Download Lab Notes

1/23/2013
Plant Tissues
• As single‐celled organisms evolved into multi‐cellular plants or animals, some cells became specialized in structure and function
• This lead to division of labor with groups of cells performing specific functions referred to as tissues
• All flowering plants are multi‐cellular and are composed of root, stem, and leaf tissues
• Meristems are regions on a plant of active cell division that give rise to the various tissue types
• Apical (apex) meristems are found at root and stem tips and cause increase in length (called primary growth)
• These give rise to leaves and non‐woody stems and roots
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• Some plants (woody plants) will have meristematic tissue that accounts for increase in diameter
• These tissues are called vascular cambium and cork cambium
and are a large part of a plant’s secondary growth
• Overall, there are 3 types of basic tissues: dermal, ground, and vascular
• Dermal tissue – the outermost cell layers of a plant
• In young plants and non‐woody plant parts, the outermost surface is the epidermis, a single layer of flattened cells
• In leaves and stems, epidermal cells secrete cutin, a waxlike
substance that constitutes the cuticle
• The cuticle prevents water loss and is so thick in some leaves that they have a shiny surface
• Some leaves will have trichomes (hairs) on their surface and most will be glandular so they release an aroma when disturbed
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• The leaf epidermis will also have pores, or stomata, for gas exchange (CO2, O2, H2O vapor)
• In woody plants, the epidermis cracks and is replaced by the periderm, which is constantly produced by the cork cambium as tree girth increases
• Cork is composed of dead cells with walls containing suberin, a water‐proofing fatty substance
Stomata
• Ground tissue – makes up the bulk of non‐woody plant tissues and there are 3 categories by function:
1. Parenchyma tissue – thin walled cells with many shapes and sizes
• Photosynthetic cells in leaves and green stems
• Storage cells in all plant organs
• Stores starch in potato tubers
• Water in cactus stem
• Sugar in sugar beets
Potato storage cells with starch granules
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2. Collenchyma cells – the primary support tissue in young plant tissue
• Elongated cells with thickened primary cell walls tightly packed just below the epidermis
• The tough strings in celery are strands of collenchyma cells
3. Sclerenchyma tissue – can be of 2 cell types:
a) Fibers – elongated cells for support but non‐living (collenchyma cells are living at maturity) and have thickened secondary walls
• These are the fibers we use for rope and clothing (hemp, cotton, etc.)
b) Sclereid cells – many shapes but usually not elongate
• For mechanical support and protection
• Extremely thick secondary cell walls account for hardness of walnut shells and grit of pear fruit
• Vascular tissue – the conductive tissues in plants
• In leaves, we can see them as veins
• Form a continuum of flow throughout the plant and there are 2 types (both composed of several cell types): xylem and phloem
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a) Xylem – transports mainly water but also some soluble nutrients from the roots through the rest of the tree
• The cells die soon after forming and harden to become wood in the interior of the tree
b) Phloem – it’s the innermost layer of bark and transports nutrients, mainly sucrose, from leaves to the rest of the tree
• In the spring, nutrients flow from roots to the rest of the tree
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Plant organs
• Angiosperms (flowering plants) are divided into 2 Classes and we see anatomical differences in stems, roots, and leaves
1. Magnoliopsida or Dicots
• 2 seed leaves
• Leaf venation – pinnate or palmate, rarely parallel
• Flower parts – usually in 4’s or 5’s, rarely 3’s
• Vascular bundles (xylem and phloem) in stems form a ring around a central pith
• Many (but not most) considered as trees
2. Liliopsida or monocots
• 1 seed leaf
• Leaf venation – almost all parallel
• Flower parts – mostly in 3’s, rarely 4’s or 5’s
• Vascular bundles are scattered throughout stem
• Only groups to reach tree size are palms, giant bamboo, and yucca‐like plants
• None have secondary xylem or wood
Monocot (left) and dicot (right) stem
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• Stems
• Composed of vascular bundles, usually surrounded by a bundle sheath of fibers, and parenchyma cells
• Dicots can be herbaceous (non‐woody) or woody
• A soft pith of parenchyma cells is found in herbaceous stems
• Woody stems are filled with more vascular tissue, especially xylem, and annual rings are formed as new xylem is added each year
• Rays of parenchyma cells transport materials radially
• Important features include:
1. Buds – found in the leaf axil (axillary) or on the tip (terminal) of the twig
• Buds can be scaly (with protective stipules) or naked (no protection for growing twig)
• Naked buds are usually found in the tropics where year‐
round high temperatures will not freeze the new tissue
• Scaled buds can have many overlapping scales (imbricate) or 2 or 3 large scales (valvate)
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Imbricate, terminal beech bud
Valvate, terminal yellow poplar
2. Leaf scars are left when a leaf falls from the twig
• We can see vascular bundle scars that show the pattern of leaf veins where they connected to the twig
3. Stipule scars (protective bud leaves) are not found on some trees and can be used for identification of species or group
Leaf scar of tupelo
Stipule scar surrounding magnolia twig
