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Chapter 21 

Evolution of Plants/origins 

Land plants evolved from green algae – the charophyceans and plants share a common ancestor.  Characteristics shared by plants and some protista including green algae: 

▪ Cell walls with cellulose 

▪ Have chloroplasts with chlorophylls a and b  ▪ Multicellularity 

 

Characteristics shared by only the charophyceans 

▪ Rose shaped complexes (arrays of proteins in plasma membrane) for cellulose synthesis  ▪ Percentage of cellulose in cell wall is higher (like plants) than in other groups of algae.  ▪ Peroxisome enzymes 

▪ Structure of flagellated sperm 

▪ Formation of phragmoplast – arrangement of organelles along cell plate during  cytokinesis. 

▪ Genetic (DNA sequence) similarities in chloroplasts.   

Charophycean algae inhabit aquatic environments – many of them in shallow waters that dry up.  Sporopollenin​ – a durable polymer prevents zygotes from drying out. Sporopollenin  encases plant spores (of spore bearing plants today) and it is likely that some 

charophycean algae in shallow waters became the first land plants to live permanently  above the waterline. 

 

Benefits to moving onto land: 

▪ Bright sunlight unfiltered by water and plankton  ▪ Abundant CO​2​ in the atmosphere. 

▪ Initially – there would have been few pathogens and herbivores.   

Boundary for the groups including Charophyceans, plants and Chlorophytes are being debated.  We will not consider Charophyceans as plants. 

 

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  Plants 

Nearly all plants have (that no other group does): (Copy page 576 and 577 for students)  ▪ Apical meristems 

▪ Alternation of generations 

▪ Walled spored produced in sporangia  ▪ Multicellular gametangia 

▪ Multicellular dependent embryos.   

Plants are​ (mostly terrestrial) multicellular eukaryotes that are photosynthetic autotrophs with  cell walls. Plants are further defined as organisms that: have exposed parts to air generally  coated with waxy cuticle, gas exchange occurs through stomata (microscopic openings on leaf  surface), cell walls are composed of cellulose and food is stored as starch. 

 

Plants adapted from an aquatic environment to one on land. Differences required between the  environments included: 

▪ Dealing with water acquisition and retention. 

▪ Supporting photosynthetic parts of plant such that they are exposed to maximum amounts of  sunlight. 

▪ Transport of water/nutrients through plant – nutrients up and products down. 

▪ Fully terrestrial plants must be able to reproduce in areas with no standing water – protecting  against zygote desiccation as well as dealing with lack of medium for movement. 

 

Plants reproduce sexually – most are capable of asexual propagation (grafting). Gametes  produced within ​gametangia​, (organs having protective jackets of non-repro cells that prevent  drying out of gametes during development). 

An alternation of generations occurs in the life cycles of all plants. In all (except bryophytes) the  diploid sporophyte is the prominent individual. 

 

Highlights of Plant Evolution 

▪ Vascularization – cells joined into tubes that transport water and nutrients throughout plant  body. (vascular plants – not mosses and relatives) 

▪ Development of seeds (primitive form – gymnosperms)  ▪ Devlopment of flowers (Angiosperms) 

 

Plant groups 

Division (Phylum) Bryophyta – Liverworts and mosses; are non-vascular  Vascular plants 

Seedless 

Phylum Psilophyta – Whiskferns  Phylum Lycophyta – Club mosses  Phylum Sphenophyta – Horsetails  Phylum Pterophyta – Ferns 

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Seed plants 

Gymnosperms​ (naked-seeded) 

Phylum Coniferophyta – Conifers  Phylum Cycadophyta – Cycads  Phylum Ginkgophyta – Ginkgo  Phylum Gnetophyta – Gnetae   

Angiosperms​ (flowering plants)  Phylum Anthophyta   

Division​ Bryophyta 

▪ Early evolutionary branch of plants, not a stepping stone between algae and vascular plants.  ▪ Have waxy cuticle but are not vascular; can survive desiccation. 

