Plants have a hierarchical organization consisting of organs, tissues, and cells

Plants have organs composed of different tissues, which in turn are composed of different cell types

Tissue: group of cells consisting of one or more cell types that together perform a specialized function
Organ: consists of several types of tissues that together carry out particular functions

Three basic organs evolved: roots, stems, and leaves
They are organized into a root system and a shoot system

• Roots rely on sugar produced by photosynthesis in the shoot system, and shoots rely on water and minerals absorbed by the root system
• Monocots and eudicots are the two major groups of angiosperms

Roots

A root is an organ with important functions:

• Anchoring the plant
• Absorbing minerals and water
• Storing carbohydrates

Most eudicots and gymnosperms have a taproot system, which consists of:

• A taproot, the main vertical root
• Lateral roots, or branch roots, that arise from the taproot

Most monocots have a fibrous root system, which consists of:

• Adventitious roots that arise from stems or leaves
• Lateral roots that arise from the adventitious roots

In most plants, absorption of water and minerals occurs near the root hairs, where vast numbers of tiny root hairs increase the surface area

Stems

A stem is an organ consisting of

• An alternating system of nodes, the points at which leaves are attached
• Internodes, the stem segments between nodes
• An axillary bud is a structure that has the potential to form a lateral shoot, or branch
• An apical bud, or terminal bud, is located near the shoot tip and causes elongation of a young shoot
• Apical dominance helps to maintain dormancy in most axillary buds
• Many plants have modified stems (e.g., rhizomes, bulbs, stolons, tubers)

Leaves

• The leaf is the main photosynthetic organ of most vascular plants
• Leaves generally consist of a flattened blade and a stalk called the petiole, which joins the leaf to a node of the stem
• Monocots and eudicots differ in the arrangement of veins, the vascular tissue of leaves
  1. Most monocots have parallel veins
  2. Most eudicots have branching veins
• In classifying angiosperms, taxonomists may use leaf morphology as a criterion

 

Dermal, Ground, Vascular Tissues

• Each plant organ has dermal, vascular, and ground tissues
• Each of these three categories forms a tissue system
• Each tissue system is continuous throughout the plant

Dermal Tissue System

• In nonwoody plants, the dermal tissue system consists of the epidermis
• A waxy coating called the cuticle helps prevent water loss from the epidermis
• In woody plants, protective tissues called periderm replace the epidermis in older regions of stems and roots
• Trichomes are outgrowths of the shoot epidermis and can help with insect defense

Vascular Tissue System

The vascular tissue system carries out long-distance transport of materials between roots and shoots

• The two vascular tissues are xylem and phloem
• Xylem conveys water and dissolved minerals upward from roots into the shoots
• Phloem transports organic nutrients from where they are made to where they are needed
• The vascular tissue of a stem or root is collectively called the stele
• In angiosperms the stele of the root is a solid central vascular cylinder
• The stele of stems and leaves is divided into vascular bundles, strands of xylem and phloem

Ground Tissue System

• Tissues that are neither dermal nor vascular are the ground tissue system
• Ground tissue internal to the vascular tissue is pith; ground tissue external to the vascular tissue is cortex
• Ground tissue includes cells specialized for storage, photosynthesis, and support

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Types of Plant Cells

The major types of plant cells are:

• Parenchyma
• Collenchyma
• Sclerenchyma
• Water-conducting cells of the xylem
• Sugar-conducting cells of the phloem

Parenchyma Cells

Mature parenchyma cells

–Have thin and flexible primary walls
–Lack secondary walls
–Are the least specialized
–Perform the most metabolic functions
–Retain the ability to divide and differentiate

Collenchyma Cells

• Collenchyma cells are grouped in strands and help support young parts of the plant shoot
• They have thicker and uneven cell walls
• They lack secondary walls
• These cells provide flexible support without restraining growth

Sclerenchyma Cells

• Sclerenchyma cells are rigid because of thick secondary walls strengthened with lignin
• They are dead at functional maturity

There are two types:

–Sclereids are short and irregular in shape and have thick lignified secondary walls
–Fibers are long and slender and arranged in threads

Water-Conducting Cells of the Xylem

• The two types of water-conducting cells, tracheids and vessel elements, are dead at maturity
• Tracheids are found in the xylem of all vascular plants
• Vessel elements are common to most angiosperms and a few gymnosperms
• Vessel elements align end to end to form long micropipes called vessels

Sugar-Conducting Cells of the Phloem

• Sieve-tube elements are alive at functional maturity, though they lack organelles
• 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

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Meristems generate cells for primary and secondary growth

• A plant can grow throughout its life; this is called indeterminate growth
• Some plant organs cease to grow at a certain size; this is called determinate growth

