Plant Science 9.1 Plant structure and growth 9.2 transport in angiospermophytes 9.3 reproduction in angiospermophytes
Remember… • Plant cell!
Plant Evolution Plants originated from green algae that lived in ponds that occasionally dried out.
Angiosperms • Angiosperms have dominated the land for over 100 million years. • Known as “flowering plants” • There are about 250,000 known species of flowering plants living today. • Most of our food comes from flowering plants • Roots, such as beets and carrots • Fruits of trees and vines, such as apples, nuts, berries, and squashes • Fruits and seeds of legumes, such as peas and beans; • Grains, such as rice, wheat, and corn
Angiosperms • Divided into two groups: • Names refer to the first leaves that appear on the plant embryo. • Embryonic leaves are called seed leaves, or cotyledons: • Monocots (embryo has one seed leaf) • Dicots (embryo has two seed leaves)
Angiosperms • Monocots: • Orchids, bamboos, palms, and lilies, as well as grains and other grasses • Leaves have parallel veins • Stems have vascular tissues arranged in a complex array of bundles. • Flowers have petals and other parts in multiples of three. • Roots form a fibrous system (a mat of threads) that spread out below the soil surface. • Make excellent ground cover that reduces erosion.
Angiosperms • Dicots: • True dicots include most shrubs and trees (except for conifers), as well as many food crops. • Leaves have a multibranched network of veins • Stems have vascular bundles arranged in a ring. • Flower usually has petals and other parts in multiples of four or five. • Large, vertical root (called a taproot) goes deep into the soil • You can see this if you try to pull up a dandelion
Plant Body • Composed of organs with various tissues reflective of their evolutionary history as land-dwelling organisms. • Must draw resources from two environments: • Water and minerals from soil • CO2 and light from air
Plant Body • Plant body is divided up to two main parts: • Subterranean part root • Aerial part shoot
Plant Body • Root system: • Anchors in the soil, absorbs and transports minerals and water, and stores food. • Monocots Fibrous root system consists of a mat of generally thin roots spread out shallowly in the soil • Dicots have one main vertical taproot with many small secondary lateral roots growing outward. • Both Monocots and Dicots have tiny projectsions called root hairs: • Enormously increase the root surface area for absorption of water and minerals.
Plant Body • Shoot system: • Made up of stems, leaves, and adaptations for reproduction (flowers) • Stems are parts of the plant that are generally above ground and support the leaves and flowers. Composed of: • Nodes • Points at which leaves are attached • Internodes • Portions of the stem between nodes • Leaves are the main photosynthetic organs in most plants (green stems also perform photosynthesis) • Consist of a flattened blade and a stalk, or petiole, which joins the leaf to a node of the stem.
Plant Body • Shoot system (continued): • Two types of buds that are undeveloped shoots: • Terminal bud • Found at the apex (tip) of the stem, has developing leaves and a compact series of nodes and internodes • Axillary bud • one of each of the angles formed by a leaf and the stem, are usually dormant.
Plant Body Apical dominance • Results from the terminal bud producing hormones that inhibit growth of the axillary buds. • By concentrating resources on growing taller, apical dominance is an evolutionary adaptation that increases the plant’s exposure to light • Important where vegetation is dense. • Removing the terminal buds usually stimulates growth of the axillary buds. • Branching is important for increasing exposure the environment
Modified Roots, Stems, and Leaves • Modified roots: • Some plants have unusually large taproots that store food in carbohydrates such as startch: • Carrots, turnips, sugar beets, and sweet potatoes Sugar Beet
Modified Roots, Stems, and Leaves • Modified Stems: • Stolon • “runner”; has a horizontal stem that grows along the ground surface • Plantlets form at nodes along their length, enabling a plant to grow asexually • Example: strawberry • Rhizomes • Look like large, brownish, rootlike structures • Horizontal stems that grown just below or along the soil surface • Store food, and having buds, can also spread and form new plants • Potato plant has enlarged structures specialized for storage called tubers (the potatoes we eat)
Modified Roots, Stems, and Leaves • Modified stems (continued) • Bulbs • Modified stems that are also used for underground food storage (onions)
Modified Roots, Stems, and Leaves • Modified Leaves: • Tendrils • Tips coil around a stem, help plants climb • Examples: grapevines, peas Tendril- Pea Plant
Plant Tissues in Stems and Leaves • Each plant organ- root, stem, or leaf- is made up of three tissue systems: • Dermal • Vascular • Ground tissues
Plant Tissues in Stems and Leaves • Dermal Tissue • Forms an outer protective covering. • Acts as first line of defense against physical damage and infectious organisms. • Consists of a single layer of tightly packed cells called the epidermis: • Epidermis of leaves and most stems is covered with a waxy layer called cuticle, which helps prevent water loss. • Typical dicot leaf also has pores on its epidermis called stomata • which allow CO2 exchange between the surrounding air and the photosynthetic cells inside the leaf. • Surrounded by guard cells: • Regulate the size of the stoma
Plant Tissues in Stems and Leaves Plant Leaf
Plant Tissues in Stems and Leaves • Vascular Tissue: • Made up of: • Xylem • type of vascular tissue that is made up of cells that transport water and dissolved ions from the roots to the leaves • Phloem • type of vascular tissue that is made up of cells that transport sugars from leaves or storage tissues to other parts of the plant
