In plant morphology, hard, rigid, pointed extension of stems, leaves or roots are termed as spines, thorns and prickles. The function of these spinose structures are same, deterring animals from eating the plants. Spines and thorns are almost look alike but however they both differ in many aspects like: spines are derived from leaves either the whole leaf or some part of leaf that contains vascular margins inside and thorns are derived from shoots which may be branched or may not be branched, they may or may not have leaves and they may or may not arise from a bud.
A sharp, pointed structure that is a modified leaf or stipule is known as spine. Spines provide protection to the plant. It also helps during the dry or hot condition by reducing water loss because when a leaf is replaced by a spine its surface area is reduced. And the function of leaf that is photosynthesis is then performed by stem. Spines are not very deeply seated and they are not connected to the vascular system of the plant. Spines occupies the same position as the leaves and bear bud in the axil. So we can say that the spines are exogenous part of the plant which is formed from the tissues that are present externally. Examples of spines are: ocotillo forms a spine that is a fusion of the stem and lower portion of the leaf. In most of the euphorbia species the spines arises from the either side of the bud and leaf scar. In acacia species stipule spines are present, stipule spines are hollow and provide shelter to the pseudomyrmex ants. Spines of cactus plants are modified leaves or shoots located in the axil of a leaf. In cactus spines are found in groups which is called as areole.
Modified branches or stem are known as thorns. They can be branched or simple. Thorns are woody, sharp, straight and pointed structures. Thorns also provide protection to the plant from herbivores. They also helps in climbing. Thorns are formed in the axil of the leaves in place of a bud on an ordinary stem the same as other branches. So they can only be seen at the node of plant. Thorns are very deeply seated in to the plant therefore they have connection with vascular tissues. Thorns are endogenous in origin because they arises from deeply seated tissues of the plant. Examples of thorns are: gleditsia triacanthos (honey locust) have branched thorns. Crataegus crus-galli (cockspur hawthorn) have unbranched thorns. In alluaudia, each pair of leaf is subtended by a thorn.
- Spine is a sharp, pointed structure that is a modified leaf while thorn is a woody, sharp, straight and pointed structure which is modified stem or branches.
- Spines and thorn both defends plant from herbivore organisms.
- Spines also helps during dry condition by reducing water loss while thorn helps in climbing.
- Spines occupies the same position as the leaves and bear a bud in the axil while thorns are formed in the axil of the leaves in place of a bud.
- Spines are not deeply seated and also have no connection with vascular system of the plant while thorns are deeply seated and have connections with vascular tissues.
- Spines are exogenous because it is derived from tissues that are present externally while thorns are endogenous in origin because they arises from the deeply seated tissues of the plant.