DSU logo BIO 101 Principles of Biology II
Structure of Flowering Plants


The two major groups (classes) of flowering plants

•  Dicots

•  Embryo in the seed has two cotyledons (seed leaves)

•  Stem vascular bundles arranged in single ring

•  Flower parts are in multiples of 4 or 5 (or many of each)

•  Leaves have net venation

•  Root system is usually a taproot

•  Monocots

•  Embryo in the seed has one cotyledon

•  Vascular bundles are scattered throughout the stem

•  Flower parts are in multiples of 3

•  Leaves have parallel venation

•  Usually have a fibrous root system

General structure of the plant body

•  We start our discussion of plant structure by looking at the overall external form of a typical vascular plant. Then we will study the internal structure by examining the cells and tissues that make up the plant body.

•  Plant organs

•  A vascular plant consists of collections of organs

•  An organ is a structure composed of several types of tissues

•  The tissues of an organ function together in a coordinated way

•  There are three types of vegetative organs - root, stem, and leaf

•  “Vegetative” means “non-reproductive.” Reproductive parts are usually composed of modified forms of other organs. For example, flower parts are considered to be modified leaves.

•  A plant consists of a root system and shoot system

The Plant Body
Gernanium plant with major parts labeled
Drawing from Botany: An Introduction to Plant Biology by James Mauseth

•  Root system

•  Typically (but not always) roots are located in the soil.

•  Anchors the plant in place

•  Absorb water and minerals from the soil.

•  Storage of food compounds

•  Shoot system

•  The shoot is composed of two organs - stems and leaves

•  Stem

•  The stem supports the shoot and keeps it upright.
•  It transports water and minerals up to the leaves.
•  It transports food compounds downward to the roots and upward to growing tissues.

•  Nodes

•  Nodes are the points where leaves attaches to a stem.
•  One, two, or more leaves may be attached at each node.
•  The stem between two nodes is an internode (inter = between).

•  Axillary buds

•  Located at node in the axil of a leaf
•  The axil is the angle above the attachment of the leaf. Any structure located here is said to be axillary.
•  Axillary buds can produce branches of the shoot.

•  Leaf

•  In most species, leaves are the primary sites of photosynthesis.
•  Some plants use leaves for protection (spines), or climbing (tendrils), or other special functions.
•  Blade
•  The blade is the flat part of a leaf.
•  Most of its cells contain many chloroplasts.
•  The blade has evolved features that adapt it to carrying out photosynthesis very efficiently.
•  Petiole
•  The stalk of a single leaf is the petiole.
•  It holds the blade in a position to receive maximum light
•  It provides a connection to the vascular system of the stem.
•  In some species the leaves lack petioles, and the leaf blades are attached directly to the stem. These are called sessile leaves.

•  Apical meristems

•  Shoots and roots grow from their tips.

•  These growing tips are called apical meristems.

Cells and tissues of the plant body

•  Three types of plant cells

•  The cells of a vascular plant come in many various kinds, but all plant cells can be classified as one of three basic types depending an structure and functions. These are parenchyma cells, collenchyma cells, and sclerenchyma cells.

•  Parenchyma cells

•  Cells have thin walls

•  Cells remain alive

•  Located in parenchyma tissue, epidermis, and in vascular tissues

•  Functions include:

•  Photosynthesis

•  Storage

•  Protection - epidermis

•  Secretion

•  Transport - phloem

•  Collenchyma cells

•  Cells have thickened walls

•  Wall thickening is uneven

•  Function: support, especially while organ is still growing

•  Sclerenchyma cells

•  Cells have primary and secondary walls, evenly thickened

•  Walls are often lignified (contain lignin)

•  Cell usually dies when mature

•  Located in sclerenchyma tissue, and in vascular tissues

•  Function: support and transport (xylem)

•  Primary tissues

•  All tissues that originate from apical meristems of stem and root are called primary tissues.

•  All vascular plants have primary tissues

•  The apical meristem

•  The meristem is the tiny region of dividing cells at the very tip of a stem or root branch.

•  Behind the meristem, differentiation begins. Three meristematic regions appear and give rise to the three tissue systems.

•  Protoderm - produces dermal tissue system
•  Ground meristem - produces ground tissue system
•  Procambium - produces the vascular tissue system

•  Mature tissues

•  The farther you move away from the apical meristem, the more mature the tissues become. Eventually, there are no more cells divisions and all cells have grown to their final size. Except under special conditions, the cells of these mature tissues will not divide again.

