Flowering Plant Root System

Parts of a Flowering Plant

• A common flowering plant includes a cylindrical axis
• The axis differentiates into two systems, the root and the shoot

Root System

• Roots absorb water and nutrients
• Roots anchor plants firmly
• Roots transport water and minerals
• Roots can store foods or nutrients
• Roots can be used for climbing
• They are sometimes used for propagation
• Genetically, roots are positively geotropic and negatively phototrophic
• Roots are generally do not feature the green colour due to the absence of chlorophyll
• Roots generally lack nodes and internodes
• Lateral branches of roots are endogenous, originating within inner tissue

Root Structure (Regions/Zones)

• The apex of the root is covered by a cap-like root cap structure
• The root cap's main function is to protect the root apex
• The meristematic zone, also known as the zone of cell division, is the growing tip of the root
• Cells in the meristematic zone actively divide and continuously increase in number
• Apical meristem cells are added to the root cap and elongation region
• The zone of elongation lies above the meristematic zone
• Cells in the zone elongation increase in size
• The zone of elongation contributes to the increased length of the plant root
• The zone of maturation, also known as the zone of cell differentiation, lies above the elongation zone
• Cells in the zone of maturation differentiate into root tissues
• Root hairs are in the zone of maturation
• Root hairs are delicate, elongated epidermal cells in a small zone behind the root's growing tip
• Root hairs absorb water and minerals from the soil

Types of Root Systems

• Two types of root systems exist
• Tap root systems develop from a radical and are common in dicotyledon plants
• Adventitious root systems develop from plant parts other than the radicle
• Adventitious root systems are common in monocots
• Adventitious roots perform the primary root's normal function, and can be metamorphosed for specialized tasks
• Lateral adventitious roots arise from various regions of the plant body alongside a short-lived primary root

Root Modifications

• There are normal non-tuberous tap roots
• Tap Roots can be classified into Conical, Fusiform, or Napiform -Conical tap roots include carrots
• Fusiform tap roots include radishes
• Napiform tap roots include turnips or beets

Adventitious Roots

• Fibrous roots are thread-like and of equal size and length
• Fibrous roots are found in Gramineae like maize, sugarcane, and wheat
• Prop roots arise from the first few basal nodes of the stem
• Prop roots grow obliquely from the stem down into the soil
• Prop roots provide support against wind action
• Underground portions of prop roots can facilitate absorption
• Maize and sugarcane are examples of prop roots
• Aerial roots arise from horizontal aerial branches of large trees
• Aerial roots grow vertically downwards, penetrate the soil, and branch inside
• As aerial roots thicken, they act as pillars
• Storage roots, are adventitious roots store food and become fleshy and swollen with some species including sweet potato
• Climbing roots, also known as clasping roots or root tendrils, develop on the nodes of weak-stemmed plants

Additional Root Types

• Contractile roots are unbranched, strong coiled Adventitious roots originating from the base of an underground stem
• When stretched or contracted, contractile roots pull plants deeper to remain subterranean or protect from temperature and drying
• Respiratory or breathing roots are common among swamp plants like in Mangrove
• These roots developing branches grow vertically upwards where the soil is water-logged and without oxygen
• Respiratory roots have lenticels to exchange gases
• Many mangrove species such as Avicennia germinans and Laguncularia racemosa have respiratory roots
• Parasitic or haustorial roots are produced by parasitic flowering plants
• Parasitic roots penetrate host plant tissue and draw nutrients

Shoot System - Stem

• The stem is a part of the plant which lies above the surface of soil
• It originates from the plumule of the embryo
• Stems are negatively geotropic and positively phototropic
• Stems bear a terminal bud and axillary buds in the axils of leaves, for growth
• Stems present nodes and internodes

Stem Functions

• Stems produce and support of lateral appendages like branches, leaves, flowers, and fruits
• Stems conduct water and minerals to different parts of shoots
• Stems transport food or photosynthates to all plant parts
• Stems may be modified to perform functions such as storage, proliferation, support, perennation, or photosynthesis

