Plant Vascular Tissue: Xylem and Pholem

• Two types of water-conducting cells, tracheids and vessel elements, are dead at maturity and have thick cell walls.
• Tracheids are found in the xylem of all vascular plants.
• No energy is needed to move (conduct) the water and minerals.
• Vessel elements are common to most angiosperms and to a few gymnosperms.
• Xylem conducts inorganic ions such as nitrates, and supports the plant body.

Vascular Tissue

• Horizontal rows of parenchyma cells called rays are developed by ray initials of the vascular cambium.
• Vascular tissue functions in lateral conduction of water and minerals and food movement and storage.
• Secondary phloem carries food to secondary xylem.
• Secondary xylem carries water to secondary pholem.

Sugar-Conducting Cells of the Phloem

• Sieve-tube elements are alive at functional maturity, though they lack organelles They are the sugar transporting cells, especially in angiosperms.
• Sieve plates are the porous end walls that allow fluid to flow between cells along the sieve tube.
• Each sieve-tube element has a companion cell whose nucleus and ribosomes serve both cells.

Meristems

• Meristems generate cells for new organs.
• A plant can grow throughout its life; this is called indeterminate growth.
• Some plant organs stop to grow at a certain size, this is called determinate growth.
• Annuals complete their life cycle in a year or less, they include weeds, wildflowers, garden flowers and vegetables.
• Biennials require two growing seasons, snapdragons
• Perennials live for many years, like pines and firs

Apical and Lateral Meristems

• Meristems are perpetually embryonic tissue and allow for indeterminate growth.
• Apical meristems are located at the tips of roots and shoots and at the axillary buds of shoots.
• Apical meristems elongate shoots and roots, a process called primary growth.
• Lateral meristems add thickness to woody plants, a process called secondary growth.
• Two lateral meristems: the vascular cambium and the cork cambium.
• The vascular cambium adds layers of vascular tissue called secondary xylem (makes up most of the wood of a tree) and secondary phloem.
• The cork cambium replaces the epidermis with periderm, which is thicker and tougher.

Primary Growth

• Primary growth produces the primary plant body.
• The primary growth of roots produces the epidermis, ground tissue, and vascular tissue.
• This includes the parts of the root and shoot systems produced by apical meristems.

Secondary Growth

• Secondary growth adds girth to stems and roots in woody plants.
• Secondary growth occurs in stems and roots of woody plants but rarely in leaves.
• The secondary plant body consists of the tissues produced by the vascular cambium and cork cambium.
• Secondary growth is characteristic of gymnosperms and many eudicots, but not monocots.

Heartwood and Sapwood

• As a tree or woody shrub ages, the older layers of secondary xylem, the heartwood, no longer transport water and minerals.
• The outer layers, known as sapwood, still transport materials through the xylem.
• Older secondary phloem sloughs off and does not accumulate.

Cork Cambium and Periderm

• The cork cambium gives rise to the secondary plant body's protective covering, or periderm.
• Periderm consists of the cork cambium plus the layers of cork cells it produces.
• Bark consists of all the tissues external to the vascular cambium, including secondary phloem and periderm.
• Lenticels in the periderm allow for gas exchange between living stem or root cells and the outside air.

Fungi and Soil

• Soil fungi are naturally found in soil unless fungicides or sterilization has altered the soil.
• Roots and the hyphae of soil fungi form symbiotic associations called mycorrhizae.
• The mycorrhizae provide more surface area for absorption of minerals and water
• Mutualisms with fungi helped plants colonize land.

Water Potential

• Water potential (water activity) is a measurement that combines the effects of solute concentration and pressure.
• Describes the amount and direction of water movement.
• Water flows from regions of higher water potential to regions of lower water potential.
• Water potential is abbreviated as Ψ and measured in units of pressure called megapascals (MPa).
• Ψ = 0 MPa for pure water at sea level and room temperature.
• The total potential energy of water in the cell is the sum of its pressure potential and solute potential.
• Represents the total potential energy of the water in the cell.

Solute Potential

• Both pressure and solute concentration affect water potential.
• The solute potential (Ψs) of a solution is proportional to the number of dissolved molecules
• Solute potential is also called osmotic potential.
• Pressure potential (Ψp) is the physical pressure on a solution.
• Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast.

This covers the xylem and phloem, the key components of vascular tissue in plants. Xylem's tracheids and vessel elements transport water and minerals, while phloem's sieve-tube elements conduct sugars. Includes information on the cells that make up vascular tissue.