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A thorn is a modified stem and can be recognized  because it is subtended by a leaf . In Alluaudia, each pair of leaves subtends a thorn. A spine is a sharp pointed structure that is a modified leