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The Transition to Terrestrial Life

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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 plants Desiccation   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 generations probably heterotrichous. Selection favors individuals more able to withstand periods without submergence (e.g. at pond margins, on wet mud) Gametophytes need water for reproduction basal 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 water 5. Appearance of first conducting tissues 6. 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 coat Evolution of gymnosperms (coexist with ferns for 200 MY Cretaceous (~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 water Transport 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 body Ventilation system - evolution of stomata -pores in the plant surface - and intercellular air spaces for transport of gasses Anchorage 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 sporangia Zosterophyllum - Devonian ancestor of club mosses and ground pines with lateral sporangia near stem tips Rhynia - Devonian ~ 400 MYA (probably the ancestor of all other extant plants) Early 1900's, fossils were found Called Rhynia (after the quarry in Scotland). erect dichotomously branched stems sporangia borne at the tips. spread by an underground rhizome bearing rhizoids. 

6 Summary of Bryophytes Embryophytes (Like other land plants, produce an embryo); distinct lineage from other land plants. Did not give rise to the vascular plants but 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 plants multicellular sex organs, ( gametes enclosed by a sterile jacket of cell)s are parenchymatous, not filamentous retain the zygote within the female sex organ for development have cutin (a cuticle) on the plant and spores Differences from land plants no lignin (usually) small, low-lying, (generally) Depend on free standing water for reproduction no true roots, only filamentous rhizoids dominant 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 - examples surrounded by sterile hairs born on the head of a stalk At 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" them chemical exuded by the egg cell attracts the sperm which swims down the neck of the archegonium fertilizes 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 meiosis Stalk - holds above the body of the gametophyte.  HAPLOID spores - dispersed by air currents Germinate after landing somewhere moist Gametophyte generation begins when spore germinates First have filamentous stage (protonema) Cells are full of chloroplasts. Grows into dominant gametophyte

Evolution from aquatic to terrestrial habitats Supply of water System of delivery Exchange rate of water for carbon is poor Evolutionary trend is innovations for greater efficiency

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