The term evolution is derived from Latin word evolve which means unfolding or unrolling which implies the process of gradual and orderly change from one condition to another through succession or a series f changing events.

By organic evolution it is understood that the gradual and orderly changes are taking place in all life forms on the earth’s crust, since the origin of first life.

According to the doctrine of organic evolution as propounded by Charles Darwin the plants and animals of present day form are the descendants of the plants and animals of the past and are the progenitors of plants and animals of the future. The chef postulates of doctrine of organic evolution are:

i. The first or primitive living organisms were very simple in their form and structure.

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ii. All plants and animals which have appeared on the earth, are the ultimate descendants of the simple primordial organisms.

iii. During the course of evolutionary change there occurred an ever increasing complexity of structure and diversifications.

iv. Evolution proceeds either on progressive or on retrogressive liner. Both these processes are proceeding simultaneously.

v. Evolution, through which new species of plants and animals are generated, advances slowly which requires millions of years.

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Evolution in plants is different from animal evolution. During prehistoric period of life the divergency of plants from animals takes place from ancestral prtophytes collectively called “protista” plants show adaptive responses and specialization just as animals but line of evolution differs. Organic evolution in plants is a truthful event and a fact. It is supported by following evidences.

I) Evidences from palaeobotany:

The story of evolution of plants is written in rocks in the alphabets of fossils. Thus the fossil history of plants shows the following procession of plant’s life through the ages.

i. Algae are the primitive group of plants originated in autic habitat as the age of fossil algae “Stromatolites” was trace back to pre-cambrian peruod.

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ii. Nematophyton are the first true vascular plants invading and evolved from algae in paaexzoic rock.

iii. Psophytales are common indevonian rock fossil lycopods are the next steps of evolution.

iv. Proto lepidodendron, follows the lepidodendron are the huge tree like ferns ancestors of gynnosperms and anglosperm.

v. Pteridosperms of caboniferous rock provided evidence for first seed plant with frond like leaves and seeds bornne in sporophyll.

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vi. Nympaceae is the angiospermic family which has close relation with fossil ancestos of jurassoc rpck.

vii. The wood fossil ancestors of Jurassic rock.

viii. The wood fossil of homoxylon from rajmahal hills of India is Jurassic fossil resemble the xylemwith gymnosperms and pollen grains resemble nelumbo of anigioperns.

II) Evidences from comparative morphology:

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There is marked morphological resemblances in body form of different plant taxo show evolutionary tendenciesn.

i. The leaves are good photosynthesizing machines which show selective advantage in the variation in leaf structure in different group of plants and modified in relation to its utility. Simple leaves are primitive over compound leaves. Pinnate compound leaves are advanced over palmate type. Parallel venation is advanced over neted veins.

Leaves are evolutionary diverged under different adaptive types in the sequence of bud scales of woody plants spines of cacti scale leaves of onion tendrils of pea pitcher vensufly trap succlent leaves etc shows evolutionary plasticity.

ii. Stem morphogy: Stems whether erect creeping or climbing do same functions. From transport and mechanical support herbaceous stemis ancestors of woody stems erect stems are advanced over prostate or creeping type.

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iii. Floral morphology: Morphology of flowers shows striking instances of evolutions in relation to pollination and fertilization or verbalization. Floral colour number and nature of petals and sepals, nectaries shows evolutionary corresponding. Mognoliacocs flower is primitive over orchidian flowers.

iv. Homology and analogy: Organs which similar in structure and differs in the development are homologous but the organs which are differs embryologically in development and similar in function but differs in morphology are analogous organs.

The phylloclade of ruscus and cladades of asparagus are simolar in function and origin and homologous in nature. But both phylloclade and cladodes are anlogous to the normal leaves. Flowers are homologous to vegetative to the normal leaves. Flowers are homologous to vegetative shoot.

Homology and analogy shows strong evidences to the line of evolution.

v. Vestigel organs :

The structures in the body of plants which are reduced in structure and less used are called vestigial. In course of evolution these organs reduce retroessively and become vestigeal.

Staminodes in flowers, specialiization of stamens into nectaries, rudimentary leaflets in apricot, reduction of leaflets in to glands in apricot are the signs & evolution.

III) Evidences from comparative anatomy:

a. Cambium comparatively less active in herbaceous stem than the woody stem.

b. Limited secondary growth in herbaceous plants than woody plants.

c. Absence of heart wood in herbs and more heart wood in woody stems.

d. Lack of definite arrangement of vascular bundles in monocots and in ring in monocots and in ting in dicots.

e. Polyrach stele of monocot is advanced over diarch or triarch stele of dicot.

IV) Evidences from comparative physiology:

Photosynhesis in the fundamental natural process in green plants, respiration is the common feature of all life forms. Physiological similarity among simplest plants to complex form indicates their common origine from chlorella to higher trees.

V) Evidences from comparative biochemistry:

Presence of characteristic chemical substance like aromatic oils, alkaloids , carbohydrates,viz, aromatic ail in labiata, aljaloids in solanceae, oily seeds of eurherbiaceai, resins in pinaceae, establishes relationship among different group of plants.

VI) Evidences from developmental biology:

According to biogenetic law or theory of recapitulation by haeckel developmental pattern of different group of plants establish the phylogenetic relations.

i. Protonema stage of moss resembles algae indicating the evolution of moss from algae.

j. Filamentous prothallus of ferns found close relation with algae.

k. Cilliated sperm in modern cycads shows pteridophytean ancestry and recalled broyphytes.

VII) Evidence from plant geography:

Similar species occupying different geographical area likes to develop divergent lines of evolution longer period of isolation of one region from other the greater is the degree of difference which brought about evolution of new character.

Thus flora of one geographic region differs from other even they are indigenous to a same scope of land.

VIII) Evidences from ecology:

From ecological succession studies it is evident that, hydric or hydrophytic is primitive over the mesophytic and mesophytic is primitive over xerophtic habit.

The plants originated on rock surface like lichens are pioneers of mesophytic forest in aeithoseral community.