▪ Rely on diffusion and capillary action for distribution of water/nutrients in plant  ▪ Gametes develop in a gametangia; 

Male gametangia – called the antheridium, produces flagellated sperm  Female gametangium – called the archegonium, produces eggs 

Eggs fertilized in archegonium, zygote develops within protective jacket of female organ.  Presence of water is required for fertilization. 

▪ Have no woody tissue – are limited to a small height. 

Life cycle​ – Gametophyte dominant, developing from protomata – outgrowth on spores;  Archegonia and anteridia develop on female/male plants, fertilization occurs when water  allows sperm to move to archegonium. Diploid sporophyte develops in archegonium and  sends out a stalk – the capsule contains the sporangium where meiosis produces spores.  ▪ Life cycle similar to moss – but can also reproduce by releasing little bundles of cells called 

gemmae. 

▪ Rootlike structures on moss called rhizoids 

▪ Areas where moss has existed for quite some time are referred to as “peat bogs”; peat is dug  up and burned for energy. 

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Vascular plants 

Further adaptations of include: 

▪ Lignin – hard material embedded in the cellulose matrix – (woody material); this allows for  larger individuals. 

▪ Body divided up into a subterranean root system (for nutrient/water uptake) without a cuticle  and aerial shoot (for photosynthesis) with cuticle. Aerial shoot divided into stems and leaves.  ▪ Transport in individuals is via the ​Xylem​ (dead cells that form a network of tubes that carry 

water and nutrients up) and ​phloem​ (living cells that distributes sugars and organic nutrients  throughout the plant). 

▪ Very common in carboniferous period. Photosynthesis increased with these organisms and  they consumed a great quantity of CO2, causing global cooling and widespread glacier  formation. 

▪ The first seedless plants are what formed coal – they did not decay fully in the stagnant  swamps, was covered by peat and then the sea…sediments built up, heat and pressure  converted them to coal and other hydrocarbons. 

 

Early Vascular Plants. 

Phylum Lycophyta​ – Club Mosses 

▪ ancient group present during Devonian, prominent during Carboniferous Period (340 mya to  280 mya). 

▪ Split into two evolutionary lines – woody trees that later became extinct, and the small  herbaceous plants that are found today (although in a reduced number of species!)  ▪ many are epiphytes, plants that use another organism as a substrate but aren’t parasites, 

others grow on ground 

▪ has a horizontal rhizome that gives rise to roots and true (vascularized) leaves. 

▪ Sporophyte is dominant with spores being born on sporophylls – specialized leaves in shape  of “clubs” – these haploid spores germinate into gametophytes that reside with symbiotic  fungi. 

▪ Some are homosporous – producing bisexual spores that develop into gametophytes with  both male and female organs (antheridia and archegonia); others are heterosporous, they  produce spores that develop either an antheridia or archegonia. 

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Phylum Pterophyta 

▪ No longer considered to be separate phylums due to molecular comparisons.  ▪ Nearly all species are homosporous. 

▪ Ferns are the most numerous of the primitive vascular plants.   

Whisk ferns (old name) 

▪ True roots and leaves are absent because they contain no vascular tissue; subterranean part  has a rhizome with tiny branches called rhizoids. 

▪ Stem has some vascular tissue. 

▪ Stems have sporangia that release haploid spores that germinate in soil; flagellated sperm  move from antheridia to archegonia of the gametophytes; gametophyte resides with  symbiotic fungi - as soon as a sporophyte emerges from the gametophyte – the latter dies.   

Horsetails 

▪ Ancient group present during Devonian, being quite common during the Carboniferous  Period, one genus (​Equisetum​) w/20 species survives today. 

▪ Has underground rhizome that has roots and gives rise to vertical stems. Whorls of leaves  are at the joints of the hollow stems. 

▪ Silica is imbedded in the cell wall. 

▪ Sporophyte generation dominant; sporangia develop at tips of some stems and produce  homosporous spores. 

▪ Gametophytes are photosynthetic and free-living.   

Ferns 

▪ Largest group of the seedless plants w/12,000 species or so. 

▪ Leaves – called fronds – are compound (divided into several leaflets) and unfurl from  fiddleheads as they grow. 

