Each
member of an androecium is known as stamen. Hence the collection or aggregation
of stamen constitutes the androecium. The number of stamens of a flower may
range from one to numerous, i.e., definite to indefinite. The arrangement of
stamens on the receptacle may be spiral, whorled or fasciculate/clustered. The
spiral arrangements of stamens are found in the members of primitive families
like Magnoliaceae, Degeneraceae, Nymphiaceae, Annonaceae, etc.
The
stamen, like other floral members is a reproductive modified leaf known as
microsporophyll i.e., microsporophyll i.e., microsporangia-bearing leaf. It is
typically a slender organ, but basically stamen is a modified leaf and is
similar to the leaf in ontogeny, morphological and anatomical characters. In
most of the families, the stamen is highly specialized organ with varieties of
forms and structures according to the methods of pollination. A stamen
typically consists of two parts, viz., the proximal sterile part called ‘filament’ which constitutes the main
body of the sporophyll and the distal fertile part called the ‘anther’ or microsporangia which bears
pollen or microspores. The median strip of the sterile tissue of the sporophyll
between microsporsngia or pollen sacs or anther lobes is known as ‘connective’.
STRUCTURE OF
PRIMITIVE STEMENS (POSSIBLE TRENDS ON THE DEVELOPMENT/SPECIALIZATION OR EVOLUTION
OF STAMENS
The
stamen in course of evolution, have undergone various phylogenetic
developments. The structure of stamens regarding the nature of filaments, and
the structure and position of anther lobe or sporangia indicate the
primitiveness or advanced stage of taxa to some extent.
The
primitive stamen has little distinction between the filament and the anther.
The most primitive stamen in living angiosperms belongs to the members of
Magnoliales-Ranales. In these members the stamens are broad, more or less leaf
like, without or with little distinction between fertile or sterile parts, and
also in the position of the anther lobes. In primitive members like Michelia, Magnolia, Degeneria, Himantandre,
Nymphaea, etc., the stamens are petaloid broad with leaf-like filaments
known as laminar stamens. In these taxa the anther lobes or sporangia are borne
near the centre of the sporophyll. It is on the abaxial side in Annonaceae,
Himatandraceae and Degeneriaceae, and adaxial side in Magnolia and Austrobalaileya
and other primitive taxa.
Stamen
with semi-laminar form occurs in the primitive and failry primitive families
like Ceratophyllaceae, Eupomataceae, Illiciaceae, Nymphaceae, etc.
In
the primitive angiospermic families like Magnoliaceae, Degeneriaceae,
Himantandraceae, Nymphaceae, etc., the connective constitutes a major part of
the anther and hardly separable from the filament and the sporangia and anther
is a minute structures. In
In
advanced families, the connective is a slender median part, sometimes
comprising only a point of a thread like structure between the anther lobes. In
course of specialization and as a result of progressive reduction the
connective which are broad and massive in some of the members of
Magnoliales-Ranales, become almost absent in the advanced families like
Poaceae. The wider and shorter types of stamen and are more primitive.
The
distal appendage of the connective, a typical feature of the anther is found in
more or less primitive gamilies like Magnoliaceae, Nymphaceae, etc.
The
characteristic sporangium number in which anther is four (tetrasporangiate).
More than four sporangia are found in Rhizophoraceae, Loranthaceae,
Gentianaceae, etc. Two sporangia members is characteristics of the families
like Onagraceae, Lamiaceae, Lemnaceae, etc. The multisporangiate condition is
due to the formation of sterile partition walls and less than four sporangia in
the anther represent a reduction from the basic four. The monosporangiate
monothecal (single lobed) anther are considered more advanced stage and meet in
the families like Malvaceae and a few members of Amaranthaceae, etc.
The
position and nature of sporangia, whether ventral or dorsal, protuberant or
sunken also indicate the primitiveness of the taxa. The ventral position of
sporangium is primitive over a dorsal one.
The
phyllotaxy, i.e., mode of arrangement of stamens on the receptacle may be
spiral, whorled and fasciculate. The spiral arrangement of the stamens is
primitive, which in course of evolution gives rise to whorled and fasciculate
types of arrangement. Spiral arrangement of stamens is found in many primitive
families like Nymphaceae, Annonaceae, some members of Magnoliaceae, etc.
