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CHAPTER 35

PLANT STRUCTURE AND GROWTH

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Plants are the first link in the food chain through photosynthesis. This affects all animals.

I. Concept 35.1: Plant Structure Plants, like multicellular animals, have organs

composed of different tissues, which in turn are composed of cells

A. Three Basic Plant Organs: Roots, Stems, LeavesThey are organized into a root system and a shoot systemRoots rely on sugar produced by photosynthesis in the shoot system, and shoots rely on water and minerals absorbed by the root system

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1. Roots Are multicellular organs with important functions:

1) Anchoring the plant2) Absorbing minerals and water3) Storing organic nutrients

A taproot system consists of one main vertical root that gives rise to lateral roots, or branch roots

Adventitious roots arise from stems or leaves Seedless vascular plants and monocots have a

fibrous root system characterized by thin lateral roots with no main root

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

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2. StemsA stem is an organ consisting of:

a. An alternating system of nodes, the points at which leaves are attached

b. 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

nonapical buds Many plants have modified stems

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Modified stems include:a.Stolons—horizontal stems growing along surface of

ground (strawberry)b.Rhizomes—horizontal stems growing underground

(iris)c. Tubers—some end in enlarged tubes where food is

stored (potatoes)d.Bulbs—vertical, underground shoots with leaves

modified for food storage (onion)

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3. Leavesa. Are the main photosynthetic organ of most vascular plantsb. Leaves generally consist of a flattened blade and a stalk

called the petiole, which joins the leaf to a node of the stemc. Some plant species have evolved modified leaves that

serve various functions

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Types of Leaves

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MODIFIED LEAVES

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B. Tissues: Dermal, Vascular, and GroundEach plant organ has dermal, vascular, and ground tissuesEach of these three categories forms a tissue system

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1. Dermal Tissue SystemOuter protective covering that secretes the cuticleIn nonwoody plants consists of the epidermisIn woody plants periderm replaces the epidermis in older regions of stems and roots

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2. Vascular Tissue SystemCarries out long-distance transport of material between roots and shootsTwo types of Vascular Tissue:

a. xylem-transports water and dissolved mineral upward from root into shoots b. phloem-transports organic nutrients from where

they are made to where they are neededVascular tissue of a stem or root is collectively called the steleThe stele of stems and leaves is divided into vascular bundles (strands of xylem and phloem)

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3. Ground Tissue System•Refers to any tissue not dermal or vascular•Include cells specialized for photosynthesis, storage

and support

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C. Common Types of Plant CellsParenchymaCollenchymeSclerenchymaWater–conducting cells of the xylemSugar—conducting cells of phloem

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•Protoplast—refers to cell contents exclusive of the cell wall

1. Parenchyma Cells•Most abundant plant tissue•Primary walls are thin and flexible•Lack secondary walls•Store food and water•Protoplast has large central vacuole•Perform most of plant’s metabolic functions such as PS and food storage•Least specialized—typical plant cell•Retains the ability to divide and differentiate into other cell types under special conditions (wound healing)

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Parenchyma Cells

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2. Collenchyma•Living support tissue with unevenly thickened cell walls•No secondary walls (no lignin)•Usually grouped in strands or cylinders for support for young parts of plant•Located just beneath the epidermis and along the midrib of a leaf•Provide flexible support

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Collenchyma Cells

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Parenchyma and Collenchyma Cells

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3.Sclerenchyma

•Sclerenchyma cells are rigid because of thick secondary walls strengthened with lignin

•They are dead at functional maturity

•There are two types:

a. Sclereids are short and irregular in shape and have thick lignified secondary walls

b. Fibers are long and slender and arranged in threads

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Sclereid and Fiber Cells

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Sclerenchyma

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4. Xylem•Water conducting cells•Dead cells with secondary walls•Stain red•Consists of two cell types:

a. Tracheids-long, thin, tapered cells with lignin-hardened

secondary walls with pits-water moves from cell to cell through pits-function in support and water transport-more primitive -found in xylem of all vascular plants

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b. Vessel elements-wider, shorter, thinner-walled-less tapered-long chains of vessel elements called xylem vessels-more efficient water conductors than tracheids-more advanced-found in most angiosperms and a fewgymnosperms

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XYLEM

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5. Phloem•Sugar-conducting cells•Stain green•Alive at functional maturity•Composed of:a. Sieve tube elements

-long narrow cells-lack nucleus, ribosomes, and vacuole, but are alive at maturity-sieve plates (end walls) are perforated as well as side wallsb. Companion cells

-lie adjacent to sieve tubes and direct their activity -connected to sieve tube by plasmodesmata (thin streams of cytoplasm)

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Phloem

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Xylem and Phloem

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II. Concept 35.2: Meristematic TissuesA. A plant can grow throughout its life; this is called

indeterminate growthB. Some plant organs cease to grow at a certain size; this

is called determinate growthC. Life span of flowering plants:

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

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D. Meristems are perpetually embryonic tissue and allow for indeterminate growthApical meristems are located at the tips of roots and

shoots and at the axillary buds of shootsApical meristems elongate shoots and roots, a process

called primary growthLateral meristems add thickness to woody plants, a

process called secondary growthThere are two lateral meristems: the vascular cambium

and the cork cambiumThe vascular cambium adds layers of vascular tissue called

secondary xylem (wood) and secondary phloemThe cork cambium replaces the epidermis with periderm,

which is thicker and tougher

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III. Concept 35.3: Primary growth lengthens roots and shoots

Primary growth produces the primary plant body, the parts of the root and shoot systems produced by apical meristems

A. Primary Growth of RootsThe root tip is covered by a root cap, which protects

the apical meristem as the root pushes through soilGrowth occurs just behind the root tip, in three zones

of cells:1.Zone of cell division•Near tip and includes the apical meristem•Quiescent center is made up of reserve replacement cells

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2. Zone of elongation•Cells elongate and push root through the soil

3. Zone of differentiation•Elongated cells complete their differentiation and

become functionally mature •Region where root hairs develop• Farthest from root tip

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The primary growth of roots produces the epidermis, ground tissue, and vascular tissue

In most roots, the stele is a vascular cylinder The ground tissue fills the cortex, the region between

the vascular cylinder and epidermis The innermost layer of the cortex is called the

endodermis Lateral roots arise from within the pericycle, the

outermost cell layer in the vascular cylinder

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Eudicot Root

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Stele—Vascular Cylinder

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Monocot Root

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Lateral Roots

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B. 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

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1. Tissue Organization of Stems Lateral shoots develop from axillary buds on the

stem’s surface In most eudicot stems, the vascular tissue consists

of vascular bundles that are arranged in a ring In most monocot stems, the vascular bundles are

scattered throughout the ground tissue, rather than forming a ring

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Stems

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2. Tissue Organization of LeavesThe epidermis in leaves is interrupted by stomata,

which allow CO2 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

Below the palisade mesophyll in the upper part of the leaf is loosely arranged spongy mesophyll, where gas exchange occurs

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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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Leaves

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IV. Concept 35.4: Secondary growth in woody plantsA. Secondary growth occurs in stems and roots of woody

plants but rarely in leavesB. The secondary plant body consists of the tissues

produced by the vascular cambium and cork cambiumC. Secondary growth is characteristic of gymnosperms

and many eudicots, but not monocots

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Secondary Growth

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D. The Cork Cambium and the Production of Periderm The cork cambium gives rise to the secondary plant

body’s protective covering, or periderm Periderm consists of the cork cambium plus the

layers of cork cells it produces Bark consists of all the tissues external to the

vascular cambium, including secondary phloem and periderm