =
ca. 256 in the northern O. vulgatum, η = ca. 370 in the tropical O.
pendulum, and η — ca. 510 in the tropical O. petiolatum.
She believes
that the grade of polyploidy is a more significant index to past evolutionary activity in a flora than is the mere percentage of polyploid species.
A careful study of the Appalachian Aspleniums discloses that these
ferns represent a polyploid complex, with 11 recognized taxa, such as is
found in the higher plants. Three are the original diploid species, the
other eight have arisen through hybridization and all are believed to be
of amphiploid origin (Wagner, 1954).
95
by similarity in chromosome morphology, that the relatively primitive
species are marked by the similarity in both the size and morphology of
their chromosomes, and that these primitive types tend to have median
or near-median constrictions, but that the more advanced species have
chromosomes increasingly unequal in size and more frequently with
subterminal constrictions (Babcock et al. 1937). Levan (1935) found a
similar situation in Allium.
Speciation in the Vascular Cryptogams.
Turning to an ancient group
of plants, the cytological story in the pteridophytes is most illuminating.
A monumental work on this subject has been produced by Mantón
(1950). In contrast to the low chromosome numbers that Babcock
found in Crepis, which is certainly a relatively modern genus, Mantón
shows that the prevailing generic chromosome numbers among the
leptosporangiate ferns of temperate latitudes fall, with few exceptions,
between 29 and 42, and that the 47 species of these ferns in the British
Isles exhibit a high degree of polyploidy. Hybridization within the latter
is also shown to be one of the causes of the polyploidy, and opportunities
for it have increased during the climatic changes attendant upon the
series of glacial and interglacial periods. The question then arose as to
whether or not the ferns of Britain could be a representative sample of
the ferns of the world. Mantón (1953) provided an answer to this by
making a cytological study of about 160 items from Ceylon, representing
over half the known fern flora of that island. She found that the same
base numbers prevail there as in Britain, and that "each and all of the
various detectable expressions of cytological evolution which were encountered in Europe have been found again in Ceylon on a larger scale."
An interesting conclusion drawn by Mantón from this comparison of the
two floras was that evolution must be proceeding faster in the tropics than
in temperate latitudes.
The very high polyploidy in the Psilotales seems to Mantón to suggest
that these are the end-products of very ancient polyploid series. The
genus Ophioglossum, one of the most primitive genera of true ferns, exhibits perhaps the highest known chromosome numbers in plants, η =
ca. 256 in the northern O. vulgatum, η = ca. 370 in the tropical O.
pendulum, and η — ca. 510 in the tropical O. petiolatum.
She believes
that the grade of polyploidy is a more significant index to past evolutionary activity in a flora than is the mere percentage of polyploid species.
A careful study of the Appalachian Aspleniums discloses that these
ferns represent a polyploid complex, with 11 recognized taxa, such as is
found in the higher plants. Three are the original diploid species, the
other eight have arisen through hybridization and all are believed to be
of amphiploid origin (Wagner, 1954).