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Chapter 22
SEEDLESS VASCULAR PLANTS
EVOLUTION OF VASCULAR PLANTS Notes from Raven et al., 7 th. Edition. Plants had an aquatic ancestor probably a Coleochaete-like alga, of the Chlorophyta. Coleochaete algae are monobiontic – one multicellular generation. Plant evolution shows a tendency toward greater independence from water as they progressively occupied the land. ir is drier than water and less buoyant. !and plants had to develop adaptations to conserve and transport water with its solutes, absorb water from the environment, support itself facing the direction of sunlight, solve reproductive problems like fertili"ation and nourishment of embryo, and dispersal of offspring. #. $ominant $ominant sporophy sporophyte te and and reduced reduced gametophy gametophyte. te. • • • •
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%n bryophytes, the gametophyte is dominant generation. &ater is required for fertili"ation. Pollen and embryo sac are much reduced gametophytes. 'he occupation of the land by the bryophytes bryophytes was undertaken with emphasis on the gamete producing generation, generation, which requires water for for fertili"ation. $ibiontic( two multicellular generations.
2. $evelopme $evelopment nt of fluid fluid-tran -transpor sportt system, system, the the )ylem )ylem and phloem phloem.. • •
quatic plants take water throughout their entire body. body. *n land, soil is the water reservoir+ the air is dry in comparison to cells.
. 'he abili ability ty to to synth synthesi esi"e "e lign lignin. in. • •
arly land plants were small and probably stayed upright by means of turgor pressure. !ignin adds rigidity to the cell wall and allows the plant to reach greater heights.
. $evelo $evelopme pment nt of apic apical al meri meriste stems ms.. • •
/ryophyte sporophyte growth is subapical and unbranched. %t allows the sporophyte to branch many times.
0. bilit bility y to produc producee many many spor sporang angia. ia. • •
*nly one sporangium is produced the bryophyte sporophyte. 'he many branches of vascular plants became capable of bearing many sporangia.
1. ore diverse diverse plant plant body body through through the the developm development ent of roots roots stems stems and leaves. leaves.
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3oots for absorption, storage and anchorage. 4tems for support above ground, transport and growth toward the light. !eaves for photosynthesis. photosynthesis.
5. vol volut utio ion n of of seed seeds. s. • •
Provides the embryo with food and protection. $ispersal of the species to new locations.
HYPOTHESES ABOUT THE ORIGIN OF THE ALTERNATION OF GENERATIONS
Notes from Mauseth. Mauseth. %6'3P*!'%*6 '7*38 'his theory presumes that the earliest land plants did not have a sporophyte generation+ the "ygote germinated by meiosis that produced haploid spores that grew into new haploid gametophytes. •
small sporophyte came into e)istence when a "ygote germinated mitotically instead of meiotically.
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'he sporophyte generations would have gradually evolved in comple)ity while the gametophyte generation remained small.
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sporophyte sporophyte generation was inserted 9interpolated: into the monobiontic life cycle.
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4ome bryophytes may represent an intermediate stage in the progression from green algae to vascular plants.
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3iccia and 3icciocarpus have simple, almost alga-like gametophyte, and their sporophytes consist of ;ust a small globose sporangium with not foot or seta. 'he "ygote undergoes several mitotic divisions and then some cells undergo meiosis.
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4peculation( the sporophyte then evolved more comple) with a foot and seta, and later developed an apical meristem to branch b ranch and live free of the gametophyte.
'364osterophylls >osterophylls were homosporous.
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ylem matured from the periphery toward the center( centripetal differentiation differentiation also known as an e$arc !roto"te#e.
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'hey are considered to be the ancestral group that gave rise to the !ycopods+ the sporangia of both groups are borne laterally laterally and are similar in shape, shape, and the )ylem in both groups groups differentiates centripetally. centripetally.
'he sporangia of e(!'##o!'te" 9ferns, gymnosperms and angiosperms: are borne terminal on the a)is. 'hat indicates that >osterophyllum with its lateral sporangia is probably not the ancestor of euphyllophytes euphyllophytes but of lycophytes, which bear lateral sporangia. *ther general of "osterophyllophytes "osterophyllophytes showed significant morphological changes( enations with a vascular trace that ended at the base of the enation, trunk-like shoots bore smaller short lateral shoots. 'hese are the morphological bases for producing cones and trunks.