Plant Tissues in Stems and Leaves • Vascular Tissue (continued): • In the stem.. • Vascular tissue forms vascular bundles • Dicots arranged in a circle
Plant Tissues in Stems and Leaves • Vascular Tissue (continued): • In the leaf… • Vascular tissue form network of veins • In the veins, the xylem and phloem are continuous with the vascular bundles of the stem. • Allows them to be in close contact with photosynthetic tissues, ensuring water and mineral nutrients from the soil are supplied, and that sugars made in the leaves are transported throughout the plant
Plant Tissues in Stems and Leaves • Ground Tissue (continued): • Accounts for the bulk of a young plant, by filling in spaces between the epidermis and vascular tissue. • Functions include photosynthesis, storage, and support. • Ground tissue inside vascular tissue is called pith • Ground tissue external to vascular tissue is called cortex Dicot Stem
Plant Tissue in Stems and Leaves • Ground Tissue (continued): • Ground tissue of dicot stems… • consists of both a cortex region and pith region • Ground tissue of the leaf… • Is called Mesophyll : • Sandwiched between the upper and lower epidermis • Consists mainly of photosynthesis cells • Loosely arranged to provide air spaces which CO2 and O2 can circulate • Main location of photosynthesis
Plant Growth • Growth in plants is made possible by tissues called meristems. • A meristem consists of cells that divide frequently, generating additional cells. • Some products of this division remain in the meristem and produce still more cells, while others differentiate and are incorporated into tissues and organs of the growing plant.
Plant Growth • Apical Meristems • Meristems at the tips of roots and in the buds of shoots • Cell division in the apical meristems produces the new cells that enable a plant cell to grow in length primary growth • Enables roots to push through the soil and allows shoots to increase exposure to light and CO2. • Growth occurs behind the root tip in three zones of primary growth: • Zone of cell division, zone of elongation, and zone of maturation • Zone of maturation brings about the three tissue systems (dermal, ground, and vascular)
Plant Growth Primary Growth of a Root
Plant Growth • Lateral meristems • Associated with the increase in thickness of stems and roots secondary growth • Caused by the activity of two cylinders of dividing cells that extend along the length of roots and stems: • Vascular cambium • Secondary growth adds layers of vascular tissue on both sides of the vascular cambium wood • Cork cambium • Outer cambium that forms the secondary growth of the epidermis cork
Control of Plant Growth • Auxin is a term used for any chemical substance that promotes seedling elongation. • Apical meristem at the tip of a shoot is a major site of auxin synthesis. • As auxin moves downward, it stimulates growth of the stem by making cells elongate. • Concentration of auxin determines its effect • Too low to stimulate shoot cells will cause root cells to elongate • High conc. stimulates shoots cell and inhibits root cell elongation. • Stimulates stem elongation and root growth, differentiation, and branching.
Control of Plant Growth • Auxins also play a part in phototropism, an occurrence that involves plants bending or moving away from light. • The shoot tip is responsible for directional movement by the plant in response to sunlight, as this is the area where auxins can be found. • Sunlight eradicates auxin, meaning that the part of the shoot tip of the plant which is receiving direct sunlight will have the least amount of auxin. • The extra auxin present on the shaded side promotes more cell division and elongation, causing the plant to bend towards the sunlight after this lop-sided growth.
Control of Plant Growth Cells on the darker side are larger and have elongated faster; causes the shoot to bend towards the light. If a plant receives sunlight uniformly from all sides or is kept in the dark, the cells all elongate at a similar rate. Effect of Auxin on Phototropism
Transport in Plants • Several factors necessary for plant growth: • CO2 from airabsorbed by leaves • O2 from air or soilabsorbed by leaves or roots • H2O from soil absorbed by the roots • Minerals from the soil absorbed by the roots • Sugars are made in the leaves from the absorbed molecules and ions and used to build the plant’s body and provide energy
Solute Uptake From The Roots • Mineral ions from the soil can get into the root of plants by three different ways: • 1. Diffusion • If the concentration of certain ions is lower inside of the root hair cells, they can simply diffuse into the root hair cells from the soil • 2. Fungal hyphae • Some plants live in a symbiotic relationship with fungi and use fungal hyphae to increase the surface of the root even more. The combination of plant root and fungal fibers are called mycorrhiza. The fungus benefit from a constant supply of sugar while the plant benefit from the increased surface area that the fungal hyphae provide, they also excrete growth factors and antibiotics • 3. Mass flow of water into the root can also carry ions passively in dissolved form
Solute Uptake From the Roots • Roots hairs are extensions of epidermal cells that cover the root and form a huge surface area • Allows the plant to absorb the water and minerals it needs for growth • Watery solution has to be transported from the soil to epidermal cells to cortex of the root to the xylem (water-conducting vascular tissue) • Plasma membrane of the xylem cells are selectively permeable, which helps regulate the mineral composition of a plant’s vascular system.
