Modifications of Leaves: Spines, Tendrils, Traps and Storage

A leaf is built to make food. Its flat blade catches light, and its pores take in carbon dioxide. Most of the sugar a plant lives on is made there.

That job can be set aside. In many plants a leaf, or part of one, is rebuilt for defense, for climbing, for storing water and food, or for catching animals. The structure is still a leaf. The work is different.

This article covers the main kinds of modified leaf, and how to tell a spine from a thorn. It also asks why one organ can end up looking like a hook, a jug or a stone.

What Counts as a Modified Leaf

A modification is a change in shape and tissue, not a new organ. A tendril, a spine and a bulb scale all start from the same leaf bud.

Two clues identify one. The first is position: a modified leaf still sits where a leaf would sit, in the angle between the stem and a bud. The second is the veins, which usually betray a blade that has been folded, narrowed or thickened.

Botanists sort these forms by what they are made of, not by what they do. Two structures that look alike can come from different organs, which is why a cactus spine and a hawthorn thorn are not the same thing.

One common conversion is the phyllode. In many Australian acacias the blade is dropped after the seedling stage, and the flattened leaf stalk takes over photosynthesis. The plant ends up with a leaf that has no blade at all.

Modification	The job it does	An example
Spine	Defense, and less surface for water loss	Cactus, barberry
Tendril	Climbing, by gripping a support	Pea, grapevine
Storage scale	Holding sugar or water for later	Onion, aloe
Pitcher or trap	Catching animals for nitrogen	Nepenthes, flytrap
Window	Letting light into a buried leaf	Lithops
Bract	Advertising a flower	Poinsettia, bougainvillea
Phyllode	Making food with a flattened stalk	Many acacias

The main leaf modifications, the job each one does, and a plant that shows it.

Spines, Thorns and Prickles

All three are sharp, and all three keep animals away. Only one of them is a leaf.

A spine is a modified leaf or part of one. In a cactus the spine is the whole leaf, reduced to a hard point. It is carried on an areole, which is a short shoot sitting in the angle of the leaf.

A spine does more than stab. It shades the stem, breaks up the still layer of air that holds heat, and in some desert cacti it catches fog that then drips to the soil. Fewer and smaller leaves also mean less surface through which to lose water.

Spines on Echinopsis tubiflora. Each one is a modified leaf, carried on an areole. Image: André Karwath, CC BY-SA 2.5, via Wikimedia Commons

A thorn is a modified stem. Hawthorn and honey locust carry thorns, and a thorn grows where a branch would, sometimes with leaves or buds of its own.

A prickle is neither. It is an outgrowth of the outer tissue, with no deep connection into the stem. Rose prickles peel away with the skin of the stem, which is the quickest way to tell them from thorns.

Structure	What it is made from	A familiar example	How to tell
Spine	A whole leaf, or part of one, reduced to a point	Cactus, barberry	Grows in the leaf position, often on an areole
Thorn	A stem, modified into a point	Hawthorn, honey locust	Grows where a branch would, and may carry leaves or buds
Prickle	An outgrowth of the outer tissue only	Rose, blackberry	Peels off with the surface, with no deep connection

Spines, thorns and prickles: what each is made from, an example, and how to tell them apart.

Spines That Come From Stipules

Not every spine is a whole blade. In black locust, Robinia pseudoacacia, the pair of stipules at the base of each leaf becomes a pair of sharp spines, while the leaf itself stays normal.

Barberry, Berberis, goes the other way and turns whole leaves into spines, often in a three-pronged shape. Its short shoots grow in the angle behind those spines.

Black locust (Robinia pseudoacacia). The pair of spines at the base of each leaf are modified stipules. Image: Kenraiz, CC BY-SA 3.0, via Wikimedia Commons

Tendrils and the Climbing Habit

A tendril is a thin, touch-sensitive structure that wraps around a support and pulls the shoot up toward the light.

Leaf tendrils come from leaflets or from the tip of a leaf. In the garden pea, the upper leaflets are reduced to tendrils, and the plant climbs by them.

Not every tendril is a leaf. A grapevine's tendril is a modified stem, and so is a cucumber's. The two look alike and are built from different organs, which is why position matters more than shape.

How a Tendril Coils

A growing tendril sweeps through the air until it touches something. Contact on one side slows growth there, the far side keeps extending, and the tendril curves around the support.

The response is called thigmotropism: growth that follows touch. Once the tendril has a grip, it coils into a spring, which shortens it and lifts the shoot. The coil also absorbs movement without snapping.