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4. Lenticels are small, sometimes wartlike patches where some species get aeration (CO2 and O2 exchange)
5. The pith is the central part of the twig and is relatively soft in consistency in newly grown twigs
• Most of our native trees have a solid pith but some have diaphragmed pith (yellow poplar) with obvious elongated cells or chambered pith (walnut) with empty chambers
Lenticels
Walnut chambered pith
6. Thorns, spines, and prickles
• Some species have pointed protective structures to deter browsing animals and they are derived from various twig tissue depending on group or species
• Types include:
a) Spines – modified leaves or stipules (black locust)
• Because they are from structures only found at the node they can only be found at the node and nowhere else on the twig
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b) Prickles – formed from epidermal tissue so can be found anywhere on the twig (devil’s walking stick, blackberry)
c) Thorns – modified stems so must occur at leaf axil and may have small leaves themselves (honeylocust, hawthorn, osage‐orange)
Black locust spines
Devil’s walking stick
Osage‐orange
• Roots can form 2 types of root systems
1. Taproot systems consist of one large main root with small lateral branch roots
• They can be used for storage (carrots, turnips, beets)
2. Fibrous root systems are highly branched and lack a central root (most grasses)
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• All roots have a thimble‐shaped root cap to protect the meristem as they grow
• Vascular tissue is found in the center, or stele, of the root with phloem surrounding xylem
• Parenchyma cells compose the surrounding cortex (the bulk of the root) and are for storage
• Leaves
• The primary photosynthetic organs of trees and one of the most variable structures
• They grow at regular intervals along a stem and the point where one or more leaves arise is called a node (the portion of the stem between the nodes is the internode)
• The wide portion of the leaf is the blade (or lamina) and the supporting stalk is the petiole
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• Some leaves have a pair of scales or leaf‐like structures called stipules that are attached to the petiole base or the twig
• These usually drop as the leaf grows but we may be able to see the scars where they were attached
• Some stipules have evolved into protective structures (spines)
• If all leaves drop after one growing season it is deciduous, and if all leaves do not drop at once and the tree appears green all year it is evergreen
• Some typically deciduous trees may keep their mostly dead, brown leaves into the winter (marcescent)
• This is common in beech and some oak species in our area (further north the species may be truly deciduous)
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• Leaf complexity – often a very good trait to divide groups
• If there is a single blade it is simple
• If the leaf is divided into ≥2 parts it is compound
• If a leaf is compound, the individual blades are leaflets
• The axis supporting the leaflets is the rachis
• If leaflets are attached laterally along the rachis it is pinnately
compound
• Leaves that have a secondary branching system (twice pinnate) are called bipinnate
• If a compound leaf does not have a long rachis and the leaflets seem to radiate in a fan‐like manner from the petiole, it is palmately compound
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• To distinguish a pinnately compound leaf from a twig with simple leaves, look for buds because they will only be found in the axil of the leaf (not leaflet)
• Leaf arrangements on twigs (phyllotaxy)
• Usually in 3 patterns of attachment at nodes:
1. Opposite – paired at the same height on each side of the twig
2. Whorled ‐ >2 leaves at the same node
3. Alternate – a single leaf attached at each node
• Leaf venation – pattern of veins within leaves
• 4 basic types:
1. Pinnate – single midrib with secondary veins branching off
2. Palmate ‐ ≥3 primary veins arising near the base of the blade and spreading out like a fan
3. Pinnipalmate – intermediate between the 2 above with lowermost pairs of secondary veins arising near the base of the midrib that are larger than the other secondary or tertiary veins
4. Parallel – many equal sized veins running parallel to one another that usually meet at the apex of the leaf
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• Leaf shape – several features of leaf shape can be used for ID
• Overall blade shape
• Shape of apices (apex or end of the leaf)
• Shape of base
• Form of the leaf margin (or edge shape)
• Degree of lobing (deepness of lobes)
• Lobed – margins indented 1/4 to 1/2 distance to midrib
• Cleft – indented just over 1/2
• Incised – deeply indented to near midrib
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Overall leaf shapes
Leaf margins
Leaf lobing
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Reproductive Morphology
• More stable traits than vegetative parts (always same number, color, length, etc.) so more reliable to ID to species
• 2 problems for gathering: 1. Usually high in the crown
2. And/or very short‐lived in most cases
• Primary male reproductive structure is the pollen grain that carries the sperm
• Primary female reproductive structure is the ovule that carries the egg
• Basic steps and terminology for reproduction:
1. Pollination – pollen must be delivered to the ovule (insects, wind)
2. Fertilization – pollen grain reaches ovule then the egg and they combine
3. Seed – develops from ovule and contains an embryo which can grow into a tree under the right conditions
• There are several levels of sexual differentiation
• Flowers that contain both male and female parts are perfect (or bisexual or synoecious; syn = together, oecious = house)
