Plant Diversity I Level 1 Biological Diversity Jim Provan Campbell: Chapter 29

Main groups of land plants Four main groups: Bryophytes (mosses, liverworts and hornworts) — Differ from algae due to adaptation to terrestrial life — Mainly reproductive differences e.g. developing embryo attached to mother plant Pteridophytes (ferns, lycophytes, horsetails etc.) — Vascular system not found in bryophytes — Some bryophytes have a rudimentary water-conducting system Gymnosperms (conifers etc.) — Seed plants: embryo, food supply and protective covering Angiosperms — Flowering plants

Highlights of plant evolution Simple charophyceans Complex charophyceans Mosses Liverworts Hornworts Lycophytes Ferns, horsetails, whisk ferns Gymnosperms Angiosperms

Ten phyla of extant plants Common name Approximate number of extant species Approximate number of extant species Nonvascular plants (bryophytes) Division Bryophyta Division Hepatophyta Division Anthocerophyta Vascular plants Seedless vascular plants Division Lycophyta Division Pterophyta Seed plants Gymnosperms Division Coniferophyta Division Cycadophyta Division Gingkophyta Division Gnetophyta Angiosperms Division Anthophyta Nonvascular plants (bryophytes) Division Bryophyta Division Hepatophyta Division Anthocerophyta Vascular plants Seedless vascular plants Division Lycophyta Division Pterophyta Seed plants Gymnosperms Division Coniferophyta Division Cycadophyta Division Gingkophyta Division Gnetophyta Angiosperms Division Anthophyta Mosses Liverworts Hornworts Lycophytes Ferns Conifers Cycads Gingko Gnetae Flowering plants Mosses Liverworts Hornworts Lycophytes Ferns Conifers Cycads Gingko Gnetae Flowering plants 12,000 6, ,000 12, ,000 12,000 6, ,000 12, ,000

Plants probably evolved from green algae called charophytes Evidence suggests plants and charophytes evolved from a common ancestor: Homologous chloroplasts: accessory pigments, stacked thylakoids, DNA evidence Biochemical: cellulose and peroxisomes Cell division: dispersed nuclear envelope, persistence of spindle etc. Sperm ultrastructure Phylogenetics

Differences between plants and charophycean algae Apical meristems Light and CO 2 found above ground Water and nutrients mainly found below ground Multicellular, dependent embryos Alternation of generations: Occurs in some algae but not charophyceans (no sporophyte generation): evolved independently in land plants Differs from standard haploid/diploid life cycles in that both stages are represented by multicellular bodies Walled spores produces in sporangia Multicellular gametangia

Alternation of generations

Alternation of generations (continued) Most plants reproduce sexually and most are also capable of asexual reproduction: all plants have life cycles with an alteration of generations: Haploid gametophyte generation produces and alternates with a diploid sporophyte generation  gametophyte Life cycles are heteromorphic i.e. have different morphology Sporophyte is larger and more noticeable, except in mosses Comparison of life cycles among divisions is instructive: Points to an important trend in plant evolution - reduction of gametophyte generation and dominance of diploid sporophyte Some features are adaptations to terrestrial environment - replacement of flagellated sperm by pollen

Alternation of generations in plants may have originated by delayed meiosis Evolved independently in various groups of algae: Does not occur among modern charophytes - occurrence in plants had a separate origin from alternation of generations in algae Appearance in plants analogous to occurrence in algae Coleochate may hold clues to evolution: Parental thallus retains eggs - zygote also attached to parent Thallus cells grow around zygote which enlarges, undergoes meiosis and releases haploid swimming spores Haploid spores develop into new individuals Only diploid stage is zygote - no multicellular diploid stage. This would arise if meiosis was delayed until after mitotic division of zygote

Alternation of generations in plants may have originated by delayed meiosis Meiosisdelayed Zygotic mitosis produces multicellular sporophyte

Adaptations to shallow water preadapted plants for living on land Many modern charophytes live in shallow water, as did their ancestors ~440mya (Ordovician  Silurian), climatic changes caused fluctuations in water levels Natural selection favoured plants tolerant to periodic drying - preadaptation to terrestrial life: Waxy cuticles Protection of gametes Protection of developing embryos Eventually, accumulated adaptations made it possible for ancestral plants to live above water line

Bryophytes

The embryophyte adaptation evolved in bryophytes The embryophyte condition was a pivotal adaptation - : Antheridium (male gametangium) produces flagellated sperm cells Archegonium (female gametangium) produces egg - fertilisation/embryo development occur within female organ Bryophytes are not totally independent of aquatic habitat: Need water for sperm to swim in Have no vascular tissue - diffusion

The gametophyte is the dominant generation in the life cycle of bryophytes

Mosses (Division Bryophyta) A tight pack of many moss plants forms a spongy mat that can absorb water Each plant grips the substratum with rhizoids Photosynthesis occurs in the upper parts of plants Cover about 3% of land surface and contain vast amounts of organic carbon

Liverworts (Division Hepatophyta) Less conspicuous than mosses Sometimes have bodies divided into lobes Live cycle similar to mosses: sporangia have elaters to aid dispersal Can reproduce asexually from gemmae Most diverse in tropical forests

Hornworts (Division Anthocerophyta) Resemble liverworts but sporophytes are horn- shaped and grow from mat-like gametophyte Photosynthetic cells have a single, large chloroplast Most closely related to vascular plants

Additional terrestrial adaptations in vascular plants Regional specialisation of the plant body: evolution of roots to absorb water and stems/leaves to make food Structural support: no buoyancy from water, so lignin became embedded into cell walls Vascular system: Xylem: conducts water and minerals from roots. Composed of dead, tube-shaped cells and provides structural support Phloem: conducts food throughout the plant. Composed of living cells organised into tubules and distributes sugars etc. Pollen and seeds: to transport gametes Increased dominance of the diploid sporophyte

Pteridophytes

The sporophyte-dominant life cycle Exemplified by ferns: Familiar leafy plant is the sporophyte Gametophytes are small and grow on or below the soil surface Vascular plants display two distinct reproductive strategies: Sporophyte of homosporous plants (e.g. ferns) produces a single type of spore which develops into a bisexual gametophyte with both male and female sex organs Sporophyte of heterosporous plants produces two types of spore: — Megaspores develop into female gametophytes (archegonia) — Microspores develop into male gametophytes (antheridia)

A sporophyte-dominant life cycle evolved in seedless vascular plants MITOSIS

Lycophytes (Division Lycophyta) Includes club mosses and ground pines Dominated land through the Carboniferous Period ( mya) Some species of Lycopodium are epiphytes: Sporangia borne on sporophylls Spores develop into inconspicuous gametophytes Mostly homosporous

Horsetails (Division Sphenophyta) Survived through Devonian and at peak during Carboniferous Only genus is Equisetum: Lives in damp locations and has flagellated sperm Homosporous Conspicuous sporophyte generation Has photosynthetic, free- living gametophytes

Ferns (Division Pterophyta) Co-existed with lycopods and horsetails in Carboniferous forests Larger leaves with branched system of veins - fronds are compound leaves Homosporous: Specialised sporophylls Gametophyte is small and fragile Embryo develops within archegonium