Plant Body: Structure and Growth

Plant Body Organization

Plant Organs and Systems

• Plant organs are organized into two systems:
  • Root system: Anchors the plant, absorbs water and minerals, and stores photosynthetic products. Branching increases surface area.
  • Shoot system:
    • Leaves: Main photosynthetic organs.
    • Stems: Hold leaves up in the sunlight and connect roots and leaves.

Vegetative Plant Organs

• The shoot system consists of stems and leaves, where photosynthesis takes place.
• The root system anchors the plant and provides water and nutrients for the shoot system.
• Phytomer: Repeating units that shoots consist of. Each consists of a node plus leaf, and internode.
• Shoot apical meristem: The tip group of cells that has the capacity to produce new organs.
• Axillary buds: Contain a shoot meristem tightly enclosed in leaves; can produce a new branch.
• Root apical meristem: Meristem at the apex of the root that first emerged from the seed.

Monocots vs. Eudicots

• Most angiosperms are in two clades:
  • Monocots: Narrow-leaved plants such as grasses, lilies, orchids, palms.
  • Eudicots: Generally broad-leaved plants such as soybeans, roses, sunflowers, maples.
• The two clades differ in characteristics such as root system morphology.

Key Differences

Plant Tissue Systems

• Plant organs are organized into three tissue types:
  • Dermal tissue system: The outermost layer of cells, or epidermis.
  • Ground tissue system: Between dermal and vascular tissue; makes up most of the plant body.
  • Vascular tissue system: The plumbing, or transport, system.

Function of Tissue Systems

• Dermal tissue system: Forms the outer covering of the plant.
• Ground tissue system: Carries out photosynthesis, stores photosynthetic products, and helps support the plant.
• Vascular tissue system: Conducts water and solutes throughout the plant.

Plant Cell Walls

• Plants have cell walls made of cellulose.
• Cell walls serve the important structural function of holding the plant upright.
• Strong walls can prevent cell elongation, while thin walls permit growth, provide flexibility, and conserve resources.
• Each species must strike a balance between the two functions.

Ground Tissue Cell Types

• Parenchyma cells: Most abundant cells in a plant.
  • Large vacuoles and thin cell walls.
  • Perform a variety of functions—transport, photosynthesis, synthesis and storage of metabolites, storage of protein and starch.
  • Depend on water content to be turgid.
  • Have primary cell walls—material is added only while the cell is expanding.
• Collenchyma cells
  • Elongate, with unevenly thickened cell walls
  • Firm enough to provide support but flexible enough to permit growth.
  • Have primary cell walls
  • When a plant gets dry, cells lose turgor and wilting occurs in parts of the plant that are composed of parenchyma and collenchyma.
• Sclerenchyma cells
  • Thick secondary cell walls reinforced with lignin
  • Undergo programmed cell death; the cell walls remain to provide support
  • Fibers—organized into bundles; provide rigid support
  • Sclereids—various shapes; may pack together densely, as in a nut shells or seed coats.

Vascular Tissue Cell Types

• Xylem: Carries water and minerals from roots to all cells of roots and shoots.
  • Two types of cells—tracheids and vessel elements—lignified, dead when mature, provide structural support.
    • Tracheids: have pits in cell walls that allow movement of water.
    • Vessel elements: are large diameter, form a pipeline.
• Phloem: mostly living cells
  • Moves carbohydrates from production sites (sources) to sites where they are used or stored (sinks).
  • Sieve tube elements meet end-to-end, forming sieve tubes connected by a set of pores called a sieve plate.
  • They are parenchyma cells which, although still alive, have lost much of their cellular contents.
  • Companion cells are connected to sieve tube elements by plasmodesmata and perform many of the phloem’s metabolic functions.
  • Phloem fibers are sclerenchyma cells that provide support for the plant

Stem Cross Sections

• The vascular tissues in stems are organized into bundles.

Leaf Anatomy

• Leaves have all three tissue systems.
• Leaf anatomy is well adapted to:
  • Carry out photosynthesis
  • Exchange with the environment
  • Limit evaporative water loss
  • Export products of photosynthesis to the rest of the plant
• Leaf surfaces are covered with nonphotosynthetic epidermal cells that secrete a waxy waterproof cuticle.
• Water vapor and gases are exchanged through pores called stomata.
• Specialized cells called stomatal guard cells control the extent to which the stoma is open.