P'#(m Tr Trimero!'to!'ta imero!'to!'ta Notes from Raven et al.
'his phylum probably evolved directly from the rhyniophytes. 'rimerophytes 'rimerophytes were larger and more comple) plants than the rhyniophytes or "osterophyllophytes. "osterophyllophytes. 'rimerophytes 'rimerophytes appeared in the arly $evonian about A0 million years ago and had become e)tinct by the end of the mid $evonian, about 2? million years later – a relative short period of e)istence. • • • • • •
'hey lacked leaves. !ateral branches forked dichotomously several times. 'hey were homosporous. 4ome branches were vegetative while others bore elongated sporangia. =ascular strand was more massive than that of the rhyniophytes. 'he )ylem differentiated centrifugally.
Di&i"ion L'co!odio!'ta *3P7*!*B8 'here are about #0 genera of lycophytes and appro)imately #2?? living species. 'his evolutionary line e)tends back into the $evonian but was most prevalent in the wet swamps of the Carboniferous period. 'he earliest lycophytes were members of the general &epranophy#us and &epranophy#us and 'arawagnathia 'arawagnathia.. 'hey were similar to their ancestors the "osterophylls, but with the difference of having large, up to cm long enations with a single well-developed trace of vascular tissue, called mi#rophylls . any e)tinct lycophytes such !epidodendron, !epidodendron, "igillaria "igillaria and "tigmaria had "tigmaria had a vascular cambium and secondary growth with pith rays and elongated tracheids. 'heir cambium could not undergo radial longitudinal division+ division+ there fore, new fusiform initials could not be produced. s s the wood grew to a larger larger circumference, cambial cells became became increasingly wider tangentially and may have stretched so much that they could not function. 6o fossil has been found with wood more than about #? cm thick. 7'3*4P*38 %n many e)tinct and e)tant lycophytes, sporangia are clustered in FconesG or "trobi#i 9sing. "trobi#(": 4ome species became heterosporous producing heterosporous producing megaspores and microspores. •
4pores of 2 types 9plants etero"!oro(":, me+a"!ore" 9#--2--:, large+ micro"!ore" numerous 9hundreds:, very small.
Bametophytes Bametophytes unise)ual( micro+ameto!'te" 9males: and me+a+ameto!'te" 9females:. 7eterospory is a necessary condition for the evolution of seeds.
%n some the megaspore developed into a megagametophyte megagametophyte without enlarging( enlarging( the megagametophyte megagametophyte e)isted completely within the wall of the original megaspore, which was up to #? mm long. 'he megaspore of some species was retained within the sporophyll, protected by the thick wall of the sporangium. 'his is very similar similar to ovules and seeds. 'he sporangium dehisced dehisced and sperms could swim to the egg.
E,TANT ORDERS AND GENERA
Notes from Raven et al. olecular and morphological evidence indicate that they split up into two evolutionary lines before ?? million years ago, in the arly – id $evonian. • •
'he #'co!'te c#ade includes the modern lycophytes. 'he e(!'##o!'te" c#ade includes whisk ferns, horsetails and seed plants.
'he e)tinct lycophytes lycophytes include very large woody trees that did not survive survive in the drier climate at the end of and after the Carboniferous age. %n the Carboniferous some lycophytes were forest-forming trees more than 0 meters tall. &oody &oody lycophytes became e)tinct before the end of the Paleo"oic era, 2@ million years ago. 'he second and the surviving group of !ycopods are the small and herbaceous plants. !ycophyta !ycophyta remains became the largest coal deposits of all geologic time. !ycophyta !ycophyta are characteri"ed by • • •
microphyllous leaves, a special spore producing body called a strobilus, the presence of true vascular stems, roots and leaves.
'here are three prominent orders of !ycophyta( !ycophyta( • • •
!ycopodiales, or club mosses+ 4elaginellales, or spike mosses+ %soetales, or quillworts.
