Plants and the Colonization of Land AP Biology – Chapter 29

Evolution of Plants There were NO PLANTS until about 500 million years ago. Until that time all life had been essentially aquatic… Terrestrial Earth – barren It’s hard to Imagine…

Evolution of Plants WATER TO LAND!!!!!!! The evolutionary history of plants is a story of ADAPTATION TO TERRESTRIAL CONDITIONS from water. WATER TO LAND!!!!!!! Water… To Land

Since their beginnings, Plants have been very successful… Plants have diversified into about 290,000 living species.

The Ancestors of Land Plants… Were most likely some type of Green Algae

Relatives to ancient plants – Green Alage Chara Coleochaete “Kingdom” Protists Spirogyra Volvox

The closest algal relatives of land plants today… Charophyceans (Charophytes) It is likely that plants and charophytes share a common ancestor. In other words, charophytes did not give rise to plants, but share a common ancestor with them. This makes charophytes the nearest living algal relative to plants.

Plant and Algae shared characteristics These characteristics are shared with many algae. Multicellular Eukaryotic Photosynthetic autotrophs Cellulose cell walls Chlorophylls a and b present Store surplus carbs in form of STARCH What then is it that links plants specifically to charophytes? Evidence for the green algae/plant connection

What links the plants to the charophytes? 4 additional characteristics Shape of the complexes that make cellulose (rose – shaped) Enzymes contained in the peroxisomes Sperm structure is similar Certain details of cell division processes Also…the DNA!

Ancient Algae and the Water to Land Transition Some ancient algae (like modern charophytes) inhabited shallow waters at edges of ponds/lakes Survived occasional drying Had coatings on spores to protect from drying Natural selection would favor individual algae that can survive periods OUT of water. Led to ADAPTIVE RADIATION of plant ancestors onto LAND

Why move to Land? Land has some big assets! Bright sunlight unfiltered by water Abundance of CO2 in atmosphere Soil rich in mineral nutrients Few herbivores and pathogens (at least at first!)

Problems in taking advantage of Land’s assets… However…to take advantage of these assets plants had to figure out how to do without water!! This means: Prevent Bodies from drying out… Prevent Gametes from drying out Fertilization without water Prevent Embryos from drying out PLANTS have solutions to these problems. ALGAE do NOT.

So, to distinguish plants from Algae we use… …the solutions to the problems of land living that plants developed but that water-living algae did NOT develop. Adaptations to land

Problem 1 – accessing resources on land Location of Resources in TWO Very Different Places Above ground – light, Below ground – water and minerals How to access these two different environments simultaneously is a huge problem Remember – algae do NOT have this problem Continually bathed in uniform water environment No need to protect tissues from drying out All tissues have equal access to light, dissolved nutrients, etc.

Problem 1 – accessing resources on land: SOLUTION Apical Meristems Small regions of growth in tips of roots and shoots increase access to resources in different directions: soil/light. Cells produced in these meristems differentiate into tissues suited for these two different environments. Example: Protected epidermis for leaves exposed to air Cuticle; stomata Roots unprotected and designed for absorption Apical Meristems

Problem 1 – accessing resources on land: SOLUTION Access to light and soil, generally require a plant to gain length in both directions But how to get substances transported across long distances within the plant’s body? VASCULAR TISSUE Tissue that can move LARGE QUANTITIES of material through a plant Without it, osmosis is the only alternative SLOW; limits size a land organism can attain.

Apical Meristems

Problem 2: Evaporation of water from tissues on Land - Algae is constantly bathed in water Evaporation from tissues is NOT a concern Also, algae tissue freely accepts nutrients from water thus no need for access to nutrients in soil No need for support – water supports the algae and floats it near the surface to receive light Algae on land would rapidly lose water from tissues with no way to replenish; also, access to light would be limited without support structures

Problem 2: Evaporation of water from tissues on Land Solution Protective tissues Epidermis and secretion of waxy cuticle Prevents water loss Stomata Allows gas exchange even though cuticle is covering the plant Cross-section through a leaf

Problem 3 – Drying out of sex cells and embryos Aquatic organisms do NOT have to “worry” about Gametes drying out during development Gametes drying out when released Embryos drying out during development Embryos drying out when released LAND organisms DO have to “worry” about these things. Water > Land transition required solving these problems

Problem 3 – Drying out of sex cells and embryos: Solution Protection of Gametes during Production Gamete production is protected inside structures called gametangia Prevents drying out of sperm/egg Dispersal In more advanced plants, gametes are protected during dispersal in structures (pollen) that can travel through air. NOT dependent on swimming sperm.

