Diversity of Specialised Tissues

Lessons

Senior 4 · Biology

Diversity of Specialised Tissues

4 lessons

Lesson 1 of 4

Plant tissues: meristems and simple permanent tissues

25 min

Grass that grows back from the bottom

A farmer who cuts fodder grass such as Napier grass for a cow can cut the same clump again a few weeks later. If a plant grew only from the tips of its shoots, cutting off the tips would stop it. Grasses keep growing because they have dividing tissue near the base of each leaf and stem segment, below the blade of the sickle or the teeth of a grazing animal. A cassava stem cutting, on the other hand, grows new roots and shoots from buds that also hold dividing cells.

These dividing tissues are meristems. Every other plant tissue comes from them. This lesson describes the meristems and the simple tissues they produce.

Definition

Tissue: a group of similar cells, together with the substances between them, that work together to carry out a particular function.

Meristematic tissue (meristem): plant tissue made of small, unspecialised cells that keep dividing by mitosis. Its cells have thin walls, dense cytoplasm, large nuclei and few or no vacuoles.

Permanent tissue: tissue whose cells have stopped dividing and have differentiated. Simple permanent tissues contain one type of cell; complex tissues (xylem and phloem, Lesson 2) contain several.

Lignin: a hard, waterproof polymer deposited in some plant cell walls, making them rigid; cells with lignified walls usually die.

Meristems

Meristem	Where	What it does
Apical meristem	Tips of roots and shoots (the root tip is protected by a root cap)	Primary growth: increases length
Lateral meristem (cambium)	In a cylinder inside stems and roots of dicots and conifers: vascular cambium between xylem and phloem, cork cambium near the surface	Secondary growth: increases girth (thickness); makes wood and bark
Intercalary meristem	At the bases of leaves and of stem segments (internodes) in grasses	Lets grasses and bamboo regrow after cutting or grazing

Behind an apical meristem, new cells first elongate (their vacuoles take up water and they grow longer) and then differentiate into permanent tissues. So a root tip has a zone of division, a zone of elongation and a zone of differentiation, in that order from the tip.

Simple permanent tissues

Tissue	Structure	Functions	Examples
Parenchyma	Living, thin cellulose walls, large vacuoles, often air spaces between cells	Packing tissue; storage of starch, water and oils; photosynthesis when it contains chloroplasts (chlorenchyma); gas exchange through air spaces; support when turgid	Cortex and pith of stems; flesh of cassava roots and potato tubers; leaf mesophyll; aerenchyma (large air spaces) in water plants such as papyrus
Collenchyma	Living, elongated; cellulose walls thickened at the corners; no air spaces	Flexible support for young, growing organs: strong but still able to stretch	Just under the epidermis of young stems and leaf stalks; the "strings" of celery
Sclerenchyma	Dead when mature; very thick lignified walls; may be long fibres or short sclereids (stone cells)	Rigid support and protection of parts that have stopped growing	Fibres in stems and leaves (sisal and jute fibres); sclereids in the hard shells of nuts and the gritty flesh of pears
Epidermis	Usually one layer of tightly fitting living cells; in shoots covered by a waxy cuticle; includes guard cells and hairs	Protection; reduces water loss (cuticle); gas exchange through stomata; absorption by root hairs	Surface of leaves, young stems and roots

Worked example 1: identifying tissues from a description

Name the tissue described: (a) living cells with cellulose walls thickened only at the corners, found just under the epidermis of a young leaf stalk; (b) dead cells with very thick walls that stain red with safranin, in clusters in the flesh of a fruit; (c) small cells with large nuclei and no vacuoles, 1 mm behind a root tip.

(a) Collenchyma: corner thickening and living cells are its defining features; its position supports a still-growing organ.
(b) Sclerenchyma, as sclereids (stone cells): safranin stains lignin red, and lignified cells die.
(c) Meristematic tissue (the apical meristem): dense cytoplasm, large nuclei and no vacuoles are the marks of dividing cells.

Worked example 2: why young stems bend and old stems do not

A young bean stem bends in the wind and straightens again; a two-year-old coffee stem is rigid. A young stem that is not watered wilts. Explain these observations using the tissues of this lesson.

