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Plant Growth Summary Primary Growth Primary growth is the lengthening of the stem and roots. All plant growth occurs by cell division and cell elongation. Cell division occurs primarily in regions of undifferentiated cells known as meristems. The meristems make it possible for plants to have indeterminate growth. Cell division in the apical meristems and subsequent elongation and maturation of the new cells produces primary growth. Secondary Growth Secondary growth is the increase in girth of stems and roots. Cell division occurs in the lateral meristem; hence, this phase is also referred to as lateral growth

Primary Growth of Stems The apical meristem produces the three primary meristems, protoderm, procambium, and ground meristem, which develop into dermal tissues, vascular tissues, and ground tissues respectively.

Primary Growth of Roots In roots, the three primary meristems, protoderm, procambium, and ground meristem, develop from the apical meristem and differentiate into epidermis, vascular tissues, and ground tissues.

Lateral Meristems In woody plants, secondary growth of stems and roots occurs through the activity of two lateral meristems: the vascular cambium and the cork cambium. Secondary growth occurs in all gymnosperms and most angiosperms, including most dicots but few monocots.

Secondary Growth Each time a cambium cell divides, one daughter cell, the initial, remains part of the cambium and the other daughter cell, the derivative, undergoes differentiation. Secondary xylem develops into the wood of a tree trunk. The secondary phloem is a thin layer to the outside of the vascular cambium.

Gymnosperms Gymnosperms are the non-flowering seed plants Gymnosperms are woody plants that bear "naked seeds." They are called naked because their seeds develop exposed on the upper surfaces of cone scales, such as in pine cones A pollen grain is carried by wind currents to the appropriate "egg" where the growth of the pollen tubes through this tissue brings the sperm to the egg Gymnosperms are usually of large size with much secondary growth, the leaves are usually evergreen needles or scales

Angiosperm Angiosperms have flowers and bear seeds enclosed in a protective covering called a fruit Angiosperms are further divided into monocots and dicots. There are at least 250,000 species of angiosperms ranging from small flowers to enormous wood trees. Pollination is accomplished by wind, insects, and other animals. The male part is the pollen grain, and the female part is the ovary.

Characteristic Monocots Dicots Cotyledons (seed leaves) One Two Vascular bundles in stem Scattered In a ring Leaf venation Parallel Netlike Floral parts Usually in 3s Usually in 4s and 5s Roots Fibrous roots Taproots Additional Fewer than 10% of species are woody About 50% of species are woody

The Flower Petals: brightly colored, modified leaves found just inside the circle of sepals; attract animals that will pollinate the plant Sepals: outermost circle of leaves; are green and closely resemble ordinary leaves; enclose the bud before it opens and protects the flower while it develops Pistils or carpels: female part of the flower; produce the female gametophytes; each consists of an ovary, stigma and style Ovule: the structure within the ovary where the ova (female gametophytes) are produced Ovary: swollen part of pistil that contains the ovule, where one or more ova are produced Style: long, usually thin stalk of pistil Stigma: sticky top of the style where pollen lands and germinates Stamen: male part of the flower, made up of anther and filament Anther: male part of the flower where sperm (pollen) are produced by meiosis Filament: threadlike structure that supports the anther

Gymnosperm vs Angiosperm Characteristics Gymnosperms Angiosperm Naked seeds X Seeds inside a fruit X Flowering plants X Produce cones X Produce fruits X Wind pollination X X Insect pollination X Examples Ginkgo, pine, redwood, hemlock, firs Corn, grasses, rose, tomatoes, apples

Control of Growth in Plants Plant hormones are also known as phytohormones . They are found in plants in very low quantities. Hormones help in different activities of a plant like flowering, senescence (aging) and ripening of fruits. Each hormone in a plant has a specific function to perform. There are five hormones which help in different processes in plant growth and development. They are: Auxins Gibberellins Cytokinins Ethylene Abscisic acid

Auxin Auxins are the plant hormones which help in maintaining apical dominance in plant This hormone helps in stem elongation in a plant by stopping lateral buds to grow Auxins are always produced in the root, shoot and bud tip Lateral buds remain inactive because of auxins and apical buds grow fast So, if the apex or tip of a plant is taken out or cut, no auxin is produced in that plant after that for some time and this promotes lateral growth of the plant Anti-auxins regulate the activity of auxins Controlled by phototropism and geotropism

Phototropism The tendency of the shoots of plants to bend toward light sources due to unequal distribution of auxins This enhances apical dominance, the preferential growth of a plant upward (toward the sun) It also stimulates stem elongation and growth by softening the cell wall It is the first plant hormone discovered Indoleacetic acid (IAA) is a naturally occurring auxin A human-made auxin, 2,4-D, is used as a weed killer

