Root Structure and Function

Introduction to Plant Growth and Development

• Plant growth and development relies on processes that give organs their shape.
• Allows generation of specialized cells and tissues within organs.
• Growth is the irreversible increase in mass from cell division and expansion.
• Development is the sum of changes that progressively elaborate a plant's body.
• Plants exhibit indeterminate growth, growing throughout their life.
• Flowers and leaves undergo determinate growth, ceasing to grow after a size threshold is reached.
• Annual plants complete their life cycle (germination to death) in one year or less, like wildflowers, cereals, and legumes.
• Biennial plant's life spans two years, with a cold period between vegetative growth and flowering.
• Perennials live for many years, including trees, shrubs, and some grasses; they often die from infection or environmental trauma, not old age.

Meristems and Plant Growth

• Indeterminate plant growth comes from perpetually embryonic tissues called meristems.
• Meristem cells divide, adding new cells.
• Some cells remain in the meristem, while others specialize into tissues of the growing plant.
• Initials are cells that remain as a source of new cells within meristems.
• Cells displaced from the meristem divide for some time, then specialize into developing tissues.
• Plant growth pattern depends on where the meristems are located.
• Apical meristems are located at root tips and shoot buds, providing cells for growth in length.
• Apical meristems enable primary growth; roots explore the soil and shoots increase light exposure.
• Woody plants also undergo secondary growth, progressively thickening roots and shoots..
• Secondary growth is the product of lateral meristems, which are cylinders of dividing cells along the length of roots and shoots.
• Primary growth is restricted to the youngest parts of woody plants, like root and shoot tips.
• Lateral meristems develop in older regions of roots and shoots.
• Each growing season, primary growth produces young extensions of roots and shoots.
• Secondary growth thickens and strengthens the older plant parts.
• The dormant terminal bud at the tip of a winter twig of a deciduous tree is enclosed by scales, protecting the apical meristem.
• In spring, the bud sheds its scales and starts a new spurt of primary growth.
• Nodes are marked by scars where leaves fell in autumn along each growth segment.
• An axillary bud or a branch twig is above each leaf scar.
• Scars from the scales that enclosed the terminal bud during the previous winter are farther down the twig.
• Primary growth extends the shoot each spring and summer as the secondary growth thickens the parts formed in previous years.

Primary Growth and Root Structure

• Apical meristems extend roots and shoots, creating the primary plant body.
• Primary growth results in the parts of the root and shoot systems made by the apical meristems.
• A herbaceous plant and the youngest parts of a woody plant represent primary plant body.
• A thimble-like root cap covers and protects the meristem during primary growth as the root pushes through the soil.
• The root cap also secretes a lubricating slime.
• Length growth is concentrated near the root's tip.
• The zones of cell division, elongation, and maturation are located near the root tip.
• The zone of cell division includes the apical meristem and its derivatives, the primary meristems.
• The apical meristem produces primary meristems and replaces cells of the sloughed-off root cap.
• The quiescent center, with cells that divide more slowly than meristematic cells, is near the middle of the root.
• These quiescent center cells resist radiation and toxic chemicals and may act as a reserve to restore the meristem if damaged.
• The cell division zone blends into the zone of elongation, where cells elongate to more than ten times their original length.
• Elongation of cells pushes the root tip and meristem ahead.
• The meristem sustains growth, continuously adding cells to the youngest end of the zone of elongation.
• In the maturation zone, cells specialize in structure and function; the three tissue systems produced by primary growth complete their differentiation.
• The cortex and stele are major components of a cross-section of the root.
• The root zones include maturation, elongation, cell division (containing apical and primary meristems), a quiescent center, and an apical meristem protected by a root cap.
• Three primary meristems give rise to the three primary tissues of roots.
• Dermal tissue makes the epidermis (rhizodermis)
• Ground tissue makes cortex with endodermis.
• Vascular tissue makes the xylem and phloem.

Root Tissues and the Stele

• The protoderm produces the epidermis.
• Water and minerals enter through the epidermis.
• Root hairs enhance absorption by greatly increasing the surface area.
• Protoderm, ground meristem, and procambium produce the roots.
• The procambium develops into the stele, which is the central cylinder of vascular tissue where xylem and phloem develop.
• In dicot roots, the stele is a cylinder of xylem and phloem cells, while in monocot roots, the stele's central cells remain as parenchyma cells, called pith.
• Xylem cells radiate from the stele's center in two or more spokes, with phloem in the wedges between spokes in dicots.
• In monocots xylem and pholem alternate and the pith of the stele is generally ringed.
• The ground meristem between the protoderm and procambium becomes the ground tissue system, which stores food and is active in the absorption of minerals and the cortex of the cells.
• The endodermis is the cortex's innermost cylinder of thick cells, forming a boundary between the cortex and stele.
• An established root may sprout lateral roots from the pericycle.
• The pericycle, located just inside the endodermis, is a layer of cells that may become meristematic and divide; through mitosis, the lateral root elongates and pushes through the cortex until emerging.
• The stele of the lateral root maintains its connection to the primary root's stele.

