Orchidaceae as seen today has developed from a primitive
stock and traversed through several lines of evolution at various stages of
geological and geographical changes. Unlike in many plant families orchids do
not have a fossil record. However several features in the living plants
themselves, less complicated as they, are indicate the evolutionary track in
this highly advanced group of plants.
The present day orchids are almost all monandrous i.e. with
a single fertile anther. These have been derived from their counterpart with
two or three fertile anthers, which are considered to be primitive. The
evolutionary trend from the former form to the latter form can be seen in the
vegetative form, floral structure (like the formation and the characters of the
reproductive organs in the pollinia), the seed structure etc.
The orchids started living as terrestrials. In due course of
time probably competetion for food in the land prompted some of them to migrate
to the nearby bare tree trunks and branches for a spacious accommodation. The
epiphytes were thus born at a later stage. These gradually perfected their
modification in the several organs, mostly in the stem and the leaf, for better
living.
Like in the general monocots, in the primitive orchids the
leaves were mainly arranged spirally along the stem. The leaves gradually got
reduced in numbers and they were arranged distichously and became articulate.
Storage tissues were developed in the groups where it was necessary to store
food, by making the leaves thick and leathery. The stem also developed special
tissues for storage of food and became fleshy. In the terrestrials corms,
tuberoids and rootstocks were the result of modification of the stems. In
course of time the primitive multi-noded corms (Acanthephippium, Calanthe) gave
rise to a form with a single node (Didicea, Tainia). In the epiphytic the
sympodial form with uniformly thickened multinoded stem in Dendrobium, Eria and
Thunia etc., gave rise to stems with a single node as in Bulbophyllum,
Pholidota, etc.
The sympodial growth form of the stem with a limited or
defmite annual growth and with formation of new lateral stem every year gave
rise to the monopodial form of stem with unlimited or indefinite growth and a
single growing point. Vanilla is the only terrestrial genus that is an
extensive creeper with monopodial growth form. The usual terminal inflorescence
was transformed to the lateral form. In the advanced monopodial form, lateral
inflorescence became the rule.
In the orchid flower organization of the anther has played a
very crucial role. In the immediate predecesser of Orchidaceae the six anthers
were arranged in two alternating whorls of three each. The primitive orchids
were basically of two types: (i) where the anthers of the outer whorl were
suppressed - the triandrae and (ii) where the anthers in the inner whorl were
suppressed — the diandrae in the lady's slipper orchids. With advancement there
was further reduction of the anther and the monandrous form with only one
fertile anther in the outer whorl came into being. Barring a few genera like
Apostasia, Cypripedium and Paphiopedilum, all other genera found in India are
monandrae.
The pollen grains in orchids, as has been said earlier, are
united to definite masses called pollinia. The unicellular pollen grains have
been variously united to give rise to simple, soft, mealy or granular pollinia,
to sectile pollinia; or waxy to hard pollinia, naked or with various pollinial
appendages, in the most advanced form. These characters are used in
classification of Orchidaceae.
In the primitive form the single-celled pollen grains are
free and contained in the four-locular anther in Apostasia (monads) or held by
a viscous fluid as in Paphiopedilum and Cypripedium. In most cases the pollen
grains are bundled into small units called tetrads. These tetrads in the
primitive form are held by elastic threads and remain in a mealy or granular
form under Spiranthoideae . Sometimes they are organized into granular packets
known as sectile pollinia. Bases of these packets are prolonged into short or
long tails that are pollinial in origin. Sectile pollinia are met in
Spiranthoideae and Orchidoideae.
From a soft consistency in the primitive forms, the pollen
tetrads tend to become gradually hard. In the less advanced form the tetrads
are collected to firm masses called waxy pollinia. There are usually eight
clavate pollen masses divided completely halfway between top and bottom, four
masses in each cell, two smaller than the other two. This is met in
Epidendroideae under the terrestrial genera Acanthephippium, Calanthe, Phaius
etc. The pollinia is prolonged at base and held by a viscous or a pad-like mass
formed from part of the rostellum. In the epiphytic form these waxy pollinia
remain naked without prolongation or any appendages as in Eria (eight pollinia)
or Bulbophyllum, Dendrobium, Oberonia .
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In the process of advancement there was a tendency for
further reduction of the number of pollen masses in the pollinium while getting
harder. This is the case with Vandoideae. In the first case the pollinia got
reduced to four laterally compressed discoid masses by the loss of four
terminal masses. The pollinia here may be provided with appendages like
caudicle, stipe or viscidium. The pollen masses were further reduced to hard or
bony masses. They were usually notched, porate, foveolate or variously cleft;
occasionally they were entire i.e. solid masses. These masses were provided
with a well- developed appendage — the pollen kit consisting of caudicle, stipe
and viscidium.
The epiphytic form in orchids is of later development. In
this the monopodial growth form is of recent origin. Common ancestral form
among the monopodial is a plant with strap-shaped or channelled thick leaves
with usually large flowers. The lip here is without much ornamentation and the
pollen apparatus is simple as seen in Vanda. From this form are evolved the
advanced genera with reduction in size of the plant, leaf and of the flower.
The lip developed complex outgrowths or callosities of various types; the
pollen apparatus became more complex and specialized, the notched pollinia
giving way to solid ones. With the complexity in floral organs, the flower has
a tendency to become ephemeral. Chiloschista, Taeniophyllum etc. are a few
genera where such reduction in the size of the plant, leaves and flower is
observed. The plants have become aphyllous, the roots taking the functions of
the leaves.
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