Diversity of Specialised Tissues

Lessons

Senior 4 · Biology

Diversity of Specialised Tissues

4 lessons

Lesson 2 of 4

Complex plant tissues: xylem, phloem and their arrangement in organs

25 min

Why a ring of missing bark kills a tree

Strip a complete ring of bark from a tree trunk, a practice called ring-barking or girdling, and the leaves stay green for weeks or even months. Water still reaches them. But in the end the tree dies, and over time the trunk just above the ring often swells. The explanation lies in two tissues that sit only millimetres apart: the phloem, which is in the bark and is removed with it, and the xylem, the wood, which is left behind.

Xylem and phloem are complex tissues, made of several kinds of cell working together. This lesson describes them and shows how plant tissues are arranged in leaves, stems and roots.

Definition

Complex tissue: a tissue made of more than one type of cell.

Xylem: the complex tissue that carries water and dissolved mineral ions up from the roots and supports the plant. Its main conducting cells are vessel elements and tracheids.

Phloem: the complex tissue that carries organic substances, mainly sucrose and amino acids, from where they are made or stored to where they are used (translocation). Its main cells are sieve tube elements and companion cells.

Vascular bundle: a strand containing xylem and phloem, often with fibres and, in dicots, cambium between them.

Xylem and phloem compared

Feature	Xylem	Phloem
Cells	Vessel elements, tracheids, xylem fibres, xylem parenchyma	Sieve tube elements, companion cells, phloem fibres, phloem parenchyma
Conducting cells alive?	No: vessel elements and tracheids are dead and empty	Yes: sieve tube elements are living but have no nucleus
Walls	Thickened with lignin in rings, spirals or networks; pits let water pass sideways	Cellulose; end walls form perforated sieve plates
Carries	Water and mineral ions	Sucrose, amino acids and other organic solutes
Direction	Upwards only, from roots to leaves	Up or down, from sources (leaves, storage organs) to sinks (roots, fruits, growing tips)
Other role	Mechanical support (wood)	Carries signals such as some hormones

A vessel is a long, continuous tube formed from many vessel elements joined end to end, whose end walls have broken down. Because it has no cytoplasm or cross walls, water flows through it with little resistance. A sieve tube is a column of sieve tube elements joined by sieve plates; each is served by a companion cell, which has a nucleus, dense cytoplasm and many mitochondria and is linked to it by many plasmodesmata. The companion cell supplies the proteins and the ATP the sieve tube element cannot make for itself.

Worked example 1: explaining ring-barking

Use your knowledge of xylem and phloem to explain why (a) the leaves of a ring-barked tree stay green for weeks; (b) the trunk swells just above the ring; (c) the tree eventually dies.

(a) The ring removes the bark, including the phloem, but leaves the xylem (wood) intact. Water and mineral ions still travel up the xylem, so the leaves get enough water and keep photosynthesising for a while.
(b) Sugars made in the leaves move down the phloem towards the roots, but the phloem is cut at the ring. Sugar accumulates above the ring, and cells there use it to grow, so the bark above the ring swells.
(c) The roots receive no sugar. Root cells use up their stored starch, cannot respire, and die; dead roots cannot absorb water, so in the end the leaves wilt and the whole tree dies.
This classic experiment is evidence that sugars move in the phloem, not the xylem.

How tissues are arranged in a leaf

A leaf is an organ: a structure made of several tissues working together, here to carry out photosynthesis. Figure 1 shows the tissues in a transverse section of the blade.

Tissue in the leaf	How it helps photosynthesis
Cuticle and epidermis	Transparent, so light reaches the mesophyll; the waxy cuticle reduces water loss
Palisade mesophyll	Tall cells packed with chloroplasts, near the upper surface where light is strongest: the main site of photosynthesis
Spongy mesophyll	Large air spaces let carbon dioxide and oxygen diffuse between the stomata and every cell
Vascular bundle (vein)	Xylem brings water and ions; phloem carries away the sugar made; veins also support the blade
Lower epidermis with stomata and guard cells	Stomata let gases in and out; guard cells close them to save water

Arrangement in stems and roots

Dicot stem (for example bean, sunflower): vascular bundles arranged in a ring near the outside, with phloem towards the outside, xylem towards the centre, and cambium between them; a central pith of parenchyma; collenchyma under the epidermis. The ring of strong tissue resists bending, like a hollow pipe.
Monocot stem (for example maize, sugarcane): many vascular bundles scattered through the ground tissue, each wrapped in a sheath of sclerenchyma; no cambium, so no secondary thickening.
Dicot root: xylem in the centre, often star-shaped, with phloem between its arms; around them a ring of cells, the endodermis, which controls what enters the xylem; a wide cortex of parenchyma that stores starch; root hairs on the epidermis. A central core of vascular tissue resists pulling, as the root anchors the plant.