THELOME THEORY
Telome
theory describes the makeup and nature of angiosperm leaf (megasporophyll) in
relation to sporangia bearing organs, i.e., sporophylls (stamens and carpels of
the angiosperms). According to this theory all the vascular plants evolved from
a very simple, leafless and a dichotomously branched ancestral type like Rhynia. In early pteridophytes (fossil
forms) like Horneophyton, Rhynia, Psylophyton,
etc., the vegetative plant body was entirely leafless and was made up of axis
with similar dichotomous branching. The terminal portion of such dichotomous
branch is known as ‘telome’.
The
telome is divided into sterile or vegetative telome (also called as phyllode) and fertile telome, bearing sporangium at the tip. The dicothomously
branched meet each other at the point of forking. As evolution preceded, the
change of dichotomous branching to sympoidal branching, which result in the fusion
of two or more telomes called ‘syntelomes’,
e.g., sporangiophore of Equisetum.
Telome
theory of Zimmermann (1930) put forward to explain the process of elaboration
of the primitive sporophyte into the modern sporophyte, has attracted great
attention and has a large number of supporters.
According
to Zimmermann, the primitive vascular cryptogams originated from the green
algae. The unicellular green algae divided in all planes to form a
parenchymatous thallus. Later meristematic tissues develop and erect radially
constructed branches came into existence. It was followed by the appearance of
distinct alternating generations. The sporophyte branched dichotomously and
possessed a central conducting strand. Such algal ancestors according to
Zimmermann led to the evolution of early vascular cryptogams of the upper
Silurian and Devonian periods like Horneophyton,
Rhynia, Psilophyton, etc. Such sporophytes are dichotomously branched and
some branches are terminated by sporangia. They posses no leaves and roots. The
function of roots was performed by rhizoids. These sporophytes had subterranean
portion called rhizome.
According
to the theory of Zimmermann (1930, 1953), the vegetative shoot of megaphyllous
plants or Filicophyta or present day angiosperms arose as a result of some
changes and processes during the course of evolution. The detailed process is
discussed below –
1.
Overtopping:- The equally dichotomizing axes developed unequal
dichotomy. This resulted in the formation of short and long branches. The short
branches was overtopped by long branches, and appeared as lateral shoots which
metamorphosed into leaves.
2.
Planation:- The equal dichotomies were originally in more than one
plane. They were arranged in planes successively at right angles. During the
process of planation, the dichotomies became arranged in a single plane. It is
an important process that led to the evolution of the leaf.
3.
Webbing or Syngenesis:- During this process, the adjacent telomes or
mesomes were connected with each other by the development of a parenchymatous
tissue between them. This is also called parenchymatous webbing. Webbing was
considered to be of two types by Zimmermann, viz., foliar webbing and axial
webbing.
4.
Reduction:- It is supposed to have brought about the evolution of
simple and unbranched microphyllous leaves of the Lycopods e.g., Lycopodium,
Selaginella, Isoetes, etc. In this, the reduction of syntelome to the single
needle like lateral appendages taken place.
5.
Recurvation:- During this process, the fertile telomes were supposed to
become reflexed. As a result the sporangium assumes an inverted position
(incurvation). Two processes can be noticed recurvation
and incurvation. In recurvation, the sporangia bent
downwards as in Sphenophyta. In incurvation,
the shifting of sporangia to ventral surfaces of foliar appendages, took place
as in Ferns.
MERITS AND
DEMERITS OF TELOME THEORY
Merits:
1.
It is simple concept and explains most of the morphological problems about
different organs of plant. According to Bierhorst (1971), this theory is too
simple and too easily applicable, but unfortunately its excessive use has
greatly diminished its value.
Demerits:
1.
Telome has been considered as readymade unit. This difficulty was realized by
Zimmermann (1949, 1952), and subsequently he recognized several other
elementary processes. However, these elementary processes do not satisfy the
plant morphologists like Puri (1956).
2.
Many other fossil plants of much greater complexity, other than Rhynia have
been discovered in the beds of same age or even earlier, e.g., Zosterophyllum,
Asterophylon. These fossil members have lateral sporangia instead of terminal
sporangia.
3.
The theory has got little attention by angiosperm morphologists. Its
application to stamens (Puri, 1947, 1951, 1955), venation pattern of leaves
(Foster, 1950), morphological nature of angiosperm leave and sporophylls and
carpels (Eames, 1961), have been criticized from time to time.
4.
The vascular structure of stamens doesnot supports the telome theory of stamen.
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