Meristems: perpetually embryonic tissue and allow for indeterminate growth
Apical meristems: 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
There are two lateral meristems: the vascular cambium and the cork cambium

1. The vascular cambium adds layers of vascular tissue called secondary xylem (wood) and secondary phloem
2. The cork cambium replaces the epidermis with periderm, which is thicker and tougher

Meristems give rise to:

–Initials, also called stem cells, which remain in the meristem
–Derivatives, which become specialized in mature tissues

• In woody plants, primary growth and secondary growth occur simultaneously but in different locations

Flowering plants can be categorized based on the length of their life cycle

–Annuals complete their life cycle in a year or less
–Biennials require two growing seasons
–Perennials live for many years

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primary growth

Primary growth produces the parts of the root and shoot systems produced by apical meristems

Primary Growth of Roots

The root tip is covered by a root cap, which protects the apical meristem as the root pushes through soil

Growth occurs just behind the root tip, in three zones of cells:

1. Zone of cell division
2. Zone of elongation
3. Zone of differentiation, or maturation

• The primary growth of roots produces the epidermis, ground tissue, and vascular tissue
• In angiosperm roots, the stele is a vascular cylinder
• In most eudicots, the xylem is starlike in appearance with phloem between the “arms”
• In many monocots, a core of parenchyma cells is surrounded by rings of xylem then phloem

• The ground tissue, mostly parenchyma cells, fills the cortex, the region between the vascular cylinder and epidermis
• The innermost layer of the cortex is called the endodermis
• The endodermis regulates passage of substances from the soil into the vascular cylinder
• Lateral roots arise from within the pericycle, the outermost cell layer in the vascular cylinder

 

Primary Growth of Shoots

• A shoot apical meristem is a dome-shaped mass of dividing cells at the shoot tip
• Leaves develop from leaf primordia along the sides of the apical meristem
• Axillary buds develop from meristematic cells left at the bases of leaf primordia

Tissue Organization of Stems

Lateral shoots develop from axillary buds on the stem’s surface
In most eudicots, the vascular tissue consists of vascular bundles arranged in a ring

 

In most monocot stems, the vascular bundles are scattered throughout the ground tissue, rather than forming a ring

Tissue Organization of Leaves

• The epidermis in leaves is interrupted by stomata, which allow CO2 and O2 exchange between the air and the photosynthetic cells in a leaf
• Each stomatal pore is flanked by two guard cells, which regulate its opening and closing
• The ground tissue in a leaf, called mesophyll, is sandwiched between the upper and lower epidermis

The mesophyll of eudicots has two layers:

1. The palisade mesophyll in the upper part of the leaf
2. The spongy mesophyll in the lower part of the leaf; the loose arrangement allows for gas exchange

• The vascular tissue of each leaf is continuous with the vascular tissue of the stem
• Veins are the leaf’s vascular bundles and function as the leaf’s skeleton
• Each vein in a leaf is enclosed by a protective bundle sheath

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Secondary style

• 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

The Vascular Cambium and Secondary Vascular Tissue

• The vascular cambium is a cylinder of meristematic cells one cell layer thick
• It develops from undifferentiated parenchyma cells
• In cross section, the vascular cambium appears as a ring of initials (stem cells)
• The initials increase the vascular cambium’s circumference and add secondary xylem to the inside and secondary phloem to the outside

 

• Elongated initials produce tracheids, vessel elements, fibers of xylem, sieve-tube elements, companion cells, axially oriented parenchyma, and fibers of the phloem
• Shorter initials produce vascular rays, radial files of parenchyma cells that connect secondary xylem and phloem
• Secondary xylem accumulates as wood and consists of tracheids, vessel elements (only in angiosperms), and fibers
• Early wood, formed in the spring, has thin cell walls to maximize water delivery
• Late wood, formed in late summer, has thick-walled cells and contributes more to stem support
• In temperate regions, the vascular cambium of perennials is inactive through the winter
• Tree rings are visible where late and early wood meet, and can be used to estimate a tree’s age

Dendrochronology: the analysis of tree ring growth patterns and can be used to study past climate change

• 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

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The Cork Cambium and the Production of Periderm

Cork cambium gives rise to two tissues:

1. Phelloderm is a thin layer of parenchyma cells that forms to the interior of the cork cambium
2. Cork cells accumulate to the exterior of the cork cambium

• Cork cells deposit waxy suberin in their walls, then die
• Periderm consists of the cork cambium, phelloderm, and cork cells it produces

Lenticels: in the periderm allow for gas exchange between living stem or root cells and the outside air
Bark: consists of all the tissues external to the vascular cambium, including secondary phloem and periderm