Early land plants
THE FIRST VASCULAR PLANTS AND THEIR LIVING RELATIVES (The origin of vascular plants on land) Origin of Land Plants (Embryophytes) Ancestors = Gree algae = Chlorophyta Why? (shared derived characters) 1. Chlorophyll a & b-used in photosynthesis 2. Carotenoids & xanthophylls-accessory pigments 3. Starch (stored carbohydrate) in chloroplasts 4. Cellulose cell walls 5. Oogamy 6. Phragmoplast (Cell plate) 7. Alternation of generations life cycle – sporophyte & gametophyte 8. Green algae very diverse unicells, colonies spherical balls of cells, coencytic filaments, 2D & 3D branched forms Which group of green algae is the closest to land plants? Class: Charophyceae Order: Coleochaetales Family: Coleochaetaceae Genus: Coleochaete 1. phragmoplast (cell plate) 2. plasmodesmata (multicellular growth) 3. lignin-like compounds (found in secondary wall thickenings in vascular plants) 4. sporopollenin (found in pollen & spores of vascular plants) 5. suberin-like compounds (found in casparian strip of endodermis in vascular plants and in bark (phellem) Origin of Vascular Plants 2 – Vascular Plants 1 - Polysporangiophytes Origin of Vascular Plants 2 – Vascular Plants 1 - Polysporangiophytes Lycophytina Zosterophylls Euphyllophytina Trimerophytes Rhyniophytes Phylogeny of basal Polysporangiophytes Lycophytes Lycopsida Zosterophyllopsida Lycophytina Vascular Tissue (Tracheids) Euphyllophytes Trimerophytes Seed Plants Progymnosperms Equisetophytes Ferns Psilotophytes Euphyllophytina Trimerophytes Zosterophylls Rhyniophytes Aglaophyton major Branching axes with sporangia Banks, 1968 T Z R Upper Silurian landscape-Cooksonia (the first vascular plant) Cooksonia Upper Silurian – earliest vascular plant How do we know that Cooksonia is a VASCULAR plant? Answer: It has vascular tissue Annular secondary wall thickenings in tracheids Tetrads of four spores Products of meiosis Spores have trilete marks (trilete sutures)=Y-shaped mark Therefore the plant is a sporophyte (2n) diploid with meiosis in the sporangia First Polysporangiophytes Cooksonia sp. Rothwell 1994 Sciadophyton sp. Mosses & basal polysporangiophytes 1. Dependent on gametophyte for a. Support b. Nutrition 2.Consist of sporangia borne on “axis or axes” Sporangium Sporangium (capsule) Unbranched seta Foot Sporophyte Foot Rhynie Chert • • • • • • A deposit of rocks (SiO2 . H2O)-chert – a glassy rock like quartz From Rhynie, Scotland Devonian age Studied using thin sections of rock Permineralized plants preserved in 3D All anatomical details present Rhynie Chert http://www.abdn.ac.uk/rhynie/ Rhynie Chert Taylor et al., 2009 Aglaophyton major 1. Is this a vascular plant? 2. Which class of plants? Taylor et al., 2009 Early land plants Classification developed by Banks, 1968, and Kenrick and Crane, 1997 Division: Tracheophyta Subdivision: Rhyniophytina* Class: Rhyniopsida* Subdivision: Lycophytina Class: Zosterophyllopsida* Subdivision: Euphyllophytina Class: Trimerophytopsida* Class: Psilophytopsida (Psilotum and Tmesipteris) Rhyniopsida (Rhyniophytes) Vascular plants in the Rhynie Chert Rhynia gwynne-vaughanii Named after the town of Rhynie Rhynia gwynne-vaughanii x.s. of axis Rhynia gwynne-vaughanii Tracheids with annular & helical secondary wall thickenings Stomatal apparatus spores Sporangium with abscission zone Rhynia gwynne-vaughanii Growth habit Rhizoids on hemispherical projections New reconstruction Sporangia shed Plant branches below sprg New reconstruction 1980 Vascular plants described by Kidston and Lang 1. 2. 3. 4. Rhynia gwynne-vaughanii Aglaophyton major *(=Rhynia major) Horneophyton lignieri (=*Hornea lignieri) Asteroxylon mackiei (we’ll discuss this one with the lycopods) Aglaophyton major reconstruction Reconstruction 1986 Aglaophyton major x.s. (=Rhynia major) Aglaophyton conducting cells No secondary wall thickenings Like the hydroids & leptoids of mosses Aglaophyton is NOT a vascular plant and it is not a bryophyte! But it is a land plant and a sporophyte Aglaophyton Sporangia Meiosis in sprg therefore a sporophyte but is it vascular? Zosterophyllopsida (Zosterophyllophytes) Sawdonia Zosterophyllum Trimerophytopsida (Trimerophytes) Trimerophytopsida Trimerophyton Psilophyton Psilophyton crenulatum The first land plants Rhyniophytopsida Zosterophyllophytopsida Trimerophytopsida Psilophyton Aglaophyton Cooksonia Zosterophyllum or Sawdonia Raven et al. 1999 Psilotophytopsida (Psilotophytes) Psilotophytopsida (Psilotophytes) • Psilotum – silent P • Tmesipteris – silent T These are living plants that some think are related to the ferns while others think that they are related to early land plants. Psilotum synangium forked enation Synangium = 2 or more fused sprg Psilotum synangium forked enation 3 sporangia fused Psilotum axis x.s. (Protostele) Psilotum underground axis Psilotum gametophyte Antheridia & archegonia antheridia archegonium sperm Tmesipteris an epiphyte on tropical tree ferns Tmesipteris Tmesipteris synangium on forked enation
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