TOPIC 1 INTRODUCTION to Botany. Introduction to cytology ...
Introduction to the science of botany
Transcript of Introduction to the science of botany
OBJECTIVES
Describe the functions of roots and its two unique features
Contrast the structure of a primary eudicot and monocot root
Describe the functions of the tissues of eudicot and monocot root
Describe several roots that are modified to perform unusual functions
ROOTS
The extent of a plant’s root depth and spread varies considerably among species and even among different individuals in the same species.
Soil conditions greatly affect the extent of root growth.
Anchors a plant securely in the soil.
The root systems are adapted to obtain water in a variety of ways.
Lack nodes and internodes and do not usually produce leaves or buds.
ROOT SYSTEM
Taproot System• Consists of one main root formed from the
enlarging radicle with many smaller lateral branch roots.
• Lateral roots often occur initially in regular rows along the length of the main root.
• Characteristic of many eudicots and gymnosperms.
• Most trees, have taproots when young and later develop large, shallow lateral roots from which other root branch off and grow downward.
Gymnosperms – seed-bearing plants that do not produce
flowersEx: Cone-bearing trees such as
pines, hemlock and firs
Eudicots – angiosperms that are flowering plants
Ex: Oak, Cherry, Bean and Daisy
ROOT SYSTEM
Fibrous System• Several to many roots of the same size that
develop from the end of the stem, which smaller lateral roots branching off these roots.
• Form in plants in which the embryonic root is short -lived
• Do not arise from preexisting roots but from the stem, they are called adventitious.
Adventitious – 0rgans occur in unusual locations, such as roots that develop on a stem or buds
that develop on roots
Monocots – grass or grass-like flowering plants
Ex: Onion, Crabgrass
FUNCTION
Anchorage• Roots anchor a plant
securely in the soil.• Firm anchorage is
essential to a plant’s survival so that the stem remains upright, enabling leaves to absorb sunlight efficiently.
Absorption• Roots absorb water and
dissolved minerals (inorganic nutrients), such as nitrates, phosphates and sulfates, from the soil.
• These materials are the transported throughout the plant in the xylem.
Conduction• The water and minerals
absorbed are conducted by the various parts of the root and shoot system
FUNCTION
Storage• Surplus carbohydrates produced in
the leaves by photosynthesis are transported in the phloem, as sugar, to the roots for storage, usually as sugar or starch, until needed.
• Although roots use some photosynthetic products for their own respiratory needs, most are stored and later transported out of the roots when the plant needs it.
• Taproot and fibrous roots may be modified for storing food.
Carrot roots – have an extensive phloem for this purpose
Taproots – Beets, Carrots, Radishes and TurnipsFibrous – Sweet Potatoes
Usually biennials, as part of the strategy to survive winter, store their food reserves in the root during the 1st
year’s growth and use these reserves to reproduce during the 2nd year’s growth
*Other plants living in dry regions, possess storage roots adapted to store water
STRUCTURE
Root cap• Covers the root tip• A protective thimble like that layers many
cells thick, covers the delicate root apical meristem
• As the root grows, pushing its way through the soil, parenchyma cells of the root cap slough off by the frictional resistance of the soil particles and replaced by new cells formed by the root apical meristem toward its outer side.
• Involved in orienting the root so that it grows downward
STRUCTURE
Root hairs• Short-lived, unicellular extensions of
epidermal cells near the growing root tip. Short, typically less than 1cm or 0.4in in length, but are quite numerous.
• Form continually in the area of cell maturation closest to the root tip to replace those that are dying off at the more mature end Of the root hair zone.
• They greatly raise the absorptive capacity of the root by increasing the surface area in contact with the moist soil.
STRUCTURE
Root hairs• Soil particles are coated with a
microscopically thin layer of water in which inorganic nutrient minerals are dissolved.
• The root hairs establish intimate contact with soil particles, which allows absorption of much of the water and dissolved materials.
• Modification of the root epidermis that enables it to absorb more water from the soil.
ROOT OF HERBACEOUS EUDICOTS
The root epidermis does not secrete a thick, waxy cuticle.
The lack of a cuticle and the presence of root hairs increase absorption.
Most of the water that enters the root moves along the path of least resistance – along the cell walls rather than entering the cells.
One of the major components of cell walls is cellulose, which absorbs water as sponge does.
Lacks a pith, a ground tissue in the centers of many stems and roots
ROOT OF HERBACEOUS EUDICOTS
Cortex• Ground tissue which is composed primarily of loosely
arranged parenchyma cells with large intercellular spaces, makes up the bulk of the root.
