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Lesson 8 of 9Tissues, Organs & Systems of Living Things
Unit 1 · Lesson 8 of 9

Plant Tissues, Organs and Systems

The tissues that make up a plant, the root and shoot systems, how xylem and phloem transport water and sugar, the structure of a leaf (mesophyll, stomata, veins) and how transpiration pulls water upward.

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Key things to remember

  1. 1Plant tissues: dermal (protection), ground (photosynthesis, storage), vascular (transport) and meristematic (new cells).
  2. 2The root system anchors the plant and absorbs water and minerals; the shoot system (stems, leaves, flowers) photosynthesizes, supports and reproduces.
  3. 3Xylem carries water and minerals upward from the roots; phloem carries sugars from the leaves to wherever they are needed, up or down.
  4. 4Leaf layers: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, veins, lower epidermis with stomata and guard cells.
  5. 5Transpiration is water loss from leaves through the stomata; it pulls water up the xylem.
  6. 6Stomatal density = number of stomata ÷ field-of-view area, where area = π × radius².

A tall tree has no heart and no muscles, yet it lifts water from the soil to leaves dozens of metres up, every day, and sends sugar made in its leaves down to its roots. Plants are organized just like animals, from cells to tissues to organs to organ systems, but their systems are built around two needs: catching light to make food by photosynthesis, and moving water, minerals and sugar between roots that sit in the dark soil and leaves that sit in the sunlight.

Plant tissues

Tissue typeJobExamples
DermalCovers and protects the plant; controls gas exchange and water lossEpidermis, the waxy cuticle it makes, guard cells, root hairs
GroundMost of the plant's bulk: photosynthesis, storage and supportMesophyll in leaves; starch-storing cells in roots and potatoes
VascularTransports materials through the plantXylem (water and minerals) and phloem (sugars)
MeristematicUnspecialized cells that divide to make new cellsRoot tip, shoot tip, cambium
The four kinds of plant tissue.
Shoot systemstem, leaves,flowers, fruitRoot systemanchors theplant; absorbswater, mineralsFlowerFruitTranspirationwater vapour leavesthrough stomataLeafmakes sugars by photosynthesisStemholds leaves up to the lightRoot hairsabsorb water and mineralsXylem: water and mineralsmove up from the rootsPhloem: sugars move fromleaves to roots, fruits andgrowing tips (up or down)
Fig 1A flowering plant's shoot system (leaves, stem, flowers) and root system. Water and minerals move up the xylem from the roots, sugars move in the phloem from the leaves to the roots and fruits (in both directions), and water vapour leaves the leaves by transpiration.
ARoot system
  • Organs: the roots, usually below ground
  • Anchors the plant in the soil
  • Absorbs water and dissolved minerals, mainly through root hairs
  • Often stores food as sugar or starch (carrots, beets, sweet potatoes)
BShoot system
  • Organs: stems, leaves and flowers (which form fruits and seeds)
  • Leaves carry out photosynthesis and gas exchange
  • Stems support the leaves in the light and carry materials between roots and leaves
  • Flowers are the organs of reproduction

Transport: xylem and phloem

AXylem
  • Carries water and dissolved minerals
  • Flows upward, from the roots to the leaves
  • Made of cells that are dead and hollow when mature, joined end to end into tubes
  • Walls are thickened and stiffened, which also helps support the plant (wood is mostly xylem)
BPhloem
  • Carries sugars (and other dissolved food) made in the leaves
  • Flows from where sugar is made or stored to where it is needed, so it can go up or down
  • Made of living cells (sieve tubes) with helper companion cells
  • Feeds roots, growing tips, fruits and seeds
Fruit and shoot tipsink: sugars used for growthMature leavessource: sugars made hereXylemwater + minerals, upward onlyRoot hairstake in water by osmosisPhloem (up or down)sugars from source to sinkTranspiration pullevaporation pulls water upRootssink: sugar stored as starchwater and minerals in xylemsugars in phloemwater vapour leaving through stomataXylem flow is one way (up). Phloem flow goes wherever the sinks are, so it can run up or down the stem.
Fig 2Transport in a flowering plant: water and minerals enter the root hairs and travel only upward in the xylem to the leaves, where water vapour leaves through the stomata and pulls more water up. Sugar from the leaves travels in the phloem up to growing fruit and shoot tips and down to the roots.