▪ Some species are quite large and woody. 

▪ Sporophytes (plants we see) have sporangia on undersides arranged in clusters called sori.  Spores are “sprung” from fronds and carried by wind – germinate into a Prothallium in the  soil. the prothallium develops rhizoids and produces (in Antheridium or Archegonium)  gametes that swim using a flagella in water (needed for repro!). The zygote develops into a  sporophyte on the gametophyte (from the Archegonium). 

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Specialized tissues in Plants 

▪ Generalized parts of a plant (see figure 23-4 in text) (Include flower, apical meristem, stem,  leaf blade, petiole, shoot, root tip, root system. 

▪ Growth: Cells in plants don’t usually divide to provide growth – instead Meristematic Tissue  consists of cells that do; located where growth occurs – these cells have thin cell walls and  divide (mitosis) quickly. They are often protected by a cap that precedes the growing cells -  like in roots…so the dividing cells don’t get damaged. 

Apical meristem​: stem and root – allow elongation  Cork cambium​ allow stems/roots to grown in diameter;  vascular cambium: adding vascular tissue to stems and roots  Pericycle​: affords for root branching and incrase in thickness. 

Epidermal tissue​ – tissue on surface of plant (stem, root or leaf) function is different  depending on where found; bark vs. root surface. 

 

Parenchyma​: unspecialized cells with thin primary cell walls (and no secondary cell walls)  that have large vacuoles. Some have chlorophyll where photosyn. takes place. 

 

Collenchyma​ cells: lack secondary walls (like parenchyma), but with thicker primary walls  than parenchyma – they support but don’t restrain growth. 

 

Schlerencyma​: cells with tough cell walls that include lignin and strengthen and support  plant – but do restrain growth and are found in parts of a plant not growing. two forms;  fibers​ – in bundles (we use as rope!) or ​sclereids​ that are irregular in shape (in nutshells).   

Vascular tissue​: 

Xylem​ – called tracheids in seed plants that consists of hollow dead cells that form a tube  network for water transport. 

Vessel elements​ are also found in angiosperms – these are continuous tubes through  which water can flow. 

Phloem​ – transports nutrients/etc. throughout plant, consisting of: 

Sieve tube elements​: Cells arranged end on end with perforated cell walls (called  sieve plates) on the ends. These cells lose most of their organelles and cytoplasm  can move between them; these cells have no nucleus or ribosomes 

Companion cells​: cells that surround the sieve tube elements – believed to  control the activity of the sieve tube elements via their nucleus/ribosomes.   

Roots 

Three tissues​; 

epidermis​ (thin – takes in nutrients by active transport – requiring energy and 

oxygen…water is a problem if continuous…as is compaction of soil; water is taken in by  osmosis) 

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The Casparian strip is not permiable to water; it may pass only through them. Nutrients  pumped across, water follows – both are then trapped in the vascular cylinder. (see pg  497 in text).   

vascular cylinder​ (contains xylem and phloem). transports water/nutrients to stems. A  pressure builds up from the action of the cortex. this is sufficient to move upwards  through plants unless trees…(we’ll discuss that later). 

Primary root​; first root grown by a plant. 

Secondary root​: roots that branch out from the primary root. 

Taproot​; Large primary root grown by some plants. Often, the taproot can sprout new  stems/leaves if portion of plant above ground removed. Some use taproot for storage of  carbohydrates (like carrot – for example). Some plants have very long taproots to reach  water. 

Fibrous roots​: network of secondary roots of plants that don’t have taproots - in these, the  secondary roots also branch (like grass roots for example). 

  Stems 

▪ Functions: Hold plants up; Conduct nutrients/water between roots and leaves.  ▪ Four basic tissues: Parenchyma, vascular tissue, cambium tissue, cork. 

▪ Xylem is closest to the center, Phloem surrounds Xylem. note that it is xylem that makes up  the rings of trees – that’s why its wet (green). 

▪ Go over figure 23-15 on page 500; tree stem consists of several layers; from outside to  inside are: cork – produced by cork cambium under the dead stuff, phloem, vascular 

cambium tissue – produces xylem and phloem, sapwood (active xylem), heartwood (inactive  xylem).  