Fami#' L'co!odiaceae
family of about ?? species, mostly tropical. 'he ta)onomic boundaries of the genera are not well understood and as many as #0 genera may eventually be recogni"ed. 4even of these genera are represented in 6orth merica. •
4porophyte with true leaves, stems and roots.
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$ichotomous branching rhi"ome from which aerial branches and roots arise.
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4tems and roots are protostelic or siphonostelic. !eaf gaps absent.
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!eaves are microphyllous and spirally arranged, sometimes opposite or whorled.
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S!oro!'##", modified fertile microphylls, sometimes grouped into "trobi#i 9strobilus, cluster of overlapping non-photosynthetic sporophylls:.
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%n $uper(ia %n $uper(ia and and )hlegmariuru )hlegmariuruss , the sporophylls are similar to ordinary microphylls and are interspersed among the sterile microphylls.
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*ne sporangium per sporophyll, near the base and on the ada)ial side.
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7omosporous.
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Bametophyte bise)ual, either green or subterranean, non-photosynthetic, non-photosynthetic, mycorrhi"al structures, depending on the genus.
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Bametangia, antheridia and archegonia, may require 1 to #0 years to mature.
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4elf-fertili"ation is rare.
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/iflagellated sperm requires water to reach the archegonium.
Fami#' Se#a+ine##aceae
'here are about 5?? species in this family, most of them tropical. "elaginella is "elaginella is the only genus in the family. •
Plants herbaceous, annual or perennial, sometimes remaining green over winter.
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4tems leafy, branching dichotomously, dichotomously, regularly or irregularly forked or branched.
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4tems and roots protostelic 9sometimes with many protosteles or meristeles:, siphonostelic, or actino plectostelic. Protostele held in place by trabec(#ae. 3hi"ophores 9modified leafless shoots producing roots: present or absent, geotropic, borne on stems at branch forks, throughout, or confined to base of stems.
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!eaves microphylls, on # plant dimorphic or monomorphic, small, with ada)ial ligule near base, single-veined, rarely veins forked.
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4trobili sometimes ill-defined, terminal, cylindrical, quadrangular, or flattened.
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4porophylls 9fertile leaves: monomorphic or ad;acently different, slightly or highly differentiated from vegetative 9sterile: leaves( micro"o!oro!'##" and me+a"!oro!'##".
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4porophylls and microphylls with #i+(#e.
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4porangia short-stalked, solitary in a)il of sporophylls, sporophylls, opening by distal slits.
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4pores of 2 types 9plants etero"!oro(":, me+a"!ore" 9#--2--:, large+ micro"!ore" numerous 9hundreds:, very small.
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Bametophytes unise)ual.
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Bametophytes develop inside the spore wall( endosporic development.
icrogametophyte icrogametophyte lacks chlorophyll. t maturity, it consists of a single prothallial cell and an antheridium. ntheridium produces many of biflagellated sperms. icrospore wall ruptures to liberate the sperms. egagametophyte egagametophyte multicellular. multicellular. egagametophyte egagametophyte protrudes through a rupture in the spore wall. rchegonia develop in the e)posed area.
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4perms require water to swim to the archegonia.
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'he "("!en"or develops and pushes the developing embryo deep into the female gametophyte.
Fami#' I"oetaceae
family of one genus, *soetes genus, *soetes 9ca. 9ca. #0? spp.:, found world-wide, especially in temperate areas. second genus, "tylites, "tylites, is sometimes recogni"ed. 4ome authors put fossil representatives in the genus *soetites, *soetites, which is known from as early as the upper 'riassic. •
Plants tufted, grass-like, perennial, evergreen aquatics to ephemeral terrestrials.
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Dnderground stem brown, cormlike, lobed.
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3oots arising along central groove separating each rootstock lobe, simple or dichotomously dichotomously branched, containing eccentric vascular strand and surrounding lacuna.
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!eaves linear, simple, spirally or distichously arranged, dilated toward base, tapering to ape), covered with a cuticle, containing transversely septate longitudinal lacunae, a central collateral vascular strand, and frequently several peripheral fibrous bundles. 'hey lack stomata.
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*so+tes has *so+tes has C photosynthesis. photosynthesis.
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bsorbs carbon dio)ide from the mud surrounding the roots.