Worked example 2: identifying an organ from its section

Three transverse sections are described. Identify each organ, and say whether it is from a monocot or a dicot. (a) Many vascular bundles scattered all through the section, each with two large xylem vessels; no cambium. (b) A single central star of xylem with phloem between the arms, surrounded by an endodermis and a wide cortex; epidermis with hairs. (c) A ring of separate vascular bundles, each with phloem outside and xylem inside and a thin layer of cambium between them; pith in the centre.

(a) A monocot stem: scattered bundles and no cambium.
(b) A root, most likely a dicot root: a central vascular core with an endodermis, and root hairs on the epidermis.
(c) A dicot stem: bundles in a ring, with cambium between phloem and xylem.

FuseSchool: "Xylem and Phloem - Transport in Plants". Compare the structure of each tissue with the table above, and note how the bundles are arranged in roots and stems.

Warning

Common mistakes

Saying phloem carries "food" or "glucose". It mainly carries sucrose and amino acids.
Saying both xylem and phloem cells are dead. Only the conducting cells of xylem are dead; sieve tube elements are alive.
Placing the phloem on the upper side of a leaf vein. In a leaf, xylem is on the upper side and phloem on the lower side.
Thinking a monocot stem has a ring of bundles. Its bundles are scattered.

Tip

Memory hook. "Xylem: exits the soil, goes up, is dead and woody. Phloem: phood (sugar) goes both ways, alive, in the bark." In a dicot stem, phloem is always outside the xylem.

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Practice: xylem, phloem and organs

Give three ways in which xylem vessels are adapted to carrying water.
Explain why a sieve tube element needs a companion cell.
Explain how the arrangement of palisade and spongy mesophyll helps photosynthesis.
State two differences between the vascular tissue of a monocot stem and of a dicot stem.
A xylem vessel in a micrograph at ×400 is 32 mm wide. Calculate its actual width in µm.
Aphids feed by pushing their mouthparts into a single cell of a stem and sucking. The liquid they collect is rich in sucrose. Which tissue are they feeding from? Give a reason.
Suggest why the vascular tissue is in the centre of a root but near the outside of a stem.

Show answersHide answers

Any three: no end walls, forming continuous tubes; no cytoplasm (dead, hollow), so little resistance; lignified walls resist collapse under tension and waterproof the tube; pits allow sideways movement; narrow, which helps water rise.
The sieve tube element has no nucleus and few organelles, so it cannot make its own proteins or much ATP; the companion cell, linked by plasmodesmata, supplies them and loads sucrose into the sieve tube.
Palisade cells, with many chloroplasts, lie just under the upper surface, where they receive the most light; spongy mesophyll below has air spaces that let carbon dioxide diffuse from the stomata to all the photosynthesising cells.
Monocot: bundles scattered, no cambium (no secondary growth). Dicot: bundles in a ring, cambium between xylem and phloem.
$A = \frac{32\,\mathrm{mm}}{400} = 0.08\,\mathrm{mm} = 80\,\mu\mathrm{m}$ .
Phloem (a sieve tube): phloem sap carries sucrose; xylem sap is mostly water and mineral ions.
A root is pulled by the shoot swaying in the wind, and a central core resists pulling; a stem is bent by the wind, and strong tissue near the outside, like a hollow pipe, resists bending best.

Quick check

Why does a ring-barked tree eventually die?

Key points

Xylem (vessels, tracheids, fibres, parenchyma): dead, lignified, continuous tubes carrying water and ions upwards and giving support.
Phloem (sieve tubes with sieve plates, companion cells): living, carries sucrose and amino acids from sources to sinks.
Leaf: cuticle, epidermis, palisade and spongy mesophyll, veins and stomata, each suited to photosynthesis.
Dicot stem: bundles in a ring with cambium; monocot stem: scattered bundles; root: central vascular core with endodermis.

Sources and credits

6.1.3: Stems, 6.1.4: Roots and 6.1.5: Leaves, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for xylem, phloem and the arrangement of tissues in stems, roots and leaves. Adapted by Ganzaa.
Girdling, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for ring-barking and its effects. Adapted by Ganzaa.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Images: Lamiot and Eewilson, CC BY 3.0; Berkshire Community College Bioscience Image Library, CC0; all via Wikimedia Commons.
Video: "Xylem and Phloem - Transport in Plants", FuseSchool - Global Education, embedded from YouTube.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why a ring of missing bark kills a tree

2Xylem and phloem compared

3How tissues are arranged in a leaf

4Arrangement in stems and roots

Why a ring of missing bark kills a tree

Xylem and phloem compared

How tissues are arranged in a leaf

Arrangement in stems and roots