Plant Orientation in Space

Introduction to Plant Responses

Since plants cannot move around, they have to make do with the conditions in the habitat in which the seed or spore GERMINATES.

To some extent, however, they can respond to changing conditions by moving their organs. A plant organ can move in one of two ways:

Concept 9: Tropisms

Success Criteria & Vocabulary

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Abscisic acid (ABA): Plant hormone that prepares a plant to withstand stress. High levels promotes seed dormancy.

Auxin: Group of plant hormones that change the rate of elongation in plant cells, controlling how long they become. 

Coleoptile: The sheath surrounding a young leaf shoot.

Cytokinin: In the presence of auxin, this group of plant hormones stimulates cell division and the production of buds, roots, and shoots, and delays leaf fall.

Ethene: Plant hormone that speeds up ripening in fruit. It also stimulates cell division in wounded tissue.

Germination: Development of a plant from seed after a period of dormancy.

Gibberellin: Group of plant hormones responsible for germination and plant development.

Hormone: Chemical messengers that regulate plant and animal growth and development (among other functions).

Indole-3-acetic acid (IAA): The most common auxin. 

Nastic response: A plant response in which the direction of the plant response is independent of the stimulus direction.

Negative response: Response away from the stimulus

Petiole: Leaf stalk

Positive response: Response towards the stimulus

Stimulus: A change in an organism's environment to which it can respond.

Tendril: Specialised stem or leaf used by climbing plants for support and attachment by twining around suitable hosts found by touch. 

Tropism: A plant growth response to a directional stimulus.

Turgor: Rigidity of cells due to the absorption of fluid.


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Concept 9: Support Notes

What are Tropisms?

TROPISMS are plant growth responses to external STIMULI in which the stimulus direction determines the direction of the growth response. 

Tropisms are named depending on the stimulus involved, and are identified as POSITIVE (towards the stimulus) or NEGATIVE (away from the stimulus). 

Tropisms act to position the plant in the most favourable growth environment. 

What are some examples of Tropisms?

Positive chemitropism - growth towards a chemical stimulus. 

For example: Pollen tubes grow towards a chemical, possibly calcium ions, released by the ovule of the flower. 

Negative geotropism - growth away from gravity. 

For example: Stems and COLEOPTILE (the sheath surrounding a young leaf shoot) grow away from the direction of Earth's gravitational pull. 

Positive hydrotropism - growth response towards water (concentration). 

For example: Roots are influenced primarily by gravity but will also grow towards water. 

Positive phototropism - growth towards light (particularly directional light). 

For example: Coleoptiles, young stems, and some leaves grow towards the light. 

Positive thermotropism - growth towards the source of heat. 

For example: Rhododendron leaves droop and curl inward in cold temperatures to prevent cell damage due to rapid thawing after freezing temperatures. 

Positive thigmotropism - growth towards the source of touch or pressure. 

TENDRILS (modified leaves) will coil around an object it touches.

Parts of a Growing Seed

A seed is made up of the seed coat that surrounds the plant embryo. The plant embryo is made up of:

The bending of a stem or root results from one side elongating faster than the other. 

So, only those parts that can increase in length are capable of tropic movement. 

For the most part, this means MERISTEMS (young stems, roots tips) and PETIOLE (leaf stalks).

How did Scientists Investigate Phototropism?

In the 1880's, Charles Darwin and his son Francis investigated phototropism in very young plants that had just sprouted whose leaves and shoots were still covered by a sheath called the coleoptile

They noticed that if light is shone on a coleoptile (shoot tip) from one side the shoot bends (grows) toward the light. The ‘bending’ did not occur in the tip itself but in the elongating part just below it. 

Removing the tip or covering it with foil meant that the shoot could no longer ‘bend’ toward the light. Covering the elongating part of the shoot did not affect the response to light at all!

Darwin Concluded that: “Some influence is transmitted from the tip to the more basal regions of the shoot thereby regulating growth and inducing curvature.”

In 1913, Peter Boysen-Jensen followed up on Darwin's work by showing that a chemical signal produced at the tip was indeed responsible for the bending response: 

They first cut off the tip of a coleoptile, covered the cut section with a block of gelatin, and replaced the tip. 

