botany 103

1. THE ORIGIN AND EARLY EVOLUTION OF PLANTS ON LAND IN THE MID_PALEOZOIC
ERA
INTRODUCTION.
The origin and early evolution of plants on land it based on interval of plants life from a simple
to complex, gradually. The origin and early diversification of land plants involve specialized
sexual organs, stems, structural tissues (wood), epidermal structure for respiration and gaseous
exchange (stomata), leaves, roots and spore bearing organs . This Paleozoic eras divide into the
several periods, according to their distinctive characteristics such as Ordovian , Silurian ,
Devonian , and Permian . Within periods there were several epoch evidenced on origin and
diversification of land plants has come mainly from dispersed spore and mega fossils.These
epochs were Eoembrophytic , Eotracheophytic and Eutracheophytic.
PATTERNS IN THE EARLY FOSSIL RECORD
Evidence on the origin and diversification of land plants has come mainly from dispersed spore
and mega fossil. Pattern in the easily fossil record recognized by three new plant based on
epochs . The following are the pattern in the fossil record
EOMBRYOPHYTIC (Mid _Ordovian early lanvirn _476 Million Years ago to the Silurian 432
Million years.)
In period of mid ordovian the spore tetrads appeared overbroad geographic area ,also provides
good evidence of land plants. The combination of decay resistant wall ( imply the presence of
sporollennin ) and tetrahedral configuration (imply haploid meiotic products) is diagnostic of
the land plant.
In late Silurian and Devonian mega fossils as well as data of the spore wall ultrastructure and
the structure of fossil , cuticles , support previous suggestion of the land flora of liverworts like
plants . Some early spores and cuticles may also represents extinct transitional line ages
between charophycean algae and liverworts. So due to that charophycean algae and liverworts
are seen to the to be the plants products coilbi n this period and are member of
eoembryophytic .
EOTRACHAEOPHYTIC (early Silurian 432Myrs to early Devonian 402Myrs)
Early Silurian marks the beginning of a decline of a decline in diversities of tetrads rise to a
dominance of individually dispersed , simple spore which are found. In several basal land such
horn roots , some mosses and early vascular plant. Although tetrads remain dominant in some
early Devonian localities from North Western Europe , the elaboration of simple spore and
turnover of spore species provide evidence of increasing land plants diversity and vegetation

2. change . Although spore have been observed in Silurian , mega fossils , the most dispersed
from remain unknown indicating that substantial land plant diversity is currently un detected in
the mega fossil records. Data from N.Europe , Siberia , Podolia southwestern Ukraine) , Libya,
Bolivia Australia and Xinjiang and Yunnan (china) document increasing land plant diversity into
the base of the Devonian.
EUTRACHEOPHYTIC
This was the period of early Devonian 398M years to Mid Permian 256M years where spores
and Mega fossil increased dramatically. At that period it was where eutracheophytes become
more complex with true vascular tissues (xylemand phloem) such as ferns , conifers , and
flowering plants compared to Eotracheophytic periods the plants were classified into three
groups
a) ccoksonoids example cooksonia
b) Lycophytes example clubmosses
c) Euphyllophytes example fern
During the eutraphytic period the plants it was able to adapt in harsh condition like drought
where it has long roots for absorption of water in the ground as well as shedding of leaves to
avoid desiccation as well as they have the ability to undergo alternative generation where by

3. both gametophyte phase and sporophyte phase.
BUILDING A LAND PLANT
Origin of land plant originated from the charophycean green algae this is due to phylogenetic
studies. The fresh water origin of plants seems likely but direct evidence from fossil records