Leaf Ground Tissue

• Ground tissue in leaves has two zones of photosynthetic parenchyma tissue called mesophyll:
  • Palisade cells: Lie just under the epidermis where light is abundant; have many chloroplasts and large surface area for gas exchange.
  • Spongy mesophyll: Irregular cell shapes create spaces where water and air are next to each other, and light is scattered.

Leaf Vascular Tissue

• Vascular tissue forms a network of veins that extend to within a few cell diameters of all cells in the leaf, ensuring the mesophyll cells are supplied with water and minerals.
• Also adds mechanical support.
• Photosynthetic products are loaded into the phloem tissue of the veins for export to the rest of the plant.

Pine Needle Adaptations

• Pine needles have adaptations for a dry environment: low surface-to-volume ratio, a thick cuticle, and stomata that are recessed (sunken) below the epidermis, which decreases water loss.
• Chloroplasts are present in the mesophyll tissue surrounding the stomatal openings.

Apical Meristems and Plant Growth

• The plant embryo in the seed has apical meristems at shoot and root tips which give rise to the rest of the plant.
• Meristem cells are comparable to animal stem cells: upon cell division, one daughter cell can differentiate, the other remains undifferentiated.
• Plants have an indeterminate body plan—they can continue making organs throughout their lives.
• Apical meristems produce primary meristems: the cells and tissues that arise from them form the primary plant body.
• Secondary meristems increase the width of the plant.
• Division of cells along the sides of the meristem produces new cells that form bulges.
• Each bulge, or leaf primordium, can form a leaf.

Shoot Apical Meristem

• Clonal analysis of variegated plants has shown that the shoot apical meristem has three cell layers:
  • L1 produces the dermal layer
  • L2 produces ground tissue
  • L3 produces vascular tissue
• After apical meristem cells divide, they can expand, which pushes the apical meristem upward, elongating the stem.

Root Meristem

• The root meristem is organized around a group of cells called the quiescent center, which rarely divide.
• It is surrounded by initial cells that do divide.
• Initial cells contribute to a root cap, which protects the root as it pushes through the soil.
• The root cap secretes a mucopolysaccharide (slime) that acts as a lubricant; also detects the pull of gravity and dictates the directional growth of roots.
• Initial cells on the other side of the quiescent center divide to form the main body of the root.
• In a mature root, the pericycle is a ring of cells surrounding the xylem and phloem; contributes to thickening in some roots and gives rise to lateral roots.
• Root hairs increase the surface area of the root system for uptake of water and soil nutrients.

Plant Diversity

• Diversity of plant forms results from differences in the relative shape, size, or number of the simple modules that make up a plant.
• Minor differences in genomes or gene regulation can underlie dramatic differences in plant form.
• Example: the difference in branching in teosinte and domesticated corn is mostly due to a single gene.

Crop Plants

• Many crop plants have enlarged organs, such that is can be difficult to identify them; e.g., a potato is an underground stem (rhizome).
• Potato eyes are axillary buds.
• Axillary buds are found at the junction between leaf and stem in both simple and compound leaves.

Simple vs. Compound Leaves

• Simple leaf vs. compound leaf

Secondary Growth

• Roots and stems of some eudicots develop a secondary plant body, the wood and bark.
• These tissues are derived by secondary growth from secondary (or lateral) meristems:
  • Vascular cambium: Produces secondary xylem (wood) and secondary phloem (inner bark).
  • Cork cambium: Produces waxy-walled protective cells near the exterior of the stem that become outer bark.

Vascular Cambium

• The vascular cambium is initially a single layer of cells between the primary xylem and primary phloem in the vascular bundles.
• A stem increases in diameter when the cells of the vascular cambium divide and expand, producing secondary xylem toward the inside and secondary phloem toward the outside.
• Successive layers of secondary xylem constitute the wood.

Annual Rings

• In temperate zones, annual rings form in the wood.
• In spring, tracheids or vessel elements tend to be large in diameter and thin-walled.
• In summer, thick-walled, narrow cells are produced; the wood looks darker and more dense.

Monocots and Secondary Growth

• Monocots do not have secondary growth.
• A few have thickened stems (e.g., palms).
• Palms have a very wide apical meristem that produces a wide stem, and dead leaf bases add to the diameter of the stem.