Primary Growth in Shoots

• The apical meristem of a shoot is a dome-shaped mass of dividing cells at the terminal bud.
• From the apical meristem are the primary meristems, protoderm, procambium, and ground meristem.
• Leaves arise as leaf primordia on the apical meristem's flanks.
• Axillary buds develop from meristematic cells left by apical meristems at the leaf primordia base.
• Leaf primordia are crowded close in the bud due to short internodes.
• Most shoot elongation occurs by growth in the length of slightly older internodes below the shoot apex from cell division and elongation.
• Meristematic regions called intercalary meristems, at the base of each internode are in grasses and cause internodes to elongate.
• Axillary buds form branches of the shoot system later on.
• Lateral roots originate deep in the main root.
• Branches of the shoot system originate from axillary buds at the surface of a main shoot.
• Branches can develop with connections to the vascular tissue without originating from deep within the main shoot.
• The vascular tissue runs the length of a stem in strands called vascular bundles, unlike their root position
• At the root and shoot connection, the stem's vascular bundles converge as the root's vascular cylinder.
• Each vascular bundle of the stem is surrounded by ground tissue.
• Vascular bundles are in a ring, with pith on inside and cortex outside in stems of dicots
• Xylem faces the pith and the phloem faces the cortex in the vascular bundles.
• Thin rays of ground tissue connect the pith and cortex.
• Vascular bundles are scattered throughout the ground tissue in the stems of monocots rather than arranged in a ring.
• In monocots and dicots, the stem's ground tissue is mostly parenchyma.
• Stems are strengthened by collenchyma just beneath the epidermis.

Leaf Tissues

• The leaf epidermis is made of cells tightly locked together like pieces of a puzzle on both sides of the leaf.
• A first line of defense against physical damage and pathogenic organisms is the leaf epidermis as well as barrier to water loss is the waxy cuticle.
• Stomata, flanked by guard cells, interrupt only the epidermal barrier in leaves.
• The guard cells form the stromata
• The stoma allow gas exchange between the surrounding air and the photosynthetic cells inside, but are also avenues for evaporative water loss (transpiration).
• Mesophyll, the ground tissue of the leaf, is sandwiched between the upper and lower epidermis.
• Mesophyll consists mainly of parenchyma cells equipped with chloroplasts and specialized for photosynthesis.
• The labyrinth of air spaces around irregularly spaced cells is where oxygen and carbon dioxide circulate and where gas exchange with the outside air occurs.
• Vascular tissue of a continuous with the xylem and pholem of the stem.
• Branches of vascular bundles called Leaf traces, in the stem, pass through petioles and into leaves.
• Veins within a leaf subdivide repeatedly and branch throughout the mesophyll
• Xylem transports water and minerals to photosynthetic tissues..
• Phloem transports its products to other parts of the plant.
• The vascular infrastructure reinforces the leaf's shape.

Secondary Growth and the Vascular Cambium

• Lateral meristems add girth by creating secondary vascular tissue and periderm.
• The periderm is the protective outside layer.
• The stems and roots, but not the leaves, of most dicots increase in girth by secondary growth
• The secondary plant body consists of the tissues produced during this secondary growth in diameter.
• Vascular cambium acts as a meristem for secondary xylem and phloem production..
• Cork Cambium acts as a meristem for a tough replacement for stem epidermis.
• The Vascular cambium cylinders of meristematic cells that forms secondary vascular tissue.
• Accumulation of vascular over the years accounts for most woody plant diameter growth.
• Secondary xylem that grows is to interior and secondary phloem is exterior of the vascular cambium.
• Elongation of the stem (primary growth) occurs at the vertical meristem so increases in diameter (secondary growth) occur farther down the stem.
• As secondary growth continues, layers of secondary xylem accumulates to form wood.
• Wood is mainly tracheids, vessel elements and fibers.
• Cells dead at maturity, have thick, give wood its hardness.
• Secondary growth in perennial plants ceases during the winter.
• First trays cells and vessels formed spring larger than cells later .
• The thick walled cells allow for more physical support.
• Camby dormancy pattern of first and production rings produces annual growth rings.

Periderm and Bark

• Early in secondary growth, the epidermis dries, splits, and falls; it is replaced by tissues produced by the cork cambium.
• Cork cambium produces cork cells, accumulating at the exterior.
• Waxy material in cell walls before they die acts as a against water loss, physical damage.
• The periderm, protective layer formed from the derm.
• Lenticels or splits develop in periderm local cork cambium .
• These trunk areas are gas exchange trunk exterior.
• Bark refers to all external cambium phloem, cambium, and cork.
• Only the secondary transport
• Old stem protect stem off during secondary.
• After several growth growth, zones are visible the annual and support the .
• Annual growth rings contain two zones, which are secondary , cambrium floen, cobium and .
• Herwood doesn't transport walls rings protect fungi trees.

This lesson covers root elongation, cell differentiation, and tissue systems. It explores the origin of the epidermis, stele, and lateral roots, as well as differences between dicot and monocot roots. Secondary growth and the functions of heartwood, bark, and lenticels are also discussed.