4. which is inclusive as mid –paleozoic charophytes are found in both fresh water and more
commonly marine farcies.
These charophycean green algae had ability to produce spore pollen , cutin , phenotic
compounds and the glycolate oxidase pathway. During the transitions of absences of well
developed saprophytes, gametophytes with sexual organs of land type , cuticle and non motile ,
air borne sporopollenin walled spore were evolved. Also in this period of transition from an
aqueous to a gaseous medium exposed plants to new physical conditions that resulted in
physical change and structural changes.
Phylogenetic studies predict that early land plants were small and morphological simple, thus
early fossil bears a strong resemblance to the simple spore producing phase of living mosses
and liverworts. The gaseous exchange surface and sexual organs seems to have been primary
responses to life on land. This changes land to highly differentiated plants with stomata, multi
cellular, sexual and spore bearing organs, water conducting and other tissues system. This
morphological differentiation occurred in both phases of life cycle includes gametophyte phase
and saprophytes phase
EARLY TERRESTRIAL ECOSYSTEM.
 The earliestterrestrial ecosystemisfirstlydescribedbythe exisistence of the landplants
 Early vascularplants(whichhave modifiedprostrate stembearingrhizoidsresemblingthose of
livingbryophytes(evolvedinearlyDevonian)
 More substantial rootscapable of anchoringlarge treesevolvedindependentlyinseveral group
duringthe middle tolate Devonian
 Biosynthesisof ligninandthe originof lateral meristemsinthose groupresultinginstratified
forestcommunities.bythe endof middle Devoniantreesevolvedindependentlyinseveral
majorgroup resultinginstratifiedforestcommunity
 The earlyevolutionof lignindecomposingfungi (someare ascomycetesandbasidomycetes) is
still poorlyunderstoodbutthisgroupwouldhave beenessential forrecyclingmuchof the
organiccarbon.
 The earliestlandplantsprobablyencounteredterrestrial ecosystemthathasbeenoccupied by
Bacteriaand protists,algae,lichensandfungi sincethe late proterozoic.
 Some of the plantssuchas enigmaticplantslike protosalvinia,grew inlate Silurianandthe early
Devonianabout400 millions yearsandsome of the largestelementsmayhave beenfungi which
may have beenextractedinthe SilurianandearlyDevoniansediments.
 The discoveryof fungi arbusculae inearlyDevonianmegafossilsconfirmsthatendomycorrhizal
associationwere animportantinnovationinthe colonizationof land.
 By the endof megascopicplantswhichappearedtohave colonizedthe landonlyonce,many
animalsgroupsmade the transitiontoterrestrial existence independentlyandovercomethe
problemsof waterrelationsn differentwayslike centipedes,collembolansandsome of possibly
bristle tails.

5.  Available evidenceindicatesthatthese animalswere mainlypredatorsanddetrivoresanduntil
the appearance of vertebratesherbivoresinthe latestpalazoezoic.These arthropodfaunaare
knownfromseveral localitiesinnorthAmericagermanyandunitedkingdom.These fauna
provesthe tissuesof some fossilsplant.
FUTURE DIRECTION.
 The fossilsrecordsof sporescombinedwith phylogeneticstudiesindicatesthatgroupsrelated
to livingbryophyteswere earlycolonizerof land.bryophytesegmoss,liverwort,.Thissuggest
that several linage of vascularplantshadalreadyevolvedbythe midSilurian.The megafossilsof
landplantshoweverappears muchlaterandin these assemblagesthereisaconspicuousbias
towardthe recognitionandperhapsrepresentationof vascularplants.The source of dataof
earlymegafossilshasbeenthe northerneuropianregionandsiluanmegafossilsare frommarine
sediments
 Thisseemonsetof continental conditionsinthe Devonianof northernEurope allowed
megafosssilstobe preservedatatime whenvascularplantswere well establishedbutstill
diversifying,rapidappearance of vascularplantsleadtochangesingeological conditiondue to
Rapidbiological diversification
 Paleontological isthe evidence showsthatmajorgroupof livinglandplantsare relicts(some
remainsandothersdied) eventhoughmuchmodernspeciesdiversitywithinthesegroupsmay
have beenevolved.Suchcombinedstudiesof livingandfossilsplantsprovide animprovedbasis
for comparative studiesatplantsdevelopment.Thisindicatesforexample thatontogenyof
leavesandspore bearinginclubmossesare likelytoshare substantial similaritiesbutare
unlikelytoexhibitcommonfeatureswithleavesinseedplantssuchasspore bearingorgans
,stems,stomataandsexual organsare each underthe same kindof developmentcontrol inall
groups
 Generallymore dataare neededtoexplore thisissue furtheronmolecularbasisof plant
developmentfromabroaderselectionof landplants.