Buds

• A bud is an undeveloped, compressed young shoot
• Bud types include; Position, Development and Duation & Habit

Bud Positions

• Terminal or Apical buds are found at the apex of the main stem or branch
• Lateral or axillary buds arise in the axil of a leaf
• Adventitious buds develop in positions other than normal buds

Bud Development (Function/Activity)

• Vegetative buds grow into new branches and shoots
• Floral or reproductive buds contain embryonic flowers
• Mixed buds grow into shoots bearing both leaves and flowers

Bud Durations and Habits

• Winter buds are covered and protected by scale leaves from frost, rain, and other agents during winter
• Early during spring, the scales unfold and fall down to allow buds to open and develop quickly
• Summer buds are naked buds without scales and are common among evergreen and herbaceous plants

Stem Branching

• Unbranched stems are called caudex or columnar
• Example of plants stems being called candex of columnar includes Palm
• Branching of the stem is of two types: dichotomous and lateral

Dichotomous Branching

• In dichotomous branching, the growing point gets divided into two in the region of branching

Lateral Branching

• In lateral branching, the growing point does not get divided
• The main stem grows from an apical bud or terminal bud
• Lateral branches develop on the main axis laterally from the lateral buds

Types of Stems

• Structures that fall under Stem classification depend on the following; - structure (amount of woods) -habit

Stem Structures

• Herbaceous stems don't produce bark, and are soft and the green stems can be easily bent
• Woody stems are hard stems which cannot be easily bent

Stems and Habit

• Stems with an Erect habit are classified as Woody or Herbaceous
• Stems with an Weak habit classifed as twiners, prostrate or climbers

Types of Stems

• Location relative to the ground differentiates stems
  • Aerial or Epiterranean stem
  • Underground or Subterranean stem

Aerial Stems

• Aerial stems are found above ground and perform various functions, can be reduced, erect, or weak

Reduced Stems

• Reduced stems are stems reduced to a disc
• Examples of plants with reduced stems include: Radish, Carrot, and Turnip

Erect Stems

• Erect stems are strong and upright with sufficient lignified mechanical tissue
• They may be herbaceous or woody

Weak Stems

• Weak stems are thin, soft, and lack mechanical tissue, thus needing support
• Weak stems can be upright (climbing or twining) or prostrate (growing on soil surface)
• Climber plants have long weak stems and organs of attachment
• Hook climbers attach by hooks, while tendril climbers attach by thread-like tendrils
• Twiners' stem body twines around the support without special organs
• Examples like Cuscuta and Dolichos, are considered twiners
• Prostrate stems are weak, creeping, or running on the soil

Stem Modifications

• Aerial modifications or Underground

Aerial Stem Modifications

• Leafy (Photosynthetic) such as a-phylloclade or b- Cladode.
• Succulent stem modifications are common
• Thorn stems, are a protective stem modification

Leafy or Photosynthetic Stem Modifications

• Phylloclade stems are flat, green leaf-like modifications of entire shoots with several nodes and internodes
• Ruscus, Muehlenbeckia, and Opuntia are examples of Phylloclade stems
• Cladode are green, cylindrical or flattened stems branch of limited growth and have one internode
• Asparagus produce Cladodes that are flattened, narrow, elongated, or needle-shaped

Succulent Stems

• Succulent stems are fleshy and specialized to store water and food

Thorn Stems

• Thorn stem modifications appear as hard, woody, sharp outgrowths protecting against grazing animals
• They are common in xerophytes like Alhagi

Underground (Subterranean) Stem Modification Characteristics

• Plants develop non-green perennial underground stems which can provide vegetative propagation, perennation, and food storage
• Underground stems produce aerial shoots annually in favorable conditions
• Unfavorable conditions cause the aerial shoots die
• The underground stems remain dormant during this period
• These stems resemble roots superficially, but are can be distinguished by the presence of nodes, internodes, scale leaves etc

Additional Underground Stem Characteristics

• Four different types of underground stem are identified: Rhizome, Bulb, Corm and Tuber