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ach leaf is a potential "!oro!'## .
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!igule inserted above sporangium on each sporophyll.
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7eterosporous 9megaspores and microspores not alike:.
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egasporophylls egasporophylls and microsporophylls microsporophylls usually borne in alternating cycles+ hardened scales and phyllopodia occasionally occasionally surround leaves. leaves.
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4porangia solitary, solitary, ada)ial, embedded in basal cavity of leaf, velum 9thin flap e)tending downward over sporangium: partly to completely co mpletely covering ada)ial surface of sporangium. sporangium.
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egasporangium with several to hundreds of megaspores.
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icrosporangium with thousands of microspores.
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egagametophytes egagametophytes white, endosporic, e)posed when megaspore opens along pro)imal ridges+ archegonia # to several, indicated by quartets of brownish neck cells.
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icrogametophytes icrogametophytes A-celled, endosporic, antheridium releasing multitailed spermato"oids.
C*6=3B6' =*!D'%*6 =*!D'%*6 &%'7 4$ P!6'4 %n both lines( • • • •
laborate efficient leaves, wood, bark and roots evolved. 4porangia became separated from vegetative structures and grouped into strobili. 7eterospory evolved. ndosporial development of the gametophyte.
THE -EGAPHYLL LINE OF EVOLUTION. EUPHYLLOPHYTES Di&i"ion Trimero!'to!'ta . fossil phylum that seems to have evolved from rhyniophytes. %t was proposed to include three genera( rimerophyton, )silophyton and )erti#a and )erti#a.. %t might be the ancestor of ferns, progymnosperms and perhaps horsetails. 'hey first appeared in the early $evonian about A0 m.y.a. m.y.a. and became e)tinct by the end of the mid $evonian, at about 5? m.y.a.
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'rimerophytes 'rimerophytes were very similar to rhyniophytes rhyniophytes in that they lacked leaves and roots+ most of the plant body consisted consisted of a dichotomous dichotomous branching stems that were photosynthetic photosynthetic throughout throughout their length. 'hey were also similar to rhyniophytes in being homosporous, having sporangia that dehisce laterally, laterally,
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=ascular =ascular tissue was present, forming a solid central bundle in the center of the stem, or protostele.
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'heir protostele was more massive than that of rhyniophytes. rhyniophytes.
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'hey had a band of thick-cell-wall cells in the corte).
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ylem differentiated centrifugally like in rhyniophytes. rhyniophytes.
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'rimerophytes 'rimerophytes branched !"e(domono!odia##', that means that the branching was unequal, forming a main stem, or a)is, with several smaller lateral branches.
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3hyniophytes 3hyniophytes branched dichotomously( stems always branched into two equal branches.
!ateral branches typically branched dichotomously, dichotomously, and were often shortened to form bushy HwebsH of small, closely spaced branches. 4ome trimerophytes also bore enations on the main stems, giving them a superficially HthornyH appearance.
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Correlated with the pseudomonopodial growth habit and the numerous photosynthetic branches was an increase in the vigor and robustness of the plant.
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'rimerophytes 'rimerophytes bore sporangia at the tips of branches, like the rhyniophytes, rhyniophytes, but unlike the superficially similar "osterophylls "osterophylls from the same time period.
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'he spindle-shaped sporangia produced only one type of spore( trimerophytes were homosporous.
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4pores released from the sporangia would have germinated into gametophytes, but no fossil trimerophytes gametophytes gametophytes have been identified, and we do not know whether trimerophytes, like their later relatives the ferns, sphenopsids, and seed plants, had small, inconspicuous gametophytes.
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'rimerophytes 'rimerophytes varied in si"e from a few centimeters to nearly a meter tall+ large trimerophytes were among the largest plants of the arly $evonian.
*3%B%6 *< '7 BP78!! 9DP78!!4: – '!* '7*38 'hree distinct homoplastic 9analogous: structures called leaves occur in plants( !eaves on gametophytes of nonvascular plants. • Enation" and micro!'##" in "osterophyllophytes "osterophyllophytes and lycophytes. • -e+a!'##" that evolved from a branch system and are present in all seed plants, ferns and • equisetophytes. 'he megaphyll evolution is summari"ed in the Te#ome Teor'.
