Vascular plant organs

• Plants are composed of cells, tissues, and organs, similar to most animals
• A cell is the the most basic unit of life
• A tissue is a group of cells consisting of one or more cell types performing a specialized function
• An organ consists of several types of tissues that carry out particular functions

Basic Vascular Plant Organs

• Roots, stems, and leaves are the three basic organs of vascular plants
• These organs form a root system and a the shoot system
• The shoot system consists of stems and leaves

Roots

• Root's function is to anchor a vascular plant in the soil
• Roots absorb minerals and water
• Roots often store carbohydrates and other reserves
• Primary and lateral roots are types of roots

Tall Plants vs Small Plants

• Tall, erect plants with large shoot masses generally have a taproot system
• Taproot system contain one main vertical root, the taproot, which usually develops from the primary root
• The vascular plants with a trailing growth habit are susceptible to grazing animals who can kill it
• Small vascular plants are efficiently anchored by a fibrous root system
• The fibrous root system is a thick mat of slender roots spreading out below the soil surface
• Most plants absorb water and minerals near the tips of elongating roots
• Root hairs, which are thin, finger-like extensions of root epidermal cells, increase the surface area of the root enormously
• Most root systems form mycorrhizal associations, symbiotic interactions with soil fungi that increase a plant's ability to absorb minerals
• Roots of plants are often adapted for specialized functions

Evolutionary Adaptations of Roots

• Buttress roots give architectural support to trunks of trees and are common because of moist conditions in the tropics
• Prop roots are aerial, adventitious roots of maize (corn) that support tall, top-heavy plants
• Storage roots of many plants store food and water, for example the common beet
• Pneumatophores (air roots) are produced by trees like mangroves in tidal swamps, enabling root systems to obtain oxygen in waterlogged mud
• "Strangling" aerial roots germinate in crevices of tall trees, where they grow to the ground, wrap around the host tree and objects, and eventually shade it out, killing it.

Stems

• Stems are plant organs bearing leaves and buds
• Stems elongate and orient the shoot to maximize photosynthesis

Stem Structure

• Nodes are the points on a stem at which leaves are connected
• Internodes are the stem segments between nodes
• The Apical bud is near the growing shoot tip
• The axillary bud forms a lateral branch, thorn or flower

Modified Stems

• Some plants stems have alternative functions, for example food storage and asexual reproduction
• Many modified stems, including rhizomes, stolons, and tubers, are mistaken for roots

Leaves

• Leaves are the main photosynthetic organ
• Leaves exchange gases with the atmosphere, dissipate heat, and defend themselves from herbivores and pathogens
• In general, a leaf consists of a flattened blade and a stalk, the petiole, which joins the leaf to the stem at a node
• Grasses and many other monocots lack petioles; instead, the base of the leaf forms a sheath that envelops the stem
• In a simple leaf, the blade is single and undivided
• In a compound leaf, the blade consists of multiple leaflets
• Compound leaves may withstand strong wind with less tearing
• Compound leaves may confine some pathogens

Evolutionary Adaptations of Leafs

• Almost all leaves are specialized for photosynthesis
• Some species have leaves with adaptations that enable them to perform additional functions, such as support, protection, storage, or reproduction
• Tendrils, by which pea plants cling to support, are modified leaves
• Spines of cacti are modified leaves where photosynthesis is carried out by the fleshy green stems
• Bulbs, such as cut onion, have a short underground stem and modified storage leaves
• Some succulents leaves produce adventitious plantlets, which fall off the leaf and take root in the soil, leading to reproduction

Plant Cell Structure

• Cells that make up a plant form three main tissue systems: ground tissue, dermal tissue and vascular tissue
• Each tissue type derive its properties from a unique combination of specialized cells
• Together, these cells carry out all of the plant's functions

Ground Tissue

• Ground tissue often fills the spaces between more specialized cell types inside roots, stems, leaves, fruits & seeds
• Ground tissue is a site for photosynthesis, respiration, and storage

Vascular Tissue

• Xylem & Phloem form a continuous distribution system embedded in the ground tissue of shoots & roots
• A vascular bundle is a strand of tissue containing xylem & phloem

Dermal Tissue

• Covers the plant
• Consists of the epidermis, a single layer tightly packed transparent parenchyma cells in herbaceous plants
• The Dermal tissues include the cuticle, stomata, and guard cells

Plant Cell Types

• Ground tissue makes up the majority of the body of a herbaceous plant
• Three main types of plant cells: parenchyma, collenchyma, and sclerenchyma

Parenchyma

• Parenchyma is the most abundant tissue type
• Parenchyma is invlolved in photosynthesis, respiration, gas exchange & starch storage

Collenchyma

• Collenchyma provide elastic support without interfering with growth of young stems and expanding leaves

Sclerenchyma

• Sclerenchyma provides inelastic support to parts of a plant that are no longer growing
• Sclerenchyma contains a tough, complex component in its secondary cell wall called lignin

Conducting Cells in Xylem and Pholem

• Vascular tissues transport water, minerals, carbohydrates, and other dissolved compounds thru the plant
• Two types of vascular tissue: xylem and phloem

Xylem

• Xylem are tissues transporting water & dissolved minerals from the roots to all parts of the plant.

