Plant structure and functionPlant structure and function(parts of chapters 35, 36 and 37)(parts of chapters 35, 36 and 37)

Unlike animals, plants remain in one place and Unlike animals, plants remain in one place and produce food through photosynthesis.produce food through photosynthesis.

In the process of photosynthesis plants (and In the process of photosynthesis plants (and other photosynthetic organisms such as algae, other photosynthetic organisms such as algae, other protists, and cyanobacteria) trap the other protists, and cyanobacteria) trap the energy in sunlight and store it in chemical bonds.energy in sunlight and store it in chemical bonds.

The energy stored in chemical bonds can then The energy stored in chemical bonds can then be used to fuel metabolic processes.be used to fuel metabolic processes.

Figure 10.2

Plants and photosynthesisPlants and photosynthesis

This process is called This process is called photosynthesisphotosynthesis..

In this class we will not discuss the In this class we will not discuss the process of photosynthesis in detail. It is process of photosynthesis in detail. It is covered in depth in Bio 101.covered in depth in Bio 101.

PhotosynthesisPhotosynthesis

In photosynthesis carbon dioxide (COIn photosynthesis carbon dioxide (CO22) )

and water (Hand water (H220) and the energy provided 0) and the energy provided

by light are used to make glucose.by light are used to make glucose.

6 CO6 CO22 + 12 H + 12 H220 + energy 0 + energy C C66HH1212OO66 + 6O + 6O22

+ 6 H+ 6 H220 0

ChloroplastsChloroplasts

The organelle plants use to carry out The organelle plants use to carry out photosynthesis is the photosynthesis is the chloroplastchloroplast..

In plants chloroplasts are concentrated in In plants chloroplasts are concentrated in the leaves, which generally are thin and the leaves, which generally are thin and flat to allow maximum exposure to light.flat to allow maximum exposure to light.

Fig 10.3

Basic structure of plantsBasic structure of plants

To carry out photosynthesis plants must To carry out photosynthesis plants must obtain water and minerals from the soil, obtain water and minerals from the soil, COCO22 from the air, and light from the sun. from the air, and light from the sun.

The structure of plants reflects their need The structure of plants reflects their need to carry out these tasks. to carry out these tasks.

Basic structure of plantsBasic structure of plants

Plants have three basic organs:Plants have three basic organs: RootsRoots StemsStems LeavesLeaves

These organs are organized into two These organs are organized into two systems: the largely below-ground systems: the largely below-ground root root systemsystem and the above-ground and the above-ground shoot shoot systemsystem (stems and leaves). (stems and leaves).

35.2

RootsRoots

Roots perform several tasks. TheyRoots perform several tasks. They Anchor the plant in placeAnchor the plant in place Absorb minerals and waterAbsorb minerals and water Store organic nutrients such as sugars (e.g. Store organic nutrients such as sugars (e.g.

carrot, sugar beet, turnip).carrot, sugar beet, turnip).

RootsRoots

Roots systems may have a Roots systems may have a central central taproottaproot with lateral roots branching off with lateral roots branching off from it (e.g. dandelion).from it (e.g. dandelion).

Alternatively, a root system may have no Alternatively, a root system may have no obvious main root, but instead be a obvious main root, but instead be a fibrous systemfibrous system with many small roots with many small roots growing from the stem, each of which has growing from the stem, each of which has its own lateral roots (e.g. grasses).its own lateral roots (e.g. grasses).

RootsRoots

The entire root system anchors a plant in The entire root system anchors a plant in place, but absorption of water and place, but absorption of water and minerals occurs mainly at the root tips.minerals occurs mainly at the root tips.

At the root tips huge numbers of At the root tips huge numbers of root root hairshairs increase the surface area increase the surface area enormously.enormously.

Root hairsRoot hairs

Root hairs are extensions of individual Root hairs are extensions of individual epidermal root cells and are not epidermal root cells and are not multicellular structures multicellular structures

(as lateral roots are). (as lateral roots are).

RootsRoots

Root hairs are permeable to water and Root hairs are permeable to water and adhere closely to soil particles allowing adhere closely to soil particles allowing efficient absorption of water and nutrients.efficient absorption of water and nutrients.

Most plants forms mutually beneficial Most plants forms mutually beneficial relationships with fungi, which facilitate relationships with fungi, which facilitate absorption of water and minerals.absorption of water and minerals.