The young bean stem is supported mainly by turgid parenchyma and by collenchyma. Collenchyma has unlignified cellulose walls, so it is strong yet flexible and can stretch as the stem grows.
When water is short, parenchyma cells lose turgor: the pressure of their vacuoles against the walls falls and the stem wilts, because much of its support came from turgor.
The older coffee stem contains sclerenchyma fibres and lignified xylem (wood, from the cambium). Lignin makes these walls rigid, so the stem stays stiff even when short of water.

Miss Angler: "PLANT TISSUES". Watch the sections on meristems, parenchyma, collenchyma and sclerenchyma (xylem and phloem are covered in Lesson 2), and compare the micrographs with Figure 1.

Warning

Common mistakes

Confusing collenchyma and sclerenchyma. Collenchyma is living with uneven cellulose thickening; sclerenchyma is dead with even, lignified thickening.
Saying meristem cells have large vacuoles. Dividing cells have dense cytoplasm and few or tiny vacuoles; large vacuoles form during elongation.
Calling the cuticle a layer of cells. It is a waxy layer secreted by the epidermal cells, not made of cells.

Tip

Memory hook. "Collenchyma: Corners, Cellulose, Can bend. Sclerenchyma: Stiff, Solid lignin, Spent (dead)."

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Practice: meristems and simple tissues

Name the three kinds of meristem and state where each is found.
Describe two features of meristem cells and explain how each suits cell division.
Explain why grasses survive grazing better than most dicot plants.
Compare parenchyma and sclerenchyma in a table of four features.
Papyrus grows in waterlogged swamps. Suggest why its stems contain parenchyma with very large air spaces (aerenchyma).
A section of a stem is stained with safranin, which stains lignin red. Which tissues would you expect to stain red, and which not?
Explain why sisal leaves are a source of strong fibres for rope, while a cassava root is a source of food.

Show answersHide answers

Apical (tips of roots and shoots); lateral, or cambium (in a ring inside stems and roots of dicots); intercalary (bases of leaves and internodes of grasses).
Any two: thin walls (easy to divide and expand); large nuclei (contain DNA for frequent replication and control); dense cytoplasm with many ribosomes (much protein synthesis); few vacuoles (cells small and active).
Their intercalary meristems lie at the bases of leaves and stem segments, below the level eaten, so the leaves keep growing from the base; dicots grow from apical meristems at shoot tips, which grazing removes.
Parenchyma: living / thin cellulose walls / large vacuole / storage and packing. Sclerenchyma: dead / thick lignified walls / no contents / rigid support.
Air spaces let oxygen diffuse from the shoot down to submerged parts and roots, where waterlogged soil has little oxygen for respiration; they also help the stem float upright.
Red: sclerenchyma fibres and xylem (lignified). Not red (green with a counterstain): parenchyma, collenchyma, phloem and the epidermis, whose walls are cellulose.
Sisal leaves contain many long sclerenchyma fibres with thick lignified walls, which are strong. Cassava roots are made mostly of parenchyma whose cells store starch.

Quick check

Match each tissue to its description.

Tap one on the left, then its partner on the right.

Key points

Meristems (apical, lateral, intercalary) contain small dividing cells; all other plant tissues come from them.
Parenchyma: living, thin-walled, packing, storage and photosynthesis.
Collenchyma: living, cellulose thickened at corners, flexible support of young organs.
Sclerenchyma: dead, lignified, rigid support (fibres and sclereids). The epidermis, with its cuticle, protects.

Sources and credits

6.1.2: The Plant Body and 6.1.3: Stems, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for meristems, ground tissues and the epidermis. Adapted by Ganzaa.
Ground tissue, Wikipedia contributors, licensed under CC BY-SA 4.0. Used for parenchyma, collenchyma and sclerenchyma. Adapted by Ganzaa.
Figure 1, the worked examples, practice and quick check: written by Ganzaa.
Images: Berkshire Community College Bioscience Image Library, CC0; Forest and Kim Starr, CC BY 3.0 US; all via Wikimedia Commons.
Video: "PLANT TISSUES", Miss Angler, embedded from YouTube.

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

1Grass that grows back from the bottom

2Meristems

3Simple permanent tissues

Grass that grows back from the bottom

Meristems

Simple permanent tissues