The gelatin is permeable enough to allow a chemical signals to travel through its pores.

They observed that the coleoptile was able to bend normally when it was exposed to light. 

They tried the experiment again, using an impermeable flake of mica instead of gelatin.

The flake of mica is impermeable.

They observed that the coleoptile lost the ability to bend in response to light. 

Only the gelatin—which allowed a chemical signal to travel through its pores—could allow the necessary communication between tip and base. In another experiment, Boysen-Jensen was also able to show that the mobile signal traveled on the shaded side of the seedling. 

When the mica plate was stuck in on the illuminated side, the plant could still bend towards the light.

But when the mica flake was stuck in on the shaded side, the bending response no longer occurred. 

The results of this experiment also implied that the signal was a growth stimulant rather than a growth repressor since the phototropic response involved faster cell elongation on the shaded side than on the lit side. 

What role do Plant Hormones have on Tropic Responses?

Plant HORMONES are chemicals that act as signal molecules to regulate plant growth and responses. Alone or together, plant hormones target specific parts of a plant and produce a specific effect. Many have roles in coordinating timing responses in plants, including promoting and breaking bud dormancy, seed GERMINATION, and fruit ripening.

The following are some examples of plant hormones.


AUXINS are a family of plant hormones. They are mostly made in the tips of the growing stems and roots, and can diffuse to other parts of the stems or roots. Auxins change the rate of elongation in plant cells, controlling how long they become. The most common auxin is called indole-3-acetic acid, a.k.a IAA.

Basically, inside a cell, auxin can loosen the cell wall and allow cells to grow in size.

Stems and roots respond differently to high concentrations of auxins:

In a stem, the shaded side contains more auxin and grows longer – causing the stem to bend towards the light. Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less - causing the root to bend away from the light. 


GIBBERELLINS are a group of plant hormones responsible for growth and development. They have a wide range of effects on plant growth:


CYTOKININS are a group of plant hormones produced in root tips and are transported to the shoots. In the presence of auxin, cytokinins have the following effects:


ETHENE is a gaseous plant hormone produced in all parts of the plant, which speeds up ripening in bananas and other fruit. The effect of ethene released from bananas is clearly visible if you keep them in a bowl with other fruit, as it causes other fruits to ripen very quickly. 

Ethene also stimulates cell division in wounded tissue.

Abscisic acid (ABA)

ABSCISIC ACID is a plant hormone that prepares a plant to withstand stress, due to either cold or to a lack of water. 

Abscisic acid (ABA) accumulates in seeds during fruit production and is important in seed dormancy, as it is a powerful growth inhibitor. A high level of ABA in the seed embryo promotes dormancy.

Model answer: Discuss the growth response of a shoot, including the role of plant hormones and the adaptive advantages of this response.

The shoot shows -ve gravitropism as it grows upwards and away from gravity/towards the surface and +ve tropism once it breaks through the surface and has contact with light/grows towards the light. 

Once the shoot (plumule) germinates, auxin produced in the apical meristem tissue settles on the lower side of the shoot tissue and causes elongation of the cells, whilst the top layer of cells elongate normally. The differential cell development as the shoot (plumule) grows causes it to bend upward against gravity and towards the soil surface. 

Once the shoot breaks through the surface and encounters light, the shoot shows a +ve tropic response. Auxin is responsible for growth/elongation of shoot cells. Auxin is produced in the apical meristem tissue/shoot tip and accumulates on the shaded side of the shoot stem. It causes elongation of cells on the shaded side and differential growth, so that the shoot bends towards the light source. It does this by stimulating shoot cell growth on the lower side of shoot in ground/darkness. Auxin stimulates the shoot cell growth on the shaded side when light. 

The adaptive advantage of the gravitropic growth response is that regardless of which way the seed is orientated, the plumule will always respond by growing upwards towards the soil surface against gravity, where it will eventually access light to begin chlorophyll production and photosynthesis, becoming independent in producing its own energy supply. The -ve geotropic response enables the shoot to grow towards the surface in the absence of light. 

