Thorns and spines are modified organs of plants. They evolved from a simple leaf or shoot to function as a defensive organ to protect themselves from the predators, sometimes they used it as a climbing tool. In xerophytes, the stems are modified to phylloclades that store water for future use and change the leaf conformation into the spines. The spines are adapted because the leaves decrease the surface volume of the leaf lamina to control the evaporation and reduce transpiration rates as well. Whereas the thorns mainly control the defense. Now let’s start the further discussion..
Thorns are the axillary shoots that are modified into rigid and sharp structures.
It is a defensive organ and also helps in climbing.
They are sub-epidermal and develop from the endogenous tissues.
They are connected to vascular bundles.
They bear lateral and foliage shoots often.
Develops in the leaf axil.
Similar internal organization as stem.
Some plant stems are modified extremely to develop thorns. They may be simple, e.g. Duranta plumieri, Hygrophyla auriculata, Aegle marmelos, prunus sp., and compound or branched thorns, e.g. Carissa carandas, Flacourtia cataphracta etc.
The whole leaf lamina or may be a part such as apex, margin, axillary bud, stipule etc. of the leaf is modified into a spine.
Performs as a defensive organ to protect the plant from the herbivores.
Leaves modify to reduce the surface of the lamina to decrease the transpiration rate in xerophytic plants.
They are not associated with vascular bundles.
Developed by external tissues.
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Leaf apex adapted to spines e.g. date palm, Phoenix sylvestris, Yucca sp. etc.
Leaf margin can be modified into spines e.g. Mexican poppy, Argemone Mexican, pineapple, Ananas comosus.
Both apex and margin are adapted to spines e.g. Aloe perfoliata
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In prickly pear, the leaves are ill-developed and fall down soon. The axillary leaf buds are adapted to spines e.g. Opuntia dillenii.
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In some cases axillary buds remain normal but leaves are modified e.g. barberry, Berberis vulgaris.
When the branches are modified into thick and bulky thorns, the leaves are also adapted to spines e.g. Ulex.
Thorns and spines are self-protective weaponry of the plants to make their precious parts unaffected from the herbivores and other mammals. The pointed deterrent dispirited the hungry mammals.
The plant with most spines or thorns is proven to make the caterpillar disappointed to defoliate the plant. The spines make their journey to the leaves difficult and full of obstacles. They not only slow the caterpillar but also interfere in their footing and force them into more energetic maneuvers. They fall off so frequently and start from the start again.
Small herbivores like snails are affected and might have manipulated their evolution.
Some plants have injurious secondary metabolites (arsenal) in the leaves. Plant develops spines to prohibit animals from touching or to consume the leaves.
Spines boost the surface area to improve photosynthesis.
They help plants to mount and sprawl.
Reduces water loss by reducing transpiration and evaporation.
| Thorns | Spines |
|---|---|
| Modified stems. | Modified leaves. |
| Thorns form in the place of axillary buds in the nodes. | Spines replace leaves in apex, margin, axillary bud and stipule. |
| Climbing and defensive aid. | Modifies to reduce transpiration, evaporation and acts as defensive aid. |
| Associated with vascular tissues. | Usually not deeply linked with vascular tissues. |
| Endogenous in origin. | Exogenous in origin. |
| Example- Duranta sp., Aegle sp. etc. | Example- Opuntia sp., Agave sp. etc. |
Not at all, they are not infectious but they are harmful. They can harm you by cutting or poking holes in our skin that may endow with the pathogen entry point e.g. roses, blackberries, pyracantha, bougainvillea etc. Rose’s thorns are able to transport bacteria and fungi into the skin that can cause infection. This infection can lead to sporotrichosis, plant-thorn synovitis and mycetoma etc. the rose picker’s disease or sporotrichosis and mycetoma are the fungal infections transmitted via cut, scrape by the thorn. If the date palm thorn or spine penetrates the joint it may cause arthritis, inflammation of synovial membrane. In these diseases, a small pink bump forms first. Gradually it grows and turns into an open sore.
The stems that are adapted into strong, woody, pointed thorns are mainly acts to prevent predator attack whilst the modified spines are the leaves that reduce its surface to control water loss. It is also a defensive organ. Thorns are made up of the internal tissues associated with the vascular tissues. They are mainly found in the plant nodes where the axillary leaf buds are placed normally. The spines are not connected to the vascular tissues as they are made up of the external tissues. They are developed by modifying the leaf apex, margin, axillary buds and stipules. Though the thorns and the spines are different in origin and function, they appear identical. So, it is difficult to recognize them separately. The thorns are of two types- simple or single, unbranched and compound or branched whereas the spines are characterized depending on the modified part of the leaves.
Q1. Define prickles.
Ans. Prickles are the type of thorns used interchangeably. They are developed from the external tissues of the epidermis and cortex of the plant stem. Prickles can be projected out from the midrib of a leaf e.g. honey locust. Not only stem and leaves, prickles are also found in some fruit and adventitious roots e.g. Ariocarpus, unlike thorns. Rose prickles protrude out from its skin.
Q2. State the reason for developing thorns in plants.
Ans. The axillary shoots are modified into the thorns in some plants as they have leaves that are reduced, weakly developed and expire quickly. These stems are adapted, sclerified and grow to a strong sharp thorn for a determinate tenure. It gives physical and mechanical defense to a plant.
Q3. How does a spine reduce the rate of evaporation and transpiration?
Ans. The spines are covered with waxy cuticles that are waterproof. It reduces the water loss. The plant lacks leaves that can trap heat as they reduce their surface, instead they are having spines inhibiting the moisture to lose. As they cannot trap sufficient heat, water cannot evaporate.
Q4. Is the spine able to photosynthesize?
Ans. No, spines are unable to photosynthesize. As they have reduced surface area that cannot absorb sufficient heat energy to proceed the photosynthesis.
Q5. What is glochid?
Ans. Glochids are the hair-like spines that are barbed and found in the areoles of the cactus. They are small and deciduous, easily detached from the cacti, stuck in the skin and cause irritation.
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