Solute Uptake From the Roots • Two possible routes to the xylem: • Intracellular route • Extracellular route
Solute Uptake From the Roots • Intracellular route: • A.k.a. Symplatic route • Cells within roots are connected via plasmodesmata (channels through the walls of adjacent cells) which allows for a continuum of living cytoplasm among the root cells • Once inside epidermal cells, solution can move inward from cell to cell without crossing membranes
Solute Uptake From the Roots • Extracellular route: • Solution moves inward within the hydrophillic walls and extracellular spaces of the root cells but does not enter the cytoplasm of the epidermis or cortex cells. • Solution passes through no plasma membranes, and there is no selection of solutes until they reach the endodermis. • Endodermis has a waxy barrier called the Casparian strip which stops water and solutes from entering the xylem. • Water and ions are forced to cross a plasma membrane into an endodermal cells, then are discharged into the xylem.
Solute Uptake in the Roots • In a real plant… • Water and solutes rarely follow just the two kinds of routes • May take a combination of these routes, and may pass through numerous plasma membranes and cell walls en route to the xylem. • All water and solutes must cross a plasma membrane at some point.
Transpiration • Why transpiration? • As a plant grows upward toward sunlight, it needs to get water and minerals from the soil. • Must be able to transport resources from the roots to thrive.
Transpiration • Xylem tissue is made of two types of conducting cells: tracheids and vessel elements. • When mature, but types of cells are dead, consisting only of cell walls, and both are in the form of very thin tubes that are arranged end to end. • Because the cells have openings in their ends, a solution of water and inorganic nutrients, called xylem sap, can flow through these tubes. • Xylem sap flows all the way up from the plant’s roots through the shoot system to the tips of the leaves.
Transpiration • Forces that push xylem sap against gravity are: • Root pressure • Root cells actively pump inorganic ions into the xylem, and the root’s endodermis holds the ions there • As ions accumulate in the xylem, water tends to enter by osmosis, pushing xylem sap upward ahead of it. • Can push sap up a few meters • For the most part, however, xylem sap is not pushed from below by root pressure by pulled upward by the leaves. • Transpiration • The pulling force caused by the loss of water from the leaves and other aerial parts of a plant. • Water molecules leave the plant through the stoma of the leaf by diffusion. • When the stoma is open, water concentration is higher in the plant cells than in the surrounding atmosphere.
Transpiration • Properties of water stimulate transpiration: • Cohesion • Sticking together of molecules of the same kind. • Because water is polar, they are attracted to each other by hydrogen bonds • Water molecules form continuous strings in xylem tubes, extending all the way from the leaves down to the roots. • Adhesion • Sticking together of molecules of a different kind. • Water molecules tend to adhere via hydrogen bonds to hydrophillic cellulose molecules in the walls of xylem cells.
Transpiration • Transpiration-Cohesion-Tension Mechanism: • Before a water molecule can leave the leaf, it must break off from the end of the string • It is pulled off a steep diffusion gradient between the moist interior of the leaf and the drier surrounding air. • Cohesion resists the pulling force of the diffusion gradient, but it is not strong enough to overcome it. • The molecule breaks off, and the opposing forces of cohesion and transpiration put tension on the remainder of the string of water molecules. • As long as transpiration continues, the string is kept tense and is pulled upward as one molecule exits the leaf and the one right behind it is tugged up into its place. • Adhesion pulls the remaining water molecules upwards from the root against the downward pull of gravity. • Process does not require energy from the plant, they are all extended by physical properties of water and the surrounding molecules.
Transpiration • Summary of Transpiration-Cohesion-Tension Mechanism: • Transpiration exerts a pull that is relayed downward along a string of water molecules held together by cohesion and helped upward by adhesion. • Transpiration is an efficient means of moving large volumes of water upward from roots to shoots. • http://www.phschool.com/science/biology_place/labbench/lab9/transpull.html
Guard Cells • PROBLEM! Photosynthesis requires large leaf surfaces • Results in constant transpiration and water loss • If soil dries out, plants wilt and eventually die • SOLUTION! Leaf stomata can open and close via the control of guard cells. • Guard cells control the opening of a stoma by changing shape, widening or narrowing the gap between the two cells.
Abscisic Acid • Absicisic acid causes the closing of stomata. • ABA crucial for plants to withstand drought. • When the plant starts to wilt, ABA accumulates in the leaves and causes stomata to close.