Geotropism Growth of portions of plants towards or away from gravity Negative geotropism gravity will increase the concentration of auxin on the lower side (if turned horizontally); thus supplying unequal auxin the lower side will grow faster than the upper side; hence the shoot grows upward Positive geotropism roots grow toward the pull of gravity higher concentration of auxin inhibits growth inhibit development of lateral bud initiate formation of lateral roots

Gibberellins These are the plant hormones which carry out or help in cell division and stem elongation Gibberellins also help in breaking the dormancy of seed and can delay aging and death of leaves and fruits Work together with auxins to promote cell growth Induce bolting, the rapid growth of a floral stalk. When a plant, such as broccoli, which normally grows close to the ground, enters the reproductive stage, it sends up a very tall shoot on which the flower and fruit develop. This is a mechanism to ensure pollination and seed dispersal

Cytokinins These hormones promote cell division, growth and differentiation in a plant Most of the time they combine with other plant hormones like auxins or ethylene and regulate different metabolic activities like leaf formation, mitotic division, differentiation and branching This hormone also aids in seed germination Work anatagonistically against auxins in relation to apical dominance Delay senescence (aging) by inhibiting protein breakdown (Florists use this agent) Produced in the roots and travel upward in the plant

Ethylene Ethylene is a gaseous plant hormone Many fruits produce ethylene when they ripe or mature. Actually ethylene is a hormone that helps in ripening of fruits It also sometimes inhibits growth and triggers programmed cell death It helps in the opening of flowers Apples and pears have ethylene when they are ripe. Some farmers and fruit sellers use artificial ethylene to ripen the raw fruits

Abscisic Acid (ABA) It helps in the process of development, mainly bud dormancy ABA compound occurs naturally in plants It regulates the opening and closing of stomata This plant hormone is also known as stress hormone in plants as it helps the plant to adapt well in stress conditions of water etc. Works in opposition to the growth-promoting plant hormones Counteracts the breaking of dormancy during a winter thaw

Strategies that Enabled Plants to Move to Land Cell wall made of cellulose lend support Roots and root hairs absorb water and nutrients from the soil Stomates open to exchange photosynthetic gases and close to minimize water loss Waxy coating on the leaves, cutin, helps prevent water loss from leaves In some plants, gametes and zygotes form with a protective jacket of cells called gametagnia that prevents drying out Sporopollenin, a tough polymer, is resistant to almost all kinds of environmental damage and protects plants in a harsh terrestrial environment Seeds and pollen have a protective coating that prevents desiccation Reduction of the primitive gametophyte (n) generation occurs

Asexual Reproduction Undifferentiated tissue (meristem) in plants provide a source of cells from which new plants can develop Vegetative propagation: high reproduction rate, lack of genetic variety, and the ability to produce seedless fruit; either naturally or artificially There are two forms of vegetative propagation: Natural Artificial

Natural Forms Bulbs: parts of the root that split to form several new bulbs e.g. onion, tulip Tubers: modified underground stems have buds e.g. carrot, parsnip Runners: plant stems that run above and along the ground e.g. strawberry Rhizomes: Stolons (woody, underground stems) reproduce new upright stems

Artificial Forms Cutting: when cut, a piece of stem of some plants will develop new roots in water or moist ground e.g. geranium, willow, dahlia Layering: stems of certain plants, when bent onto the ground and covered by soil, will take root e.g. blackberry, raspberry Grafting: stem of one plant (scion) is attached to the rooted stem of another closely related plant (stock); in order to work, the cambiums of both stems must be in contact

Sexual Reproduction Diploid (sporophyte) and haploid (gametophyte) generations go through a process called the alternation of generations

Gametophyte Male Pollen grain develops from the spores Transferred from the anther to the stigma Use insects, wind, and water to cross pollinate Pollination of spore on the stigma creates a pollen tube (uses food and water) In the pollen tube, are tube nucleus and two sperm nuclei; all are haploid Female Develops in the ovule from one of four spores Embryo sac contains nuclei, including the two polar (endosperm) nuclei and an egg nucleus

Pollination We define pollination as the transfer of pollens from anther to the stigma of the same flower or different flower. Formation of fruits from flower depends on fertilization and this can be carried out only after pollination . Types of Pollination: Self pollination - There are a few plants which can pollinate themselves. This is transfer of pollen to the stigma of the same flower. Stigma is sticky and therefore receives the pollen easily. E.g. legumes, peanuts, tomatoes and peas Cross pollination -This is a process in which pollen of one flower is transferred to the stigma of another flower. Plants which are adapted to cross pollination have taller stamens than carpels, so that pollens can spread and reach different flowers for fertilization.eg-pumpkins and cucumber. Hydrophily -This is a rare form of pollination in which pollen reaches the stigma by the flow of water, so this is a transfer of pollen by water.eg-species of waterweed and pondweed. Anemophily -This is a type of pollination where pollen is distributed by wind.eg-grass, conifers and chestnut.

