Armen
Takhtajan, a Russian Botanist presented a system of classification, which was
first published in a very preliminary form in 1942. The actual system however
appeared in 1959 in German in his book ‘Die Evolution der Abgiospermen’. After
1966, his system has gone through a series of modifications and elaborations,
but the nucleus of the system and the basic features remained essentially the
same. A new version of Takhtajan’s system was published in 1980 with the
heading ‘Outline of the classification of flowering plants (Magnoliophyta)’ in
the ‘Botanical Review’.
Some
of the criteria used by Takhtajan in determining the relative degree of
advancement of flowering plants are –
1.
Woody plants are primitive compared to herbaceous plants.
2.
Deciduous woody plants are evolved from evergreen plants.
3.
Parallel venation is most advanced.
4.
Most primitive leaf arrangement is alternate.
5.
Stomata with subsidiary cells are primitive while those lacking subsidiary
cells are advanced.
6.
Unilacunar nodes are derived from trilacunar or pentalacunar nodes.
7.
Xylem fibres evolved from trachieds to labriform fibres, through fibre
trachieds.
8.
Cymose inflorescence is primitive while racemose is derived.
9.
Flowers with an indefinite or a variable number of their floral parts are
primitive.
10.
Pollen grains with their exine lacking any external sculpturing are advanced.
11.
Apocarpous gynoecium is the characteristic of primitive taxa.
12.
Unitegmic ovules developed from bitegmic ovules.
13.
Basic type of ovule is anatropous, all others are derived ones.
14.
Basic and most primitive type of female gametophyte is 8-nucleate (Polygonum)
type.
15.
Primitive condition is porogamy, and the derived conditions are misogamy and
chalazogamy.
16.
The most primitive and basic type of fruit is a many seeded follicle which
develops from mylticarpellary apocarpous gynoecium.
Takhtajan
(1980) divided division Magnoliophyta (Angiospermae) into two classes –
Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons). He further
divided Magnoliopsida into 7 sub-classes, 20 super-orders and 71 orders. He
divided Liliopsida into 3 sub-classes, 8 super-orders and 21 orders. Takhtajan
(1980) recognizes a total of 92 orders and 410 families among angiosperms.
However,
according to latest Takhtajan’s system of classification (1997), the two
classes – Magnoliopsida (Dicotyledons) and Liliopsida (Monocotyledons) have
been divided into 17 sub-classes; 11 under Magnoliopsida and 6 under
Liliopsida. He further divided the 11 sub-classes of Magnoliopsida into 51
super-orders and 168 orders. Liliopsida has been further divided into 16
super-orders and 56 orders.
A
symbiotic outline of the latest Takhtajan’s system of classification (1997) is
given in the following table –
MERITS
AND DEMERITS OF TAKHTAJAN’S SYSTEM OF CLASSIFICATION
MERITS
(a) Dicots (Magnoliopsida) are
discussed prior to monocots (Liliopsida).
(b)
Dicots begin with Magnoliales which are universally considered as the moxt
primitive living angiosperms.
(c) In this system, the families
are small homogenous units made up of closely related genera.
(d) Division of Dicots into two
traditional groups of Engler and Prantl, i.e., Archichlamydae and
Metachlamydae, has been abolished in this system.
(e)
The order Alismataleswhich are considered to be the most primitive living
monocots these days are the starting point of Monocots in this system.
DEMERITS
(a) One strong demerit of
Takhtajan system of classification is the extremely narrowly defined taxa
resulting in the unwanted splitting of the related groups.
(b) Derivation of monocotyledons from
the stocks ancestral to the Nympheales.
(c) It is very difficult to
identify the plants with the help of this system of classification.
(d) It is liable to change with
the increase of knowledge.
(e) Some of the
phylogenetic principles of Takhtajan are not accepted by modern taxonomists.
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