A pea tendril. The upper leaflets of the leaf are reduced to a coil that grips a support. Image: Rasbak, CC BY-SA 3.0, via Wikimedia Commons

Storage Leaves, Traps and Windows

Some of the most extreme leaves do work that has nothing to do with light. They store, they catch, or they guide light into a buried plant.

Storage Leaves

An onion bulb is a stack of thick, fleshy scales around a short stem. Each scale is a leaf base swollen with sugar, and the dry outer layers are the same leaves reduced to a wrapper.

A succulent leaf stores water instead. Aloe and Agave have leaves packed with large, thin-walled cells that take up water and swell, which is what makes the leaf thick.

Both kinds of storage cost the plant surface area for light. The trade pays where the growing season is short, or where rain arrives only twice a year.

An onion bulb. The fleshy scales are thickened leaf bases storing sugar for the next season. Image: H. Zell, CC BY-SA 3.0, via Wikimedia Commons

Leaves That Catch Animals

A carnivorous plant still photosynthesizes; the trap is an addition, not a replacement. What the trap supplies is nitrogen and phosphorus, which are scarce in bogs and on bare rock.

The pitcher plant Nepenthes turns the blade into a jug. The rim is smooth and slippery, the inner walls are waxy, and the fluid at the bottom digests what falls in. A lid keeps rain from filling the pitcher and diluting that fluid.

The Venus flytrap uses two hinged lobes with three trigger hairs on each. One touch arms the trap, and a second touch within about 20 seconds closes it, so a falling raindrop does not spring it.

Sundews hold drops of sticky mucilage on tentacles, and bladderworts pull small animals into bladders under water. Each leaf has been rebuilt around a different mechanism.

A lower pitcher of Nepenthes macfarlanei. The jug is the leaf blade, and the lid is part of the same leaf. Image: Alexey Yakovlev, CC BY-SA 4.0, via Wikimedia Commons

The trap of a Venus flytrap, with its trigger hairs. Two touches close the lobes. Image: Noah Elhardt, CC BY-SA 2.5, via Wikimedia Commons

Window Leaves

A window leaf is a leaf that has given up its exposed surface. In Lithops, the plant body is mostly buried, and only the flat top shows.

That top carries translucent tissue, the window, which lets light pass inside to where the green cells sit. The plant gets light without holding a large, water-losing blade up to the sun.

Fenestraria and some Haworthia species do the same, and the pattern appears in several deserts. It is a convergence: unrelated plants arriving at one solution.

Lithops salicola. Most of the plant is buried, and the translucent top is the window that lets light in. Image: H. Zell, CC BY-SA 3.0, via Wikimedia Commons

Bracts, Petals and Display

The showiest modified leaves are the ones a visitor mistakes for flowers.

A bract is a leaf at the base of a flower or an inflorescence. Usually it is small and green. In poinsettia the bracts are large and red, and the actual flowers are the tiny yellow clusters in the middle.

Bougainvillea works the same way, and so does the arum's spathe, the hood that wraps the flower spike. In each case a leaf takes on the advertising job.

Petals and sepals are modified leaves too, arranged in rings on a shoot that stopped growing. That makes the flower the most elaborate leaf modification of all.

Poinsettia. The red parts are bracts, which are modified leaves; the flowers are the small yellow clusters. Image: Vinayaraj, CC BY-SA 3.0, via Wikimedia Commons

For identification, a modified leaf is a strong clue but not a label. A spine points to a dry place, not to a family: cacti and some euphorbias both grow spines, and the two families are not close relatives. The reliable test is still position and origin, read together with the rest of the plant.

Key Terms in Plain English

These are the words in this article that would send you to a dictionary. Each one is given here the way it is actually used above.

Term	What it means
Leaf modification	A leaf rebuilt in shape and tissue for a job other than making food.
Spine	A sharp point made from a whole leaf or part of one.
Thorn	A sharp point made from a modified stem.
Prickle	A sharp outgrowth of the outer tissue, with no deep connection in the stem.
Areole	The small cushion on a cactus that carries spines; it is a short shoot.
Stipule	A small flap at the base of a leaf stalk, which can become a spine.
Tendril	A thin, touch-sensitive structure that grips a support and helps a plant climb.
Thigmotropism	Growth that turns or coils in response to touch.
Bulb	A short underground stem wrapped in thick storage leaves, as in an onion.
Scale leaf	A small or thickened leaf, such as the fleshy layers of a bulb.
Succulent	A plant with tissues that store water, often in thick leaves.
Carnivorous plant	A plant that traps and digests animals to get nitrogen and phosphorus.
Pitcher	A leaf shaped into a jug that holds fluid and traps animals.
Trigger hair	A sensitive hair on a flytrap lobe that springs the trap when touched.
Window leaf	A leaf with translucent tissue that lets light reach green cells inside.
Bract	A modified leaf at the base of a flower or inflorescence, sometimes colored.
Spathe	A large bract that wraps a flower spike, as in arum lilies.
Phyllode	A flattened leaf stalk that takes over photosynthesis when the blade is lost.
Petal	A modified leaf forming part of the flower's inner ring.
Sepal	A modified leaf forming the outer ring that covers a bud.
Convergence	The same form evolving separately in unrelated plants.
Mucilage	A sticky, water-based substance, used by sundews to hold insects.

