Presentation on theme: "The Transition to Terrestrial Life"— Presentation transcript:
1 The Transition to Terrestrial Life First plants - algae - still thrive in a range of aquatic habitats today (not primitive - simple compared to more complex groups; highly evolved and well adapted to the niche they occupy.)Aquatic environment is predictable (~stable). Why venture onto land? Selection pressure may have been competition!Selective pressures on pioneer land plantsDesiccation Water for reproduction - need free standing water for fusion of gametes Support - buoyancy supports and spreads the algal thallus. On land, plants would be plastered on the mud Water for spore dispersal - to colonize new terrestrial habitats spores would have to be released in air not water
2 Evolutionary Trends in the Transition to Land ~ 400 million years ago freshwater, green, filamentous algae invaded the land.probably isomorphic alternation of generationsprobably heterotrichous.Selection favors individuals more able to withstand periods without submergence (e.g. at pond margins, on wet mud)Gametophytes need water for reproductionbasal part of the gametophyte developed with loss of the upper portion.sterile jacket of cells evolved to protect the developing gametes during periods of exposure.4. Sporophytes - spore dispersal was originally in water.Spores need to be dispersed in air.upper spore-bearing part of the plant would need to be held above water5. Appearance of first conducting tissues6. The generations began to diverge.
3 Highlights of Plant Evolution Silurian (~425 MYA) first land plants (cuticle, gametangia, vascular tissues)Devonian (~400 MYA) seedless vascular plants (ferns)Late Devonian (~360 MYA) Seed - Plant embryo with food and a coatEvolution of gymnosperms (coexist with ferns for 200 MYCretaceous (~130 MYA) emergence of flowers
4 Changes Needed in the Sporophyte Generation Cuticle - a new non-cellular, waxy, water-proof layer Sporopollenin - a similar waxy waterproofing appeared on the surface of spores (prevented desiccation during travel through air). Multiaxial filament - (vs flimsy uniaxial filament) for erect plant retains turgor at the core of plant even when the outer cell layers lose waterTransport system - evolution of the stele, (xylem and phloem) for transporting water, minerals and organic material when simple diffusion is no longer sufficient for a large plant bodyVentilation system - evolution of stomata -pores in the plant surface - and intercellular air spaces for transport of gassesAnchorage System - the lower portion of the sporophyte developed an anchorage system that was more than a few thread-like rhizoids.
5 The Plant Fossil Missing Links Cooksonia - late Silurian with terminal sporangiaZosterophyllum - Devonian ancestor of club mosses and ground pines with lateral sporangia near stem tipsRhynia - Devonian ~ 400 MYA (probably the ancestor of all other extant plants) Early 1900's, fossils were foundCalled Rhynia (after the quarry in Scotland).erect dichotomously branched stemssporangia borne at the tips.spread by an underground rhizome bearing rhizoids.
6 Summary of BryophytesEmbryophytes (Like other land plants, produce an embryo); distinct lineage from other land plants.Did not give rise to the vascular plantsbut they probably were the earliest land plants probably evolved from green algal ancestors, closely related to the Charophytes. well-adapted to moist habitats. Similarities to land plantsmulticellular sex organs, ( gametes enclosed by a sterile jacket of cell)sare parenchymatous, not filamentousretain the zygote within the female sex organ for developmenthave cutin (a cuticle) on the plant and sporesDifferences from land plantsno lignin (usually)small, low-lying, (generally)Depend on free standing water for reproductionno true roots, only filamentous rhizoidsdominant generation is the gametophyte; sporophyte is parasitic on the gametophyte.
7 Reproduction in Bryophytes: Vegetative 1. fragmentation - pieces of the gametophyte breaking off (sole means of dispersal in the Arctic)2. gemmae - specialized propagules produced mitotically.
8 Reproduction in Bryophytes: Sexual Often dioecious - male and female sex organs are borne on separate gametophytes.sex organs in clusters - examplessurrounded by sterile hairsborn on the head of a stalkAt maturity, antheridium bursts releasing the sperm cells or antherozoids.can only swim a few cm so that if the archegonia are not adjacent the sperm rely on raindrops to "splash-launch" themchemical exuded by the egg cell attracts the spermwhich swims down the neck of the archegoniumfertilizes the egg.resulting zygote represents the beginning of the sporophyte generation
9 Sporophyte Generation Develops within the archegonium remains parasitic on this for its entire life (except a few moss sporophytes which develop photosynthetic capacity.)Sporophyte generation is short-lived.Capsule - produces spores by meiosisStalk - holds above the body of the gametophyte. HAPLOID spores - dispersed by air currentsGerminate after landing somewhere moistGametophyte generation begins when spore germinatesFirst have filamentous stage (protonema)Cells are full of chloroplasts.Grows into dominant gametophyte
10 EVOLUTION OF WATER TRANSPORT Evolution from aquatic to terrestrial habitatsSupply of waterSystem of deliveryExchange rate of water for carbon is poorEvolutionary trend is innovations for greater efficiency