MycorrhizaeMycorrhizae

The plants and fungi form The plants and fungi form mycorrhizae: mycorrhizae: symbiotic associations of plant roots united with symbiotic associations of plant roots united with fungal hyphae (hyphae are tiny filaments that fungal hyphae (hyphae are tiny filaments that form the bulk of a fungus).form the bulk of a fungus).

Most plants form these symbiotic mycorrhizal Most plants form these symbiotic mycorrhizal relationships and they greatly enhance the relationships and they greatly enhance the plants growth. [a symbiotic relationship is a plants growth. [a symbiotic relationship is a close, mutually beneficial relationship]close, mutually beneficial relationship]

36.10

Mycorrhizae (white) growing on a root

MycorrhizaeMycorrhizae

The fungal hyphae grow over the root and The fungal hyphae grow over the root and penetrate into it and may in some cases penetrate into it and may in some cases form a mantle or layer over the root. form a mantle or layer over the root.

The fungus benefits from a steady supply The fungus benefits from a steady supply of sugar donated by the host plant.of sugar donated by the host plant.

37.12

MycorrhizaeMycorrhizae

Plant receives numerous benefits:Plant receives numerous benefits: Fungus Fungus greatly increases surface areagreatly increases surface area for absorption for absorption

(can be as much as 3 meters of hyphae per cm of (can be as much as 3 meters of hyphae per cm of plant root length).plant root length).

Fungus Fungus selectively absorbs phosphateselectively absorbs phosphate and other and other nutrients and supplies them to plant.nutrients and supplies them to plant.

Fungus may Fungus may secrete growth factorssecrete growth factors that promote root that promote root growth.growth.

Fungus may Fungus may produce antibioticsproduce antibiotics that protect the plant that protect the plant from pathogenic bacteria and fungi in the soil.from pathogenic bacteria and fungi in the soil.

MycorrhizaeMycorrhizae

Plant-fungus symbiosis may have been Plant-fungus symbiosis may have been one of the early adaptations that allowed one of the early adaptations that allowed plants to colonize the land, which probably plants to colonize the land, which probably initially was quite nutrient poor.initially was quite nutrient poor.

Fossils of some of the earliest plants show Fossils of some of the earliest plants show mycorrhizae.mycorrhizae.

Shoot SystemsShoot Systems

Shoot systems consist of stems and Shoot systems consist of stems and leaves.leaves.

Stems are elongated structures comprised Stems are elongated structures comprised of nodes and internodes.of nodes and internodes.

Nodes are where leaves are attached and Nodes are where leaves are attached and internodes are the sections in between.internodes are the sections in between.

35.2

Shoot SystemsShoot Systems

Stems have a terminal bud at the tip and Stems have a terminal bud at the tip and this is where elongation takes place, this is where elongation takes place, enabling the stem to reach upwards enabling the stem to reach upwards towards the light.towards the light.

If the tip of the stem is eaten or shaded, If the tip of the stem is eaten or shaded, however, axillary buds (buds on the side) however, axillary buds (buds on the side) will begin to grow. will begin to grow.

Shoot SystemsShoot Systems

Gardeners shape plants by pruning them.Gardeners shape plants by pruning them.

By removing terminal buds a bushier plant By removing terminal buds a bushier plant can be produced or by removing lateral can be produced or by removing lateral flower buds a single large flower can be flower buds a single large flower can be produced. produced.

Shoot SystemsShoot Systems

Stems have been greatly modified in many Stems have been greatly modified in many plants to perform a variety of functions.plants to perform a variety of functions.

Rhizomes, bulbs, tubers, and stolons are Rhizomes, bulbs, tubers, and stolons are all modified stems although they are often all modified stems although they are often mistaken for roots.mistaken for roots.

Modified stemsModified stems

BulbsBulbs: vertical shoots that grow underground. : vertical shoots that grow underground. The “flesh” of a bulb (e.g. an onion) consists of The “flesh” of a bulb (e.g. an onion) consists of leaves modified for food storage.leaves modified for food storage.

Stolons and rhizomesStolons and rhizomes: are stems that grow on : are stems that grow on (stolons) or just under (rhizomes) the soil (stolons) or just under (rhizomes) the soil surface. New plantlets form periodically along surface. New plantlets form periodically along the length of these stems (asexual the length of these stems (asexual reproduction). reproduction).

35.5

Modified stemsModified stems

Tubers: are enlarged ends of rhizomes Tubers: are enlarged ends of rhizomes specialized for storing food (e.g. potato).specialized for storing food (e.g. potato).