Terms used in this article, defined in plain words.

Key Takeaways

• A modified leaf is still a leaf: it keeps the leaf's position on the stem and is rebuilt in shape and tissue for another job.
• A spine is made from a leaf or part of one, as in a cactus, where each spine sits on an areole.
• A thorn is made from a stem and grows where a branch would, as in hawthorn; a prickle is only an outgrowth of the outer tissue, as in a rose.
• Some spines are modified stipules, as in black locust, and some are whole leaves, as in barberry.
• A tendril grips a support and pulls the shoot up, and the coiling is a touch response called thigmotropism.
• Pea tendrils are modified leaflets, while grapevine tendrils are modified stems, so not every tendril is a leaf.
• Onion bulb scales are thick leaf bases storing sugar, and aloe leaves store water in large, thin-walled cells.
• Pitcher plants, flytraps and sundews add a trap to a working leaf because their habitats are poor in nitrogen and phosphorus.
• A window leaf keeps the plant buried and lets light in through translucent tissue, as Lithops does.
• Bracts and petals are modified leaves doing the advertising, and the flower itself is a shoot of modified leaves.

Frequently Asked Questions

What are leaf modifications?

They are leaves, or parts of leaves, rebuilt for a job other than photosynthesis. Common ones are spines for defense, tendrils for climbing, storage scales, traps for catching animals, window leaves and bracts.

Are cactus spines leaves?

Yes. Each cactus spine is a whole leaf reduced to a hard point. It grows on an areole, which is a short shoot in the angle of the leaf.

What is the difference between a spine, a thorn and a prickle?

A spine is made from a leaf, a thorn from a stem, and a prickle from the outer tissue only. A rose prickle peels off with the skin of the stem; a hawthorn thorn does not.

Are all tendrils modified leaves?

No. Pea tendrils are modified leaflets, but grapevine and cucumber tendrils are modified stems. The position of the tendril on the shoot is what tells them apart.

Why does a Venus flytrap close?

Two touches on its trigger hairs within about 20 seconds close the lobes. The two-touch rule stops a raindrop or a stray twig from springing the trap.

What is a window leaf?

A leaf with translucent tissue that lets light pass inside to the green cells. Lithops grows mostly buried, with only its window top showing above the soil.

Are petals modified leaves?

Yes. Petals and sepals are modified leaves arranged in rings on a shoot that has stopped growing, which is why the flower counts as the most elaborate leaf modification.

Why do plants modify their leaves?

Because a leaf is the cheapest organ to rebuild. Changing its shape and tissue lets a plant defend itself, climb, store food or water, catch animals or advertise a flower without evolving a new organ.

Key references
• Leaf — the organ and its modifications (Wikipedia)
• Thorn — spines, thorns and prickles (Wikipedia)
• Tendril — leaf and stem tendrils (Wikipedia)
• Thigmotropism — growth in response to touch (Wikipedia)
• Bulb — storage leaves around a short stem (Wikipedia)
• Carnivorous plant — traps and their habitats (Wikipedia)
• Lithops — window leaves and buried growth (Wikipedia)
• Bract — modified leaves around a flower (Wikipedia)

Sources: the Wikipedia article on leaves, for the definition of a leaf and for leaf modifications as changes in shape and tissue rather than new organs; the Wikipedia article on thorns, for the separation of spines (leaf), thorns (stem) and prickles (outer tissue), and for the areole and the stipular spines of black locust; the Wikipedia article on tendrils, for leaf tendrils in the pea and stem tendrils in the grapevine; the Wikipedia article on thigmotropism, for the touch response that coils a tendril; the Wikipedia article on bulbs, for the onion as a short stem wrapped in thick storage leaves; the Wikipedia article on carnivorous plants, for the nitrogen and phosphorus shortage that makes a trap worth building, and for the pitcher, flytrap, sundew and bladderwort mechanisms; the Wikipedia article on Lithops, for the buried body and the translucent window; and the Wikipedia article on bracts, for the poinsettia bracts and the arum spathe. Each source was opened and read in full. Figures are from Wikimedia Commons under the credit named in each caption. This article is written for education.