•  We will first discuss the tissues as the appear in a typical stem, but their general characteristics are the same in leaves and roots as well.

•  Three-dimensional view of primary tissue formation

Primary meristems
Diagram of shoot apex showing differentiation of tissues from meristem.

•  Microscopic view of shoot apex

Coleus shoot apex, l.s.
Longitudial section of Coleus shoot tip
Micrograph by Biodisc

•  Dermal tissue system

•  Dermal tissues are the covering layers which function in protection from desiccation, disease organisms, etc.

•  Epidermis

•  This is the outer layer of cells of the plant body - usually just one cell thick. It is a complex tissue containing several types of cells.
•  Ordinary epidermal cells (parenchyma)
•  Their outer cell walls are covered by the cuticle which contains the fatty compound cutin. Cutin is a water-proofing agent.
•  Stomata
•  Stomata (singular: stoma) are pores in the epidermis which allow gas exchange. Each pore is bounded by two guard cells-specialized cells that cause the pore to open and close.

•  Periderm

•  Replaces the epidermis in secondary growth.

•  Ground tissue system

•  The ground tissues carries out most functions that are not protective or transport, mainly photosynthesis, support, and storage.

•  Parenchyma tissue

•  Carries out functions that require a living cell, such as photosynthesis, storage, secretion.

•  Collenchyma and sclerenchyma tissues

•  Cells have thick walls to provide support. Collenchyma is flexible enough to allow for growth. Sclerenchyma is stronger and forms after growth is complete.

•  Vascular tissue system

•  Consist mainly of cylindrical cells that stack together to form tubes. These tubes function in transport of water and food compounds.

•  Xylem

•  Functions in water and mineral nutrient transport
•  Transporting cells
•  Tracheids
•  Vessel members

•  Phloem

•  Functions in food transport
•  Transporting cells
•  Sieve tube members
•  Companion cells. These move sugars into and out of the sieve tubes.
•  Phloem also contains parenchyma and sclerenchyma

The stem

•  The arrangement of tissues in the stem can best be seen by slicing a cross section.

•  Typical dicot stem

•  Vascular bundles are arranged in a ring

Helianthus stem, x.s.
A sunflower stem with its vascular bundles in a single ring, a feature typical of dicots
Micrograph by John Tiftickjian

•  Higher magnification micrograph showing tissues

Helianthus stem, x.s.
Cross section of Helianthus (sunflower) stem with major tissues labeled
Micrograph by Biodisc

•  Typical monocot stem

•  Vascular bundles are scattered throughout the stem.

Zea stem, x.s.
Corn stem showing scattered vascular bundles that is typical of monocots
Micrograph by John Tiftickjian

The leaf

•  Functions

•  Photosynthesis

•  For most plants, the leaf is the primary site of photosynthesis-food production. Leaf structure should be considered with this function in mind. Many structural features of a leaf have evolved in response to efficiency of photosynthesis.

•  Other functions

•  In some plants leaves are modified and carry out other specialized functions. Some examples are:

•  Tendrils. Vines use these for climbing.

•  Spines. Used for protection (e.g. cactus)

•  Reproductive leaves. These can produce plantlets at margins.

•  Insect traps. As in the Venus fly trap and pitcher plants

•  Each new leaf starts as a leaf primordium produced by the shoot apical meristem.

•  Leaf arrangement

•  Defined by how many leaves are attached to the stem at each node.

•  Alternate. One leaf per node

•  Opposite. Two leaves per node

•  Whorled. More than two leaves per node

•  Leaf parts

•  Blade. The flat part

•  Petiole. The leaf stalk

•  Leaf form

•  Simple. The blade is all one piece.

•  Compound. The blade is divided into multiple parts (leaflets).

•  Leaf anatomy

•  Epidermis is found on both sides of a leaf.

•  There are numerous stomata

•  Stomata allow carbon dioxide to enter the leaf for photosynthesis, but can close at night to reduce water loss.

•  The ground tissue of a leaf is the mesophyll

•  Vascular system = veins

•  Each vein is a vascular bundle containing xylem and phloem, just like the vascular bundles of the stem.

•  Venation pattern varies: dicots have net venation, monocots have parallel venation.