Fertilization When pollen grain of a flowering plant fuses with the ovule of a flower, it is known as fertilization. The pollen grain forms a pollen tube after reaching the stigma of a flower. This pollen tube grows down through the style and pierces one of the ovules. This process is known as fertilization. Double fertilization It is a process in flowering plants during reproduction where two sperm nuclei from each pollen fertilize two cells of the ovary. pollen tube + embryo sac 1 sperm nuclei + 1 egg nuclei = zygote = 2n embryo 1 sperm nuclei + 2 polar nuclei = 3n endosperm (food or cotyledon)

Development of the Plant Embryo Epicotyl: develops into leaves and the upper part of the stem Cotyledons/seed leaves: store food for the developing embryo Hypocotyl: develops into the lower stem and root Endosperm: grows and feeds the embryo Seed coat: outer covering of the ovule Radicle: first organ to emerge from germinating seed *embryo + seed coat = seed  ripened fruit

Fruits Fruits are actually the ripened ovaries of flowering plants. They contain seeds which are developed ovules. There are three types of fruits: Simple fruit -They are fleshy fruits.eg-berry, apple, pear. Aggregate fruit -This kind of fruit develops from flowers with many pistils.eg-strawberry, raspberry and blackberry. Multiple fruit -It is a fruit formed by a group of flowers, each flower forming a fruit.eg-pineapple and orange.

Seeds Seed -It is a small embryonic plant covered inside in a seed coat. It is actually a ripened ovule. Seeds are very important parts of a flower as they give rise to a new plant. Seed dispersal-Scattering of seeds in the surroundings by various methods is called seed dispersal It can be dispersed by animals like squirrels. Some fruits are eaten by animals and they do not digest the seed. The seed comes out through the gut of animals and gets dispersed in soil. It can also be dispersed by wind. Dispersal of seeds also takes place by water, gravity and force.

Gamete Production Genes are located on chromosomes in the cell nucleus Each organism must inherit a single copy of every gene from each parent, so each organism must carry two complete set of genes When an organism produces its own gametes, those two sets of genes must be separated from each other so that each gamete contains just one set of genes

Chromosome Number Case Example: Fruit fly, Drosophila Body cell of adult has 8 chromosomes 4 came from the male parent and 4 from the female parent These 2 sets of chromosomes are homologous , meaning they have corresponding chromosomes from each parent A cell that contains both sets of homologous chromosomes is called a diploid (meaning 2 sets) cell The number of chromosomes in a diploid cell is represented by the symbol 2n For Drosophila , the diploid number is 8, which can be written as 2n=8 Diploid cells contain 2 complete sets of chromosome and therefore 2 complete sets of genes Gametes of sexually reproducing organisms contain only 1 set of chromosomes and therefore only 1 set of genes These cells are called haploid (meaning 1 set) for Drosophila, this can be written as n=4, meaning that the haploid number is 4

Meiosis Meiosis is a process in of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Involves 2 divisions: meiosis I and meiosis II By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells

Inheritance Every living thing has a set of characteristics inherited from its parent or parents Genetics is the scientific study of heredity, inheritance passed down from one generation to the next Can you guess what the parents of these children look like? Explain how you know?

Gregor Mendel’s Peas Gregor Mendel was a monk who studied the genetics of pea plants in his monastery garden in the 1800’s Mendel knew that each flower produces pollen, which contains the plant’s male reproductive cells, sperm; the female portion of the flower, the pistil, produces egg cells Fertilization is the process in which during sexual reproduction, male and female reproductive cells ( gametes ) join This produces a new cell, which develops into a tiny embryo encased with in a seed Pea plants are normally self-pollinating and the seeds produced by self-pollination inherit all of their characteristics (i.e. flower color, seed size, etc.) from the single parent that bore them Mendel’s pea plants were true-breeding , meaning that if they were allowed to self-pollinate, they would produce offspring identical to themselves He used these true-breeding plants as the basis of his experiments

Gregor Mendel’s Peas Mendel wanted to produce peas/seeds by joining male and female gametes from 2 different plants He had to prevent self-pollination by cutting away the stamens, the pollen bearing male parts of a flower He dusted pollen from another plant on to the pistils of the stamen-less flowers This process, called cross-pollination , produced seeds that have 2 different plants as parents Cross-breeding allowed him study plants with combined characteristics