• Lacks supporting collenchyma cells, although it may develop some supporting sclerenchyma cells as it ages.
• Primary function of the root cortex is storage.• The large intercellular air spaces, provide a pathway for
water uptake and allow for aeration of the root.• The oxygen that root cells need for aerobic respiration
diffuses from air spaces in the soil to the intercellular spaces of the cortex, and from there to the cells of the root.
ROOT OF HERBACEOUS EUDICOTS
Endodermis• Inner layer of cortex which controls the
amounts and kinds of water and dissolved materials that enter the xylem in the root’s center.
• Endodermal cells fit snugly against one another and each cell has a special bandlike region, called a Casparianstrip, on its radial (side) and transverse (upper and lower) walls.
Casparian Strip – a band of waterproof material that ensure water and minerals enter
the xylem only by passing through the endodermal cells. It contains suberin
ROOT OF HERBACEOUS EUDICOTS
Endodermis• Water enters endodermal cells
by osmosis, whereas inorganic minerals enter by passing through carrier proteins in their plasma membranes.
• Thus, endodermis controls what kinds of dissolved minerals and how much of each kind move from the soil into the vascular tissue of the root and from there to the rest of the plant body.
ROOT OF HERBACEOUS EUDICOTS
Continuum consisting of the cytoplasm of many plant cells which is connected from one cell to the next by plasmodesmata.
Some dissolved mineral ions move from the epidermis through the cortex
Continuum consists of the interconnected porous cell walls of a plant, along which water and inorganic mineral ions move freely.
The water and mineral ions diffuse across without ever entering a living cell.
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Water and dissolved minerals that enter the root travel from one cell’s cytoplasm to another through
cytoplasmic connections or from cell to cell along the interconnected porous cell walls. On reaching the
endodermis, water and minerals can continue to move into the root’s center if they pass through a plasma
membrane and enter an endodermal cell. The Casparianstrip blocks the passage of water and minerals along the
cell walls between adjoining endodermal cells.
ROOT OF HERBACEOUS EUDICOTS
Pericycle• Outermost layer of the stele which is
just inside the endodermis.• Composed of parenchyma cells that
remain meristematic, gives rise to lateral roots.
• Involved in forming the lateral meristems that produce secondary growth in wood roots.
Lateral roots – originate when a portion of the pericycle starts dividing.
- pushes through several layers of root tissue before entering the soil
- has all the structures and features of the larger root from which it emerges: root hairs, root cap, epidermis, cortex, endodermis pericycle, xylem and phloem
ROOT OF HERBACEOUS EUDICOTS
Xylem• Centermost tissue of the stele, often
has two, three, four or more extensions, or “xylem arms”
• Tracheids and Vessel Elements conduct water and dissolved minerals
Phloem• Located in patches between the
xylem arms.• Sieve-tube Elements conducts
carbohydrates (sucrose)
Root Hair Epidermis Cortex Endodermis
Root XylemStem Xylem
Sugar is made in photosynthesis
Sugar is used for growth and maintenance of root tissues or stored, usually as starch
Sugar is stored as starch
Sugar is used for growth and maintenance of
tissues
Vascular Cambium gives rise to secondary tissue in woody plants, is
sandwiched between xylem and phloem
ROOT OF HERBACEOUS MONOCOTS
More varied in internal structure than eudicot roots
Its xylem does not form a solid cylinder of vascular tissues in the
center
Phloem and xylem are in separate alternating strands that in cross section are arranged in a
circle around the centrally located pith, which consists
parenchyma cells
Do not have true secondary growth, therefore, no vascular
cambium exists
Have thickened roots produced by a modified form of primary growth in which parenchyma cells in the cortex divide and
enlarge
Cortex expands in such plants
ROOT OF WOODY PLANTS
Plants that produce stems with secondary growth (wood and bark) also produce roots with secondary growth
The production of secondary tissues in the roots of woody plants occurs some distance back from the root tips is the result of the activity of two lateral meristems, the vascular and cork cambium.
Major roots of trees are often massive and have both wood and bark.
In temperate climates, the wood of both roots and stems exhibits annual rings in cross section.
ROOT WITH UNUSUAL FUNCTIONS
• Adventitious roots often arise from stem nodes (regions of the stem where leaves are attached).
• Many aerial adventitious roots are adapted for functions other than anchorage, absorption, conduction or storage.
• Prop roots are adventitious roots that develop from branches or from a vertical stem and grow downward into the soil to help support the plant in an upright position.
• Prop roots are more common in monocots.