Four Key Stem Types

• Rhizomes are horizontal, fleshy, non-green underground stems
• Rhizomes has distinct nodes and internodes
• Scale leaves can be found nodes which also protect developing axillary buds
• Adventitious roots arise from the lower side
• Cynodon and Ginger are examples of Rhizomes
• Bulbs are short discoid stems that carry a terminal bud on the upper surface
• The terminal bud of a Bulb is surrounded by fleshy, scaly leaves while many roots arise from its base
• Food is stored in flashy leaves in the bulb with examples inclduing onions and garlic
• Corms are condensed forms of rhizomes that grow vertically
• The Corm is spherical with a flattened base and bears circular nodes and internodes
• Nodes bear scale leaves and axillary buds
• Adventitious roots arise either from its base or at each node, with Colocasia as an example
• Tubers are swollen tips of special underground branches
• A common example of a Tuber is potato where the surface has "eyes", represent nodes with buds subtended by a leaf scar
• A large scar end marks stolon attachment with few to no adventitious roots

Morphology of Leaves

• Leaves are green, thin, flattened lateral stem outgrowths
• Leaves are borne at stem nodes
• Leaves are the main organs of photosynthesis

Leaf Insertion

• Cauline leaves are found on the node of the stem - (Maize)
• Radical leaves develop from the nodes of the underground stem and seem to develop from roots - (Radish, Turnip)

Venation

• Veins arranged in the leaf blade or lamina is called venation
  • Reticulate
  • Parallel
• Reticulate venation is common in dicot leaves
• Parallel venation is observed in monocot leaves

Phyllotaxy

• Leaves are arranged on the stem or branches stems
• Alternate leaves are arranged each node
• Opposite leaves are arranged two at each node
• Whorled leaves are arranged in a whorl with three or more at each node

Leaf Parts

• Leaf base
• Petiole
• Lamina/Blade

Leaf Base

• The section that allows the leaf to attach to the stem or one of its branches is called the leaf base

Petiole

• Connects the lamina with the stem or branch
• Leaves with petioles are petiolate
• Leavers without petioles are sessile

Leaf Blade Modifications

• Leaves can be simple in a leaf where the blade or lamina is entire
• Lobed leaf, where the blade is is divided into portions still connected by tissues
• Compound leaf, a leaf in which the lamina is divided into a number of leaf like lobes called leaflets

Simple Leaf Blades

• Simple leaf blades is a single leaf blade
• Have multiple shapes including elliptic, lanceolate, acicular, linear, oblong etc

Lobed Leaf Blades

• The blade is deeply divided but lobes remain united by tissue

Compound Leaf Blades

• In compound leaves, the lamina divides into leaflets borne on a common axis without axillary buds
• Compound pinnate leaves have leaflets borne on a common axis called the rachis.
• Compound palmate leaves have leaflets attached at a common point

Differences between Simple Leaves and Compound Leaves

• Simple leaves have a single lamina with a bud at the leaf axil.
• Compound leaves have several leaflets and no bud at the axils of the leaflets.

Additional differences between Compound Leaves and Branches

• A compound leaf arises directly on the stem whereas a branch is axillary to a leaf
• Compound leaves lack nodes and internodes whereas brances contain them
• Compound leaves contain axillary buds whereas a branch lacks them
• The compound leaf lacks terminal buds whereas a branch contains them
• Stipules lack at the base of the leaflets where they are at the based of a branch
• The leaf is abscissed as a whole, while leaves on branches fall at different times

Modifications of Leaves

• Leaves are modified to handle additional functions (other than photosynthesis and transpiration)
• The modifications are:
  • Leaf Tendrils
  • Leaf-spines
  • Phyllode
  • Insect Catching Leaves

Leaf Tendrils

• Weak stemmed plants grow using these leaf structures which wrap leaves or portions of leaves around plants

Leaf Spines

• Leaves become wholly or partially modified into sharp pointed structures known as spines

Phyllode

• The petiole or rachis flatten to take on the shape and green color/functionality of leaves.

Insect Catching Leaves

• Insectivorous plants catch and digest insects to fulfill their nitrogen requirement