Genes and Dominance Trait – specific characteristic that varies from one individual from another Mendel studied 7 traits that each had two contrasting characteristics He crossed plants with each of the contrasting characteristics and studied their offspring Each original pair of plants is labeled the P (parental) generation The first offspring are labeled F 1 Hybrids – offspring of crosses between parents with different traits

Genes and Dominance After cross-breeding, Mendel’s F 1 peas had the characteristic of only one of the parents The principle of dominance states that some alleles are dominant and others are recessive Genes – sequence of DNA that codes for a protein and thus determines a trait Alleles – different forms of a gene

Recessive Alleles Mendel wondered if the recessive alleles were still present in the F 1 generation, or if they disappeared completely He allowed all 7 kinds of F 1 hybrid plants to produce an F 2 generation by self-pollinating

Results of F 1 Self-pollination The traits controlled by the recessive alleles had reappeared 25% of F 2 plants showed the recessive trait

Explaining the F 2 Results When each F1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carriers only a single copy of each gene Therefore, each F1 plant produces two types of gametes – those with the allele of the dominant trait and those with the allele for the recessive trait The result of this process is an F 2 generation with new combinations of alleles Tt Tt Segregation F 1 Gametes F 2 Tt Tt T t TT tt t T T = dominant allele T = recessive allele

Genetics and Probability Probability – the likelihood that a particular event will occur Mendel realized that principles of probability could be used to explain and predict the results of genetic crosses Example: the probability of a coin landing on tails is 1 chance in 2, or 50% Example: the probability of a coin landing on tails 3 times in a row is ½ x ½ x ½ = 1/8 (1 in 8 chance) Because each flip is an independent event, past outcomes do not affect future ones The way in which alleles separate is completely random, just like a coin flip

Punnett Squares The gene combinations that result from a genetic cross can be determined by drawing a diagram called a Punnett Square The types of gametes produced by each F 1 parent are shown along the top and left sides of the square The possible gene combinations for the F 2 offspring appear in the four boxes that make up the square The letters represent alleles R = dominant and r = recessive. Combinations with the dominant trait will express that trait. What is the probability that the F 2 generation will have smooth peas? What is the probability that they will have wrinkled peas? What are the dominant and recessive traits?

Punnett Squares Punnett squares can be used to predict and compare the genetic variations that will result from a cross Organisms that have two identical alleles for a particular trait are called Homozygous (i.e. TT) True-breeding for a particular trait Organisms that have two different alleles for the same trait are called Heterozygous (i.e. Tt) Hybrid for a particular trait Phenotype refers to physical characteristics Genotype refers to the genetic makeup Example: pea plants can display the same phenotype (i.e. round pea shape), but have different genotypes (i.e. RR and Rr) Which are the homozygous individuals and which are the heterozygous individuals? Which individuals display the same phenotype? Which display the same genotype? Which display the same phenotype and different genotypes?

Punnett Squares Draw a Punnett square that represents the diagram

Probabilities Predict Averages Probabilities predict the average outcome of a large number of events, but they cannot predict the precise outcome If you flip a coin twice, you are likely to get one heads and one tails, but you might get two heads or two tails The same is true of genetics The larger the number of offspring, the closer the resulting numbers will get to predicted values based on probability If an F 2 generation contains just 3 or 4 offspring, it may not match Mendelain predicted ratios If the F 2 generation contains hundreds of thousands of individuals, the ratios usually come very close to matching expectations

Independent Assortment Mendel wondered if the segregation of one pair of alleles affected to segregation of another, in other words, if they segregated independently His experiment to test this is called a two-factor cross (F 1 generation and F 2 generation) Two-factor Cross F 1 : crossed true-breeding plants that produced only round, yellow peas (RRYY) with plants that produced wrinkled, green peas (rryy) Results: all of the F 1 offspring produced round, yellow peas (RrYy) This shows that the alleles for round and yellow are dominant over wrinkled and green It did not show if the genes segregate independently, but it provided the hybrid plants needed for the next cross (the F 1 plants to produce the F 2 plants) RRYY rryy ry ry ry ry RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy RY RrYy RrYy RrYy RrYy

Independent Assortment Two-factor Cross F 2 : F 1 plants were crossed with each other (RrYy , heterozygous for both seed texture and seed color genes) Mendel wondered if the two dominant (RY) and two recessive (ry) alleles would stay together or segregate independently so that any combination of alleles was possible Results: F2 plants produced 556 peas; 315 = round and yellow, 32 = wrinkled and green, 209 = different combinations of phenotypes, and therefore alleles, not found in either parent Very close to the 9:3:3:1 ratio predicted by the Punnett square RrYy RrYy