'he twigs of the last dichotomy are called te#ome". /eginning with a dichotomous branch, the megaphyll evolved following the steps listed below. 'his theory is based on the study of fossils made by >immermann in the #A?s. • • •
*ne branch overtops the other which remain small and lateral( o&erto!!in+. 'he lateral branch system becomes restricted in their branching to one plane( !#anation. 'he spaces between close branches developed a thin sheet of chlorophyll-containing chlorophyll-containing cells( /ebbin+.
'he euphyllophytes consists of two sister clades( moni#o!'te" and #'co!'te"% 4everal current studies suggest that megaphyllous plants are united by three synapomorphies( #. 3oot 3ootss hav havee e)ar e)arch ch )yle )ylem. m. 2. 'hey 'hey have have mega megaph phyl yll. l. . 'hey have have a ?-kilobase ?-kilobase inversio inversion n in the large large single-copy single-copy region region of their their plastid plastid $6.
-ONILOPHYTES any monilophytes are known as ferns 9!eptosporangiate, *phioglossales, *phioglossales, arattiales:+ others are known as fern allies 9equisetophytes, 9equisetophytes, Psilotales:. 'he $ivision rthrophyta rthrophyta consists of four *rders, three of which are e)tinct, and one e)tant, *rder quisetales.
E0UISETOPHYTES
*3$3 ID%4'!4 Notes -ased on Raven et al. quisetophytes quisetophytes e)tend back to the $evonian and reached their ma)imum development in the Carboniferous 9@?-2@? m.y.a.:. m.y.a.:. family of one e)tant genus, Euisetum genus, Euisetum 9ca. 9ca. #0 species:, of nearly worldwide distribution in damp habitats such as riverbanks, lakeshores, and marshes. ichigan is a center of diversity for the genus with nine native species. 'hey are known by their common name as horsetails or scouring rushes. •
'he sporophyte of Euisetum of Euisetum is is differentiated into an underground rhi"ome that bears adventitious roots and an upright, photosynthetic stem with whorls of microphylls..
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'ough 'ough perennial herbs with ;ointed, ridged aerial stems with distinct nodes.
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4tems rough, accumulating silica and metals, and comple) anatomically.
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'rue roots are present.
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'he aerial stems contain a large central pith region, which in mature plants is hollow. 'his 'his is the arrangement of a "i!ono"te#e.
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4urrounding the pith cavity are discrete bundles of vascular tissue. 4ome call this arrangement of discrete bundles around the pith a e("te#e, and not a siphonostele. siphonostele.
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'he bundles contain both )ylem and phloem, and are marked by the presence of large canals known as carinal canals 9under the ridges:, which also f unction in water conduction. 'he proto)ylem is endarch. endarch.
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ylem contains &e""e#", a rare occurrence outside angiosperms. angiosperms. )ternal to the vascular bundles is another set of canals, the &a##ec(#ar cana#" or cortical canals 9under the valleys:.
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'hese canals line up with the depressions between the ribs on the surface of the stem. ost fossil sphenophytes sphenophytes had very similar stem morphology.
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'he corte) is composed of large cells, the outer ones chlorophyllous. chlorophyllous.
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'he epidermis contains stomata and guard cells.
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!eaves are small, whorled, non-photosynthetic, non-photosynthetic, fused together to various degrees and adpressed to the stem.
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4porangia borne on peltate sphorangiophores sphorangiophores aggregated into a "trobi#(".
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4trobili consist of tightly packed appendages called "!oran+io!ore".
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7omosporous+ spores with e#ater".
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Bametophytes green, epigeal, bise)ual or unise)ual, male gametophytes smaller than female+ with rhi"oids.
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rchegonia develop before antheridia to increase the probability of cross-fertili"ation.
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4perms are multiflagellated and require water to swim to the egg.
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4everal eggs on a single gametophyte gametophyte may be fertili"ed.
'he arthrophytes can be traced back to through the trimerophytes trimerophytes to the rhyniophytes. rhyniophytes. 'he 4phenophyllales and Calamitales had secondary growth but their fusiform initials could not undergo radial longitudinal division to produce more fusiform initials.
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