• Many plants: corn, squash, dogwood, cherry, black locust
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• If cones or flowers have only one sex they are imperfect or unisexual
• When male and female flowers or cones are on same plant it is monoecious (one house)
• Most conifers, oak, hickory, birch, many more
• When male and female flowers or cones are different plant it is dioecious (two houses)
• Willow, poplar, persimmon, holly, ginkgo, juniper, ash, boxelder, only a few others
• Pollination – transfer of pollen from pollen sac to the ovule (for gymnosperms/conifers) or anther to stigma (for angiosperms/ flowering plants)
• 3 types:
1. Wind – small, lightweight, dry pollen in super‐abundance
• Small flowers with exposed anthers and stigma
• Flowers in early spring before leaves for maximum airflow among branches
• Many are allergenic: oak, hickory, pine
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• Conifers have wind‐borne pollen but much less allergenic than angiosperms
• Some pollen have air sacs or “wings” and shape is genus or species specific, so with pollen ID in soil or fossil remains we can determine what types of plants grew in the past and where
2. Insect – floral morphology correlated with a particular pollinating insect (beetle, bee, bumble bee, wasp, butterfly, moth)
• Flower petals are showy to attract and guide pollinator and flowers are usually aromatic (fragrant)
• Small amounts of large, heavy, sticky pollen grains
• Many times with a nectar supply as additional attractant (but some insects do eat pollen)
• Persimmon, magnolia, apple, cherry, sourwood, golden rod and most other small plants
• In gymnosperms, cycads are beetle pollinated
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3. Birds – similar to insect pollination, flower shape is correlated to pollinator and flowers are showy (usually red or yellow), aromatic, and full of nectar
Bee pollinates
a sourwood tree
Magnolia – beetle
Fig – fig wasp
Reproductive Structures in Gymnosperms
• Ovules are exposed (naked) and arranged in various ways depending on the group
• Ovules of cycads are found on modified leaves attached to a central stem axis and the entire structure is a strobilus
• Ovules of conifers are found in axils of bracts (a type of reduced leaf) which are attached to modified lateral branches (scales)
• Pollen cones are simple with a stem axis with modified leaves, each with ≥2 pollen sacs
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Male pollen cone of pines
Cycad with strobilus
A mature opened cypress cone and a mature unopened cone
• The exposed part of the scale in a mature, unopened cone is called the apophysis
• In pine cones, it terminates with a small pointy bump called an umbo
• Pine cones usually mature in 2 but sometimes 3 years
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• Most cones open and release seed soon after maturity but some remain closed for a long time and require fire to open
• These are called serotinous cones and are found on pines that have evolved in a fire‐maintained habitat
• This leads to rapid seed dispersal and seedling development after fire
• Seeds can be terminally or laterally winged or wingless depending on species
Reproductive Structures in Angiosperms
• Flower anatomy:
• Central stem axis (receptacle) is a modified short shoot
• Perianth – highly modified sterile leaves
• Outer whorl of leaves in perianth are sepals and together they are called the calyx
• Inner whorl of leaves are petals and together they are called the corolla
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• Stamens – male reproductive leaves are composed of anthers
(holds pollen sacs) and a filament (stalk)
• Carpels – female reproductive structure composed of:
• Ovary containing ovules
• Style (stalk) and stigma (collects pollen)
• Pistil is a term to describe the visible portion of the ovary/style/stigma complex
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• A flower with all 4 parts (sepal, petal, stamen, pistil) is considered complete and if it’s missing any one it is incomplete
• If one of the missing parts is a stamen or pistil, the flower is imperfect and is either staminate (male) or pistilate (female)
• In many flowers, these parts can be fused leading to further ID clues and the ovary will occur in different positions relative to the other parts (inferior, superior)
• Flowering is the process of floral maturation to the point when pollination is possible (anthesis)
• This is called coning in conifers because they have no flowers
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Inflorescence and Infructescence
• Flowers can be terminal on the twig or axillary in leaf axils
• An inflorescence is the arrangement of several flowers in a branching system
• Some flowers are single so have no inflorescence
• The infructescence is the same but for fruit
• The stalk of a single flower or an inflorescence is the peduncle
• The stalk of individual flowers of an inflorescence is a pedicel
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• A fruit is a ripened ovary, usually with seeds
• A seed is a ripened ovule, usually with an embryo (fused pollen and egg) inside
• Classification of fruits is based on ovary structure and form of the mature fruit
• A fruit from a single ovary in a flower is simple and one formed from several ovaries that stay together when mature is compound
• Both types may be fleshy or dry
• Fleshy fruits are classified by differentiation of the fruit wall (pericarp)
• Dry fruits are classified by whether they split open and release seeds (dehiscent) or not (indehiscent), number of carpels (ovaries), and general form
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