BIOLOGY · Stage 9 · Chapter 9: Plants and Photosynthesis
Biology 9.8 Xylem, phloem and plant pests — Vascular Bundles and Translocation
5 min read 7 parts Lesson 8 of 9
Lesson summary
9.8 Xylem, phloem and plant pests — Vascular Bundles and Translocation — Alhussan International Academy — Biology (Stages 7–9) notes: Distinguish xylem from phloem, describe translocation and explain how aphids and other pests exploit the plant's transport system.Vascular bundles…
- Phloem sieve tubes
- Sources and sinks
- Bark damage
- Insecticides
- Biological control
- Crop rotation and resistant varieties
Section 1 of 7Vascular tissue
- Vascular bundles
- flowering plants have transport tissues in bundles that run through the root, stem and leaf, and each bundle contains xylem and phloem.
- In stems the bundles form a ring.
- In roots they form a central core, which gives strength to resist pulling.
- In leaves they form the veins.
Section 2 of 7Comparing xylem and phloem
| Feature | Xylem | Phloem |
|---|---|---|
| Carries | Water and mineral ions | Dissolved sugars (sucrose) and amino acids |
| Direction | One way, upward from roots to leaves | Both ways, up to buds and flowers and down to roots |
| Structure | Dead, hollow, lignin-strengthened tubes | Living cells (sieve tubes) with companion cells |
| Other role | Support | None |
- Phloem sieve tubes
- their end walls have holes in them, so the sugary liquid flows through.
Section 3 of 7Translocation
1Translocation
= the movement of sugars and other organic substances through the phloem.
2Sources and sinks3 points
- sugars are made in the leaves (a source) and carried to places that use or store them (sinks).
- Sinks include growing shoot tips, roots, fruit and storage organs.
- In spring stored sucrose moves from roots to growing buds, and in summer from leaves to developing fruits.
3Energy2 points
- companion cells use active transport to load sucrose into the sieve tubes.
- Translocation therefore stops if the phloem is poisoned or cooled.
Section 4 of 7Evidence from ringing
- Ringing
- removing a ring of bark, including the phloem, from a tree trunk makes sugars collect above the cut, which swells, while the roots below starve.
- Xylem
- it lies deeper in the trunk, so water still rises.
- Bark damage
- if deer eat a complete ring of bark from a young tree, the phloem is cut and the tree dies.
Section 5 of 7Phloem-feeding insects
- Target
- every plant has phloem tubes full of sugar-rich liquid, and many insects use this as easy food.
- Insects
- aphids, plant hoppers, leafhoppers, cicadas, spittlebugs, scale insects and shield bugs.
- Each is small but there are enormous numbers, and they destroy billions of plants every year.
- Stylets
- their mouthparts are sharp, pointed, hollow tubes that they push into the phloem to feed.
- Cutting off an aphid and collecting the sap that exudes was used to study phloem.
- Damage
- they steal sugars, weaken the plant, reduce crop yields and ruin garden plants, and some pests also eat parts of the plant.
Section 6 of 7Plant pathogens
1Carriers2 points
- pests often carry plant pathogens, which are viruses, bacteria or fungi that cause disease.
- Aphids spread viruses.
2Spread3 points
- pathogens injected into the xylem or phloem are carried all over the plant.
- Pathogens that block the xylem stop water reaching the stems and leaves, so the plant wilts and dies.
- One fungus attacks the xylem and turns the veins yellow.
Section 7 of 7Controlling pests
- Insecticides
- they kill pests but must be used with care because they can harm useful insects such as bees.
- Biological control
- natural predators are used, for example ladybirds, which eat aphids.
- Crop rotation and resistant varieties
- both reduce pest damage.
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