Problem 3 – Drying out of sex cells and embryos: Solution Protect delicate Embryos from drying out, too! Egg fertilization occurs inside gametangium Zygote develops into embryo inside gametangium Embryo is retained and nourished inside gametangium Embryos dispersed in structures that also prevent drying out: Spores Seeds

However, solutions to all these problems were NOT achieved at once… These solutions were generally achieved in STEPS We can use these steps in evolution of land adaptations as a way to determine a “plant family tree” or PHYLOGENY

A new period of adaptive radiation followed Green Algae Moss Ferns Conifers Flowering Plants Each of these circled branch points indicates some new adaptation that led to even more success on land… A new period of adaptive radiation followed Algae/Plant Common Ancestor

Plant Classification Domain Eukarya Kingdom Plantae (currently excludes the algae, but there is DISAGREEMENT about this!) Some think green algae should be included within the plant kingdom! 10 major phyla (actually called divisions in plants) 3 of these are nonvascular (bryophytes) 7 are vascular and include the vast majority of plants (93%)

Generalizations Regarding Plant Groups Plants can be loosely and informally placed into two major groups Nonvascular (the bryophytes/mosses) More ancient / primitive Rely on cell to cell diffusion for transport Small (like mosses) No true roots, stems or shoots Vascular (everything else) More recent / advanced Use sophisticated “plumbing” (vascular tissue) to move food/water Can become quite large True roots, stems and shoots (by definition, roots/stems/shoots are all structures that contain vascular tissue)

Generalizations Regarding Plant Groups Vascular Plants can be informally grouped into Seedless Vascular Plants First to arise from nonvascular ancestors Possess (obviously) vascular tissue, but that’s about all that sets them apart from nonvascular plants Gymnosperms Naked seed plants / Conifers – 1st seeds, but seeds unprotected Angiosperms Flowering plants – seed development protected inside ovaries

FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors Living groups from this first period of plant evolution: Division Bryophyta – the mosses 10,000 species Division Hepatophyta – Liverworts 6,500 species Division Anthocerophyta – Hornworts 100 species

Typical Bryophyte (moss) life cycle Plant Life Cycles called ALTERNATION OF GENERATIONS Life cycle alternates between two different multicellular bodies, with each form producing the other. One body form is haploid The other body form is diploid Do NOT confuse with haploid/diploid stages of animals, etc. The haploid form in plants is a multicellular body, not just a gamete.

Alternation of Generations – Why do we care? ALL plants undergo this pattern called alternation of generations Why do we care? Differences in plant life cycles give us important clues about evolutionary advances of each major plant group

Haploid spores land in suitable environment Sperm must SWIM to egg DIPLOID Sporophyte grows out of haploid female gametophyte Egg/ ovum MITOSIS Fertilization In female gametophyte creates DIPLOID ZYGOTE MEIOSIS occurs in spore capsule Male Gametophyte Haploid generation Female Gametophyte Haploid generation Spore capsule opens and releases HAPLOID spores Haploid spores germinate and grow into haploid gametophytes Haploid spores land in suitable environment

FIRST Period of Plant Evolution: Origin of Plants from Aquatic Ancestors Adaptations to Terrestrial Life Cuticle Jacketed gametangia protect gametes and embryos BUT, still tied to water because… Still have swimming sperm MUST have water for fertilization to occur LACK vascular tissue Means no real means of efficiently conducting food and water Size is limited – very small No seeds – spores used for embryo dispersal Gametophyte is dominant generation Diploid generation is present ONLY as ZYGOTE!!