The “eyes” of a tuber are axillary buds.The “eyes” of a tuber are axillary buds.

35.5

LeavesLeaves

Leaves are the main photosynthetic organ Leaves are the main photosynthetic organ of plants, although green stems also of plants, although green stems also perform photosynthesis.perform photosynthesis.

Leaves vary in form, but usually have a flat Leaves vary in form, but usually have a flat bladeblade and a stalk ( and a stalk (petiolepetiole) that joins the ) that joins the leaf to the stem.leaf to the stem.

LeavesLeaves

Leaves are generally flat to maximize the Leaves are generally flat to maximize the area exposed to the sun and minimize the area exposed to the sun and minimize the distance gases must be transported to and distance gases must be transported to and from photosynthesizing cells.from photosynthesizing cells.

However, in many cases leaves have However, in many cases leaves have been substantially modified by natural been substantially modified by natural selection to perform other functions.selection to perform other functions.

Modified leavesModified leaves

TendrilsTendrils of climbing plants such as clematis are often of climbing plants such as clematis are often modified leaves.modified leaves.

SpinesSpines of cacti are modified leaves (most of cacti are modified leaves (most photosynthesis being carried out by the fleshy stem.photosynthesis being carried out by the fleshy stem.

Some leaves are modifed as Some leaves are modifed as storagestorage leavesleaves to store to store water.water.

Some leaves called Some leaves called bractsbracts look like petals (e.g. in look like petals (e.g. in dogwoods) being brightly colored and enlarged to attract dogwoods) being brightly colored and enlarged to attract pollinators to the flowers they surround. pollinators to the flowers they surround.

Some leaves produce plantlets that drop off the plant Some leaves produce plantlets that drop off the plant and take root in the soil.and take root in the soil.

35.7

Plant vascular systemPlant vascular system

Plants contain two vascular systems that Plants contain two vascular systems that transport water, minerals, and sugars transport water, minerals, and sugars around the plant.around the plant.

XylemXylem transports water and dissolved transports water and dissolved minerals from the roots into the shoots.minerals from the roots into the shoots.

PhloemPhloem transports sugars from the leaves transports sugars from the leaves to where they are needed in the plant.to where they are needed in the plant.

36.2

Plant vascular systemPlant vascular system

XylemXylem cells are dead at functional maturity cells are dead at functional maturity and form thin elongated tubes that water and form thin elongated tubes that water moves through. moves through.

PhloemPhloem cells are alive. cells are alive.

Plant secondary growthPlant secondary growth

Primary growth is stem elongation, secondary Primary growth is stem elongation, secondary growth refers to the thickening of woody plants growth refers to the thickening of woody plants over time.over time.

Xylem and phloem cells are both produced by a Xylem and phloem cells are both produced by a plant tissue called vascular cambium that is plant tissue called vascular cambium that is located under the bark.located under the bark.

This cambium produces xylem cells on the This cambium produces xylem cells on the inside and phloem on its outside.inside and phloem on its outside.

Plant secondary growthPlant secondary growth

As the plant grows older inner xylem As the plant grows older inner xylem tissue forms the heartwood of the tree. tissue forms the heartwood of the tree. This tissue no longer transports liquid.This tissue no longer transports liquid.

Xylem cells have thick lignified walls (lignin Xylem cells have thick lignified walls (lignin is a complex cross-linked polymer) that is a complex cross-linked polymer) that provide structural support for the plant.provide structural support for the plant.

The outer (more recently produced) xylem The outer (more recently produced) xylem is called sapwood and this carries liquid.is called sapwood and this carries liquid.

35.20

Plant secondary growthPlant secondary growth

On the outside of the cambium layer On the outside of the cambium layer phloem is produced. Phloem is produced phloem is produced. Phloem is produced more slowly than xylem and older phloem more slowly than xylem and older phloem is sloughed off the tree so it does not is sloughed off the tree so it does not accumulate as xylem does.accumulate as xylem does.

Plant secondary growthPlant secondary growth

Because the outer layer of phloem is Because the outer layer of phloem is essential to transportation, a tree that is essential to transportation, a tree that is “ringed” by grazers (i.e., has its outer bark “ringed” by grazers (i.e., has its outer bark removed around the circumference of the removed around the circumference of the plant) will die.plant) will die.

In contrast, a tree may be hollowed out In contrast, a tree may be hollowed out and still survive.and still survive.