  Leaves 

▪ Function as primary photosynthesis part of plant.  ▪ leaf = blade, attached by a ​petiole​. 

▪ see page 503 – generalized leaf. Leaves are covered by a cuticle to prevent moisture loss,  and have openings called stomas on the underside of the leaf to afford for gas exchange.  Stomas are closed/opened by guard cells by water pressure. Plants close stomas when  moisture loss becomes a problem. 

▪ Leaf vascular tissue is connected to the xylem and phloem of the stem. 

▪ Most of a leaf consists of mesophyll – Palisade cells – lots of chloroplasts, spongy  mesophyll, affords for gas exchange to all cells. 

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Seed Plants – Spermopsida 

Seeds are structures that protect the zygotes of seed plants. After fertilization, the zygote grows  into a tiny plant called an embryo – the embryo stops growing while in the seed – awaiting  favorable conditions. Food is stored in the seed and a seed coat surrounds it and the food,  protecting it from drying out. The seed can survive rather tough conditions…and has allowed  seed plants to thrive in areas other types of plants have not been able to survive. 

 

Includes two main groups – Gymnosperms and Angiosperms  Gymnosperms – naked seed plants 

Angiosperms – jacketed seed plants   

Both 

▪ have gametophytes that develop completely within flowers or cones. 

▪ have the male gametophyte as pollen; sperm produced by the male gametophyte don’t swim  to the female gametophyte, rather, they are carried there by wind or pollinators 

▪ The female gametophyte is also in a cone or flower, when a pollen grain reaches it – it has  been “polloninated” 

 

Gymnosperms 

Seed plants evolved from ​seed ferns​, as the Earth became increasingly dry. Seed ferns (fossils  indicate) were very similar to ferns of today – except they produced seeds rather than spores. No  seed ferns survive today. 

 

In all; some leaves have evolved into specialized reproductive structures called ​scales​. Scales  are grouped into ​cones​. Male cones produce male gametophytes called pollen, female cones  produce female gametophytes called eggs and later (after pollination) the female cone holds  seeds that sit on the scales. Because they sit, exposed on top, they are called “Naked”.   

Seed plants belong to one of three classes:  Cycadae 

Present in warm, temparate regions, they thrived during the Mesozoic Era (225 mya to 100 mya  or so). Represented by nine genera now. In the US, can be found in Florida.   

 

Ginkgoae 

Common during Mesozoic, but now only a single species, ​Gingo biloba​, survives. It is probably  the oldest seed plant alive today – looking exactly like those found in the fossil record. 

 

Coniferae 

Called conifers or evergreens, are the most abundant gymosperms today 

▪ Leaves are long and thin; leaves are shed in a cycle (usually 2-14 years), although a few  species drop all needles on an annual basis (like the Larch). 

▪ Scales that form cones contain structures called sporangia that produce gametophytes.  ▪ Male cone (Pollen cones): Sporangia cells – meiosis – pollen grains. 

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divides mieotically to become a megaspore (only one of the 4 haploid cells survives). This  megaspore divides mitotically to produce a large immature gametophyte within the nucellus  mass; within the megaspore, two or three archegonia develop – each producing an egg.  ▪ The pollen grain – usually wind carried in gymnosperms – arrives at the micropyle and 

digests its way through the nucellus through a pollen tube. After this is complete – the male  gametophyte matures and produces sperm that fertilize the egg. An embryo then develops.   

Note​: Pollination is not the same as fertilization! Pollination occurs when pollen is transferred  from a male cone to a female cone! Fertilization occurs when sperm combine with an  egg…this doesn’t occur until quite some after pollination. 

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Chap. 25 – Angiosperms. 