Monocots and herbaceous plants that produce prop roots
Examples: Corn and sorghum
Tropical and subtropical eudicot trees also produce prop rootsExamples: Red mangrove and
banyan
ROOT WITH UNUSUAL FUNCTIONS
• The roots of many tropical rainforest tress are shallow and concentrated near the surface in a mat.
• Swollen bases and braces called buttress roots hold the trees upright and aid in the extensive distribution of the shallow roots.
Examples: Moreton Bay Fig Tree and Terminalia arjuna
ROOT WITH UNUSUAL FUNCTIONS
• In swampy or tidal environments where the soil is flooded or waterlogged, some roots grow upward until they are above the high tide level.
• Even though roots live in the soil, they still require oxygen for aerobic respiration.
• A flooded soil is depleted of oxygen, so these aerial “breathing” roots, known as pneumatophores, may assist in getting oxygen to the submerged roots.
Pneumatophores anchors the plant, have a well-developed system of
internal air spaces that is continuous with the submerged parts of the root,
presumably allowing gas exchange
Examples: Back mangrove and white mangrove
ROOT WITH UNUSUAL FUNCTIONS
• Climbing plants and epiphytes, which are plants that grow attached to other plants, have aerial roots that anchor the plant to the bark, branch and other surface on which it grows.
• Some epiphytes have aerial roots specialized for functions other than anchorage. Epiphytic roots may absorb moisture as well.
• Other starts its life as an epiphyte that produces long, hanging aerial roots that eventually reach the ground and anchor the plant in the soil.
Examples: Epiphytic Orchids –Photosynthetic roots
Examples: Strangler fig - the tree where it originally grew is often killed as the fig grows around it, competing
with it for light and other resources and crushing its second phloem
Becomes a self-supporting
tree
ROOT WITH UNUSUAL FUNCTIONS
• Some parasites have roots that penetrate the host-plant tissues and absorb water and dissolved minerals from the host’s xylem.
• Other parasitic plants, take both water and sugars from their hosts
Examples: Mistletoe – does not take sugars from its host plant
ROOT WITH UNUSUAL FUNCTIONS
• Contractile roots grow into the soil and then contract (the cortical cells shorten or totally collapse on themselves), thus pulling the plant deeper into the soil.
• The contraction is irreversible.• Contractile root are necessary for
some plants because each succeeding year’s growth is on top of the preceding year’s growth.
• Are more common in monocots, but eudicots and ferns also possess them.
Examples: Corm and bulb-forming plants such as Lily and Gladiolus
Corm and bulb tend to move upward in the soil over time.
Without contractile roots, they would eventually be exposed at
the soil’s surface
ROOT WITH UNUSUAL FUNCTIONS
• Some roots reproduce asexually by producing suckers, which are aboveground stems that develop from adventitious buds on the roots.
• Each sucker grows additional roots and becomes an independent plant when the parent plant dies.
• These plants are difficult to control, because pulling the plant out of the soil seldom removes all the roots.
Examples: Black locust, pear, apple, cherry, red raspberry and
blackberry
Examples: Some weeds – field bindweed
The roots grow as deep as 3 meters in the soil. In response to a wound
the roots produce additional suckers
ROOT ASSOCIATION WITH OTHER SPECIES
• The two types of roots may grow together by secondary growth to form a natural graft.
• The root of most plant series form a mutually beneficial relationship with certain soil fungi.
• These subterranean associations known as mycorrhizae, permit the transfer of materials (such as sugars) from the roots of the fungus.
• At the same time, essential minerals (phosphorus) move from the fungus to the roots of the host plant.
• The threadlike body of the fungal partner extends into the soil, extracting minerals beyond the reach of the plant’s roots.
Examples: Between birch and maple
Because their vascular tissues are c0nnected in the graft, dissolved sugars and other materials such as hormones
pass between the two trees. Root grafts have been observed in more than 160
tree species
Ectomycorrhizae –fungal mycelium encircles the root
like sheath
Endomycorrhizae –fungus that
penetrates root cells
The relationship is mutually beneficial because when mycorrhizae are not
present, neither the fungus nor the plant grows as well
ROOT ASSOCIATION WITH OTHER SPECIES
• Certain nitrogen-fixing bacteria, collectively called rhizobia form associations with the roots of leguminous plants.
• Swellings called nodules develop on the roots and house millions of the rhizobia.
• Like mycorrhizae, the association between nitrogen-fixing bacteria and the roots of plants is mutually beneficial.
Examples: Clover, peas and soybean
The bacteria receive the products of photosynthesis from the plants while
helping the plant meet its nitrogen requirements by producing NH3 from
atmospheric nitrogen.