•  A typical leaf seen in cross section

Ligustrum leaf, x.s.
Cross section of a typical dicot leaf. Note upper and lower epidermis, palisade and spongy mesophyll, veins, stomata.
Micrograph by John Tiftickjian

•  Leaf abscission

•  At the end of a leaf's lifetime, it abscises from the stem in a controlled manner. In other words, the plant controls when the leaf falls off. This way it can absorb and recycle some leaf nutrients back into the stem before the leaf is thrown away.

•  Deciduous plants. All leaves are shed in the winter. (e.g. maple, pecan)

•  Evergreen plants. Leaves are retained all year long (e.g. pine, spruce)

•  Even though evergreens keep there leaves all year, their leaves still have a limited lifetime, typically two or three years.

Modified shoots

•  Tuber - an swollen underground stem for storage, e.g. potato

•  Rhizome - horizontal underground stem, e.g. iris

•  Bulb - underground shoot with fleshy storage leaves, e.g. onion

•  Succulent stem - fleshy stems for water storage, e.g. cactus

The root

•  Function in anchorage, absorption, and storage.

•  Root systems are adapted to efficient absorption of water and minerals

•  Tap root system

•  There is a one main root that grow downward and forms branches.

•  Tap roots are common in dicots and gymnosperms

•  Tap roots can grow deep to "mine" for water and minerals

•  Taraxacum (dandelion) root system

Tap root system
This Taraxacum (dandelion) displays a tap root system.
Photo by John Tiftickjian

•  Fibrous root system

•  Many major roots develop from the base of the stem.

•  Common in monocots and lower vascular plants.

•  Examples: grasses, ferns.

•  Fibrous roots are good at harvesting water near the soil surface before it escapes to lower levels of the soil.

•  Sedge plant with fibrous roots

Fibrous root system
This sedge displays the fibrous root system.
Photo by John Tiftickjian

•  Root hairs

•  Root hairs increase the surface area of the root providing greater contact with the soil.

•  Radish seedling with root hairs

Root hairs
Root hairs on a radish seedling
Photo by John Tiftickjian

•  Mycorrhizae

•  These symbiotic associations of roots and fungi benefit both organisms. The fungi receive food from the root; the root absorbs nutrients more efficiently.

•  Roots grow from apical meristems just as stems do

•  Root tip, longitudinal section

Root Tip Growth Regions
A root tip seen in longitudinal section
Drawing by John Tiftickjian

•  Anatomy of a typical root

Ranunculus root, x.s.
Cross section of Ranunculus root
Micrograph by Biodisc

•  Higher magnification showing vascular tissue

Ranunculus Root Stele
Cross section of Ranunculus root showing high magnification of vascular tissues
Micrograph by John Tiftickjian

Secondary growth

•  The problem of growing tall

•  As the land plants evolved, some developed the capacity to grow tall. These taller plants had certain advantages over the short ones, but they paid a price. Evolution is often about compromises.

•  Benefits of growing tall

•  Tall plants can get out of the shade of their neighbors where they will have more light for photosynthesis.

•  Disadvantages of being tall

•  Weight of the shoot increases-more support needed

•  Having more leaves requires greater transport capacity

•  The solution to the problem - Secondary growth

•  By increasing in diameter, the plant can increase both its supporting tissues and its transport tissues.

•  This requires a new kind of meristem - lateral meristems

•  Tissues derived from lateral meristems are called secondary tissues.

•  Secondary growth allows for increase in diameter of stem and root

•  Woody species have secondary growth.

•  All gymnosperms (pine, other conifers, etc.)

•  Many dicots (flowering trees and shrubs)

•  Secondary growth does not happen in:

•  Monocots

•  Lower vascular plants (ferns and their relatives)

•  Lateral meristems

•  A lateral meristem is a ring of dividing cells that makes new tissue to its inside and outside

•  Vascular cambium - produce secondary xylem and phloem

•  Cork cambium - produces protective cork tissue.

•  The vascular cambium develops between the xylem and phloem in vascular bundles, then becomes a complete cylinder.

•  Secondary xylem (wood) is produced to the inside

•  Secondary phloem is produced to the outside

•  Annual rings form in the xylem and can be used to age a tree.

•  Typical woody stem

Fraxinus stem, x.s.
Cross section of 3-year old woody stem (Ash). Note annual rings in xylem (wood).
Micrograph by Biodisc

•  Roots also have secondary growth

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