Independent Assortment Conclusion: the alleles for seed shape segregated independently of those for seed color Therefore genes that segregate independently do not influence each other’s inheritance The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes Independent assortment helps account for the many genetic variations observed in organisms

Summary of Mendel’s Principles The inheritance of biological characteristic is determined by individual units known as genes, which are passed from parents to their offspring In cases in which two or more forms (alleles) of the gene for a single trait exist, some alleles may be dominant and other may be recessive In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent These genes are segregated from each other when gametes are formed The alleles for different genes usually segregate independently of one another

Non-Mendelian Inheritance Patters

Exceptions to the Principles Genetics is more complicated because some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes Incomplete Dominance – cases in which one allele is not completely dominant over another The heterozygous phenotype is somewhere in between the two homozygous phenotypes Example: a cross btwn red-flowered (RR) and white-flowered (WW) Mirabilis plants produces an F 1 generation of pink-flowered (RW) plants

Exceptions to the Principles Codominance – a situation in which both alleles contribute to the phenotype Both alleles are shown in the phenotype but they are separate, rather than blended Example: Heterozygous chickens and cows display speckling

Blood Group Genes ABO Blood Group Has 3 alleles: I A , I B , and I I A and I B are codominant Produce molecules called antigens on the surface of RBCs i is recessive Do not produce antigens Blood type O Blood groups are often mentioned at the same time (i.e. AB-negative blood refers to alleles from both gene groups)

Exceptions to the Principles Create a Punnett square for the chicken using a black-feathered (BB) parent and a white-feathered (WW) parent. Create a Punnett square for the cow using two brown and white (BW) speckled parents. Compare your Punnett squares with your classmate.

Exceptions to the Principles Multiple Alleles – a situation in which a gene has more than two alleles Does not mean that the individual can have more than two alleles, just that more than two possible alleles can exist within a population Example: coat color in rabbits is determined by single gene that has a least four different alleles They display a pattern of simple dominance that can produce multiple coat colors Example: human gene for blood type (can also be codominant)

Exceptions to the Principles Polygenic Traits – traits controlled by two or more genes Different combinations of alleles for these genes produce certain traits Often show a wide range of phenotypes Example: skin color in humans is controlled by more than four different genes Example: eye color in fruit flies ( Drosophila ) are the result of at least three different interacting genes A popular organism for genetic research

Gene Linkage – Modification to Independent Assortment Each chromosome is actually a group of linked genes Chromosomes assort independently, not just the individual genes Genes for certain traits can almost always be inherited together if they are on the same chromosome Example: Drosophila almost always inherit certain characteristics together, such as reddish-orange eyes and miniature wings

Gene Maps 2 genes on the same chromosome are not linked forever They can be separated during crossing-over in meiosis, producing new combinations of alleles This helps generate genetic diversity The further apart 2 genes are, the more likely they are to be separated during crossing-over in meiosis Genetic maps showing the relative locations of each known gene can be made using the rates of crossing-over and recombination

Gene Linkage Linked: if two genes are located near each other on the same chromosome, then the alleles for these genes will stay together during meiosis Unlinked: the further apart the genes, the more recombination that will occur between them, and the less linkage that will be observed. Recombination frequency will display almost no linkage and assortment independently Example: A-B: 5% A-D: 2% D-C: 10% B-C: 3% A-C: 8%

Human Chromosomes To analyze chromosomes, pictures of cells in mitosis are taken when the chromosomes are easy to see The chromosomes are then cut out from the pictures and grouped together in pairs A picture of chromosomes arranged in this way is called a karyotype Each haploid gamete has 23 chromosomes which combine to make 46 in a fertilized cell (now 23 pairs of homologous chromosomes) One pair is called the sex chromosomes because they determine an individual’s sex Females have 2 copies of the large X chromosome Males have 1 copy of the large X chromosome and 1 copy of the small Y chromosome The other 44 chromosomes are called autosomes , or autosomal chromosomes This karyotype is read 46, XY

Human Chromosomes Males and females are born in a roughly 50:50 ratio because of the way in which sex chromosomes segregate during meiosis All human egg cells carry a single X chromosome (23, X) Half of all sperm cells carry an X chromosome (23, X) and half carry a Y chromosome (23, Y) This ensures that just about 50% of the zygotes (fertilized eggs) will be 46, XX and 50% 46, XY

Human Traits A pedigree chart shows the relationships within a family and can be used to study how traits are passed from one generation to the next

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