The leaf: an organ built for photosynthesis

CO₂ inO₂ and watervapour outCuticlewaxy, waterproofUpper epidermisPalisade mesophyllmost photosynthesisSpongy mesophyllloosely packed cellsAir spaceLower epidermisGuard cellsopen and close the stomaStoma (pore)Xylemupper side of the vein;brings water inPhloemlower side of the vein;carries sugars away
Fig 3Cross-section of a leaf: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, a vein with xylem above phloem, lower epidermis, and a stoma between two guard cells. Carbon dioxide enters through the stoma; oxygen and water vapour leave.
LayerStructureJob
CuticleWaxy, waterproof coating made by the epidermisReduces water loss
Upper epidermisSingle layer of clear, tightly packed cellsProtects; lets light through to the cells below
Palisade mesophyllTall, column-shaped cells packed with chloroplastsMain site of photosynthesis, near the top where light is strongest
Spongy mesophyllLoosely packed, rounded cells with air spaces between themSome photosynthesis; air spaces let gases diffuse to and from the cells
VeinBundle of xylem and phloemBrings water in (xylem) and carries sugar out (phloem)
Lower epidermis with stomataTiny pores (stomata; one is a stomate or stoma), each between two guard cellsLet carbon dioxide in and oxygen and water vapour out; guard cells open and close each pore
The layers of a typical leaf from top to bottom.
Photosynthesis: carbon dioxide (in through the stomata) and water (up the xylem) become glucose and oxygen; the sugar is carried away in the phloem, mostly as sucrose.

Every leaf faces a trade-off. The stomata must open to let in carbon dioxide for photosynthesis, but every open stomate also lets water vapour escape. This evaporation of water from the leaves is called transpiration. It is not just a loss: as water evaporates from the mesophyll and diffuses out, it pulls on the unbroken columns of water in the xylem, drawing more water up from the roots (transpiration pull). On a hot, dry, windy day transpiration is fast; if the roots cannot keep up, the guard cells close the stomata to save water, and if the cells still lose too much water, the plant wilts. In many land plants most stomata are on the lower surface of the leaf, which is shaded and cooler, so less water is lost; floating leaves like water lilies are an exception, with stomata on top.

Plant diseases and research
Diseases attack plant organ systems too. Tobacco mosaic virus mottles and damages leaf tissue, reducing photosynthesis. Wheat rust, a fungus, grows into leaves and stems and steals the plant's sugar, and has caused major crop losses; Canadian plant breeders have spent decades developing rust-resistant wheat varieties. Dutch elm disease, carried by beetles, blocks the xylem of elm trees so water cannot reach the leaves, and it killed a large share of the elms in Ontario's cities.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1
A student paints clear nail polish on the underside of a leaf, peels it off and looks at the impression at 400×. The field of view is 0.40 mm in diameter and 24 stomata are visible. Estimate the stomatal density in stomata per mm².
Example 2
Trace the path of a water molecule from the soil to the air outside a leaf.
Example 3
On a hot, dry afternoon, a tomato plant's guard cells close its stomata. Explain one benefit and one cost of this response.

Where marks get lost

  • Mixing up xylem and phloem. Xylem carries water up (think: xylem is the water pipe); phloem carries food (sugar) both ways.
  • Saying phloem only flows down. Sugar moves from where it is made or stored to where it is needed, which can be up to a fruit or a growing tip.
  • Thinking plants take in water through their leaves. Almost all water is absorbed by the roots, mainly through root hairs; leaves lose water by transpiration.
  • Saying stomata are only for water loss. Their main purpose is letting carbon dioxide in for photosynthesis; water loss is the unavoidable cost.
  • Forgetting the circle area when finding stomatal density. Divide the count by the area (π × radius²), not by the diameter.

Flashcards 8

Xylem
Phloem
Mesophyll
Stomate (stoma)
+ 4 more

Practice

1
Which tissue carries sugar from the leaves to the roots?
2
Match each leaf structure to its main job.
Cuticle
Palisade mesophyll
Spongy mesophyll
Guard cells
Xylem in a vein
3
What are the small pores in the leaf epidermis that let carbon dioxide in and water vapour out?
4
In a leaf impression, 30 stomata are counted in a field of view 0.50 mm in diameter. Estimate the stomatal density, to the nearest whole number of stomata per mm².
stomata/mm²
5
Put the steps in the path of water through a plant in order.
  1. 1Water evaporates from a mesophyll cell into an air space
  2. 2Water reaches a vein in the leaf
  3. 3Water vapour diffuses out through a stomate
  4. 4Water enters a root hair cell from the soil
  5. 5Water enters the xylem in the root
  6. 6Water travels up the xylem in the stem
6
A plant's guard cells close its stomata on a hot, dry day. Which result is most likely?
7
Which organ belongs to the root system rather than the shoot system?
8
Why are most chloroplasts found in the palisade mesophyll?

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