Unlike Cone-bearing plants, all angiosperms are in one phylum: ​Anthophyta  Formerly classified as being in one of two classes: monocotyledon or a dicotyledon 

Now, however, it seems that evolutionary relationships aren’t quite as cut and dried. (Pass out   Page 602 and 603) – 6 clades, with monocots and eudicots comprising most of the plant species.  Read over page 602 (Have students read) 

Compare and contrast moncots vs. Eudicots​; characteristics of both… 

▪ Dominant forms of plants – grow in most land environs and many water environs.  ▪ Key adaptations are the flower and fruit. 

 

Flowers (Figure 30.7) 

▪ The flower is a specialized shoot that can have up to 4 rings of modified leaves called floral  organs including the – sepals, petals, stamens, and carpels. All of these parts are attached at  the ​receptacle​. (Fig. 535) 

Sepals​: often are green and look like leaves, located under the petals. They enclose the  flower and protect it while growing. Together, the sepals form the ​calyx​. An easy  example to see are those of the dandelion….it closes up every night – inside the calyx.  ▪ Petals​: second circle of parts – together form the ​corolla​ – often colored and/or modified 

in some way to attract pollinators. 

Stamens​: consist of a ​filament​ (like a stalk) and an ​anther​ (contains ​microsporangia that  produce pollen​). Arranged inside the petals (sometimes fused to petals). 

Carpels​: Located at the center of the flower, contains ​megasporangia that produce eggs​.  Carpels are found together in the ​pistil​ (fused or unfused). The pistil consists of the  ovary (base), style (a stalk) and the stigma ​(top of style where pollination occurs).  ▪ Sometimes flowers have both male and female organs - sometimes they are separate - corn 

has two types of flowers on same plant for example. Some plants (like willows) have either  male OR female flowers. 

▪ Flowers have, in many cases, coevolved with pollinators into unique shapes and they exhibit  coloration or odors that attracts pollinators. Example, skunk cabbage, orchids, daisies.   

Fruits​ - include (for example) walnuts, tomatoes, oranges, peaches, blueberries…grapes 

▪ Consists of a mature ovary and sometimes other flower parts. Pea is a fruit with seeds (peas)  in the ripened ovary (pod). 

▪ Wall of the ovary becomes the pericarp – thickened wall of fruit.  ▪ Can be fleshy (like an apple) or dry (like wheat). 

▪ Fruits help disperse seeds – they are eaten in their entirety, with the pericarp being digested  and the seed within passing through (bear poop) and being distributed…others hitch rides.   

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Life Cycle 

Pass out Figure 30.10 campbell and reece  Male gametophyte 

Inside anthers, the microsporangia produce microspores via meiosis that become pollen grains  with tough walls. The nucleus of each pollen grain divides to become a tube nucleus 

(disintigrates) and a generative nucleus that produces two sperm. At this point – pollen grains  become dormant and only continue growth if they land on a stigma. 

 

Female gametophyte 

In the megasporangium of each ovule meiosis occurs – one megaspore survives and becomes  the embryo sac. Inside this sac is the egg and polar bodies (from meiosis). This sac is called the  female gametophyte. 

 

Pollination 

Pollen from the anther is released and arrives at the stigma. (Cross-pollination is pollination  between plants, self-pollination is of same plant.) The pollen grain then grows a pollen tube  through the style; the generative nucleus produces two sperm that arrive via the tube at the egg.   

Fertilization 

Combining of the sperm with the egg. Note that in ANGIOSPERMS ​one sperm fertilizes the  egg​ and ​the other combines with polar bodies to form a triploid endosperm​. The endosperm  grows to provide stored food for the zygote. We eat the endosperm of many…grains,  

 

After fertilization 

A seed coat develops about the embryo and food supply 

the ovary often grows larger and fuses with other flower parts to form a fruit  Embryonic seed leaves and a rudimentary root develop. 

 

Seed distribution…   

Seed germination: water results in the seed coat swelling and emergence of the cotyledon(s); a  root. 

 

Vegetative reproduction in plants: 

cuttings, layerings – cut part of a stem – put in soil or water, roots develop (some plants).  grafting – for seedless varieties, part of plant stem is cut and put into another plant’s stem in a  splice. They grow together. 

Stolons, rhizoids, bulbs (that can differentiate into several starters – like garlic), roots (potato  buds). 

References

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