Tracheids cells

• Long, narrow water-conducting cells that overlap at their tapered ends
• Water moves from one tracheid to another through pits

Vessel Elements

• Short, wide water-conducting cells that stack end to end forming long, continuous tubes
• Vessel side walls have pits, but end walls are perforated or absent

Pholem

• Phloem has main conducting cells call sieve tube elements
• Sieve tube elements align end to end to make a single functional unit called sieve tube
• Companion cell is are associated with the sieve tube
• Companion cell is specialized parenchyma that retains all of its organelles for the sieve tubes

Plant Growth

• Shoots become larger by adding ("modules") consisting of repeated nodes and internodes
• Roots branch as they explore the soil
• Determinate growth is when growth stops after a structure is made
• Indeterminate growth continues through lifetime

Modular Growth

• Plants produce most root tips in pockets of soil with richest nutrients
• Shrubs receiving most sunlight can add new branches; shaded limbs remain unchanged
• The loss of a branch or root does not harm a plant
• Neighboring branches can add modules to compensate for a broken tree limb
• Modular growth distinguishes plants from animals

Plant Growth at Meristems

• Meristems undergo active mitotic cell division
• Two types of meristems: Apical and Lateral

Apical Meristems

• When cells in the apical meristem divide, they can produce new cells that differentiate into all of the tissue types

Lateral Meristems

• When the cells divide, they typically divide to produce tissues to both the inside & outside of the meristems

Sexual and Asexual Reproduction in Angiosperms

• Sexual Reproduction produces genetically variable offspring, increasing reproductive success in a changing environment
• Asexual Reproduction produces identical clones developing from the roots, stems, or leaves of a parent plant

The Angiosperm Life Cycle

• Plant life cycles include alternation of generations
• Diploid sporophyte undergoes meiosis and produces haploid spores, which give rise to haploid gametophyte generation
• The gametophytes produce gametes
• Fertilization yields a diploid zygote, which undergoes mitosis to develop into a new sporophyte
• Flowers and fruits are two features found only in angiosperms

Flowers

• Flowers are reproductive structures with parts attached to a receptacle
• Sepals and petals are accessory structures that enclose the stamens
• Stamens have pollen-containing anthers at their tips
• Carpels occupy the center of the flower
• Each carpel's ovary contains one or more ovules
• The stigma tops the style, which extends from the ovary

Spores

• In the anther, specialized cells undergo meiosis to produces four haploid microspores
• The microspores divide mitotically to yield immature pollen grains
• In ovules, specialized cells undergo meiosis to yield four haploid cells
• One of the four haploid cells becomes a megaspore which then divides mitotically 3 times

Pollens to the Stigma

• Animals and wind carry pollen from anthers to a stigma
• Flowers are usually adapted to attract pollinators
• Flowers and their pollinators coevolve

Double Fertilization

• After a pollen grain lands on the stigma, a pollen tube and sperm nuclei grow through the tube toward the ovary
• In double fertilization, one sperm nucleus fertilizes the egg to form a diploid zygote
• In double fertilization, the other sperm fertilizes the central cell's two nuclei to make a triploid endosperm

Seed Formation

• A seed is an embryo, endosperm and seed coat
• Endosperm nourishes the developing embryo as cells divide
• As the embryo grows, one or two cotyledons develop
• Seeds enter a dormancy period of development

Fruit Formation

• After fertilization, non-essential flower parts fall off
• Hormones influence the ovary (and sometimes other plant parts) to develop into a fruit

Seed Dispersal

• Fruits protect the seeds
• Animals, wind, and water disperse seeds to new habitats

Seed Germination

• Seed germination requires oxygen, water and a favorable temperature
• The new plant's growth begins when the embryo emerges from the seed coat

Plant Hormones

• Abscisic acid maintains seed dormancy until favorable conditions arise
• Gibberellins break seed dormancy
• Cytokinins stimulate cell division in germinating seeds
• Auxins stimulate stem elongation, suppress lateral bud growth, and controldirection of growth
• Gibberellins and cytokinins stimulate cell division in shoot and roots
• Cytokinins stimulate lateral bud growth
• Abscisic acid inhibits shoot growth and maintains bud dormancy
• Cytokinins delay aging of leaves
• Ethylene hastens fruit ripening and promotes shedding of petals and leaves
• Auxins are highest at the main shoot tip & prevents the lateral buds from growing (called apical dominance
• Cytokinins stimulate cell division in actively developing plant parts

Plant Growth and Tropisms

• Gibberellins stimulate cell division and elongation, helping break dormancy
• Ethylene speeds ripening and aging
• Abscisic acid inhibits shoot growth

Tropisms

• Photoreceptors absorb light energy to influence a plant's growth and response to environment
• A tropism is a growth response toward or away from an environmental stimulus
• Light stimulates auxin to move in phototropism, causing the stem to bend towards the light
• Gravitropism is growth toward or away from the direction of gravity, as helped by starch granules
• Thigmotropism is a growth reaction to touch or a mechanical stimuli

Explore the basic organs of vascular plants: roots, stems, and leaves. Learn about root systems versus shoot systems. Understand how the taproot system helps tall plants thrive.