Moss

2nd Period of Plant Evolution – Diversification of Vascular Plants Group known as SEEDLESS VASCULAR PLANTS Adaptations From Previous Group: Cuticle and gametangia of previous plants New to SEEDLESS VASCULAR PLANTS: Sporophyte is dominant Generation VASCULAR TISSUE added!! Can efficiently move food and water throughout body Can attain larger size Still share these primitive characteristics: Still depend on swimming sperm Still no seeds – use spores for embryo dispersal

2nd Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of plant evolution Division Psilophyta – whisk ferns 10-13 species Division Lycophyta – club mosses 1,000 species Division Sphenophyta – horsetails 15 species Division Pterophyta – ferns 12,000 species

Ferns

Ferns

3rd Period of Plant Evolution – Diversification of Vascular Plants Group known as GYMNOSPERMS or “Naked Seed Plants” Adaptations From previous groups: Cuticle, gametangia, dominant sporophyte and vascular tissue NEW to GYMNOSPERMS: SEEDS – to further protect embryos from dessication (and other hazards) and aid in dispersal. Pollen – air transported sperm – no longer need water to transport swimming sperm Still primitive in that… NO Flowers NO Fruit

3rd Period of Plant Evolution – Diversification of Vascular Plants “naked seeds” refers to seeds NOT being enclosed in any specialized chambers (fruit) Seeds are exposed to environment on highly modified leaf surfaces (ex. Cones) A seed consists of Embryo Stored food (starch) Protective covering (seed coat) Pollen – airborne sperm (sort of) Highly reduced male gametophyte No longer dependent on water for fertilization

3rd Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of plant evolution Division Coniferophyta (conifers) Largest group Needle-like leaves 550 species Division Cycadophyta 100 species Division Gingkophyta 1 species Division Gnetophyta 70 species

4th Period of Plant Evolution – Diversification of Vascular Plants Emergence of Flowering Plants - Angiosperms Adaptations From previous groups: Cuticle, gametangia, dominant sporophyte, vascular tissue, seeds and pollen of previous groups NEW to ANGIOSPERMS: FLOWERS FRUITS Double fertilization Fert of egg to make embryo Fert of 2 polar nuclei to make endosperm TRIPLOID tissue (endosperm – stored food 2 polar nuclei + 1 sperm nuc. = 3n

4th Period of Plant Evolution – Diversification of Vascular Plants Advantages of flowers and fruits Flowers draw pollinators which are very efficient at transferring pollen from flower to flower Fruits aid in seed dispersal either by water, wind or animals Other plants depend on water or wind for gamete and seed dispersal which is not near as precise. Angiosperms may use wind also, but fruits allow for elaborate structures to carry seeds FURTHER by wind.

4th Period of Plant Evolution – Diversification of Vascular Plants Advantage of triploid tissue in seeds Has greater ability to pull food resources from “mother” than embryo does. Thus nutrients can be stored in the seed for later use. Also, fact that endosperm must be fertilized ensures that endosperm will not be made if fertilization has not occurred. Avoids waste.

4th Period of Plant Evolution – Diversification of Vascular Plants Living groups from this period of evolution ONE Divison – Anthophyta 235,000 species Two Important Classes Monocot Dicot

Double Fertilization!!

Alternation of Generations Haploid gametophytes and diploid sporophytes take turns producing one another. Gametophyte Plant that makes GAMETES HAPLOID PLANT MAKES HAPLOID CELLS Sporophyte Diploid plant Cells inside undergo MEIOSIS Plant that makes SPORES – HAPLOID CELLS Why make spores? Efficient at dispersal of “babies” Wind dispersed

Evolutionary Trends in Alternation of Generations Initially in the evolution of land plants, gametophytes were the dominant generation. Larger than sporophyte Sporophyte dependent on gametophyte for everything – sporophyte NOT photosynthetic

Evolutionary Trends in Alternation of Generations Trend in plant evolution was for SPOROPHYTE to become dominant and GAMETOPHYTE to become dependent Why? Diploid tissue is less subject to the negative effects of mutation than haploid tissue In diploid tissue there are two copies of genes. If one gets damaged, the other may be able to function normally.

Why Study Plant Life Cycles? Features of the life cycles are key evolutionary adaptations to terrestrial environments Example: replacement of flagellated sperm by pollen

Egg/ ovum ________ ____________ occurs in spore capsule