The advantage of phototropic growth response is that the shoot, as it develops leaves and chlorophyll, in order to photosynthesise, is able to gain maximum light exposure at the most effective angle to enable photosynthesis and therefore growth, development, and reproduction to occur. The plant has energy and raw materials to develop specialised reproductive tissues such as flowers and seeds and seed receptacles (pollination and seed dispersal). The plant is able to compete with other plants for adequate light for effective photosynthesis and starch production for continued growth and repair. The plant can orient towards the light and gain maximum light exposure on developing leaf surfaces (chlorophyll)for photosynthesis. Once the shoot grows upwards, it can produce flowers and have pollination. When the shoot grows out of the soil into a mature plant, seed dispersal occurs. 

Enables competition with other plants for light. Enables growth of specialised tissue for fertilisation and seed development. Enables development of leaf surface area to maintain adequate starch production for continued growth and differentiation/reproduction.

Concept 10: Nastic Responses

Success Criteria & Vocabulary

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Adaptive advantage: Any trait that results in an organism having a greater chance of surviving to an age where it can reproduce.

Browsing: Type of herbivory in which a herbivore feeds on leaves, soft shoots, or fruits of high-growing plants.

Diurnal: Active during the daytime.

Nastic response: A plant response in which the direction of the plant response is independent of the stimulus direction.

Nerve impulse: Electrical signals transmitted through neurons so that the central nervous system (CNS) can sense the environment and produce a response.

Stimulus: A change in an organism's environment to which it can respond.

Turgor: Rigidity of cells due to the absorption of fluid.


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Concept 10: Support Notes

What are Nastic Responses?

NASTIC RESPONSES are plant responses in which the direction of the plant response is independent of the STIMULUS direction. They are reversible and often rapid movements, often resulting in localised changes in TURGOR

Nastic responses can occur in response to temperature (thermonasty), light (photonasty), or touch (thigmonasty). 

Plant 'sleep movements', in which flowers close or leaves droop at night, are specialised DIURNAL photonasties. 

What are some examples of Nastic Responses?

NYCTINASTY/PHOTONASTY: Sleep movements in plants

Many plants show movements in relation to light and dark. Oxalis spreads its leaves out during the day to capture sunlight. During the night, the leaves are lowered and bend slightly along the midline. This helps to prevent dew accumulating on the leaf and minimises the risk of damage while the leaf is not being used to capture light. 

Is there a difference between nyctinasty and photonasty? 

These responses usually happen in plant leaves and flower petals. 

THIGMONASTY: Movements of the sensitive Mimosa plant

The sensitive mimosa plant has long leaves composed of small leaflets. When a leaf is touched, the leaflets fold together. Strong disturbances cause the entire leaf to droop from its base. This response takes only a few seconds and is caused by a rapid loss of TURGOR PRESSURE from the cells at the bases of the leaves and leaflets. 

The message that the plant has been disturbed is passed quickly around the plant by electrical signals, not by plant HORMONES (as what occurs in tropisms). The response is like the NERVE IMPULSES of animals, but it is much slower. After the disturbance is removed, turgor pressure is restored to the cells, and the leaflets slowly return to their normal state. 

The ADAPTIVE value of this nastic response is uncertain but may relate to deterring BROWSERS or reducing water loss during high winds. 

Thigmonastic responses in carnivorous plants

Some small, specialised plants obtain most of their nitrogen (but not their energy) from trapping and digesting small animals such as insects. This allows them to grow in nutrient-poor (particularly low nitrogen), high light environments, such as acidic bogs and rocky outcrops. 

Venus flytrap (Dionaea)

When an insect touches the hairs on a leaf of a Venus flytrap, the two lobes of the leaf snap shut, trapping the insect. Once the insect has been digested, the empty leaves reopen. The hairs on the leaf must be touched twice in quick succession (back-to-back) for the leaf to close. This means false alarms, such as a twig falling onto the leaf, do not set it off. 

Sundew (Drosera)

Sundews also show a thigmonastic response. An insect landing on a leaf quickly becomes trapped in the sticky hairs. The hairs fold around the insect and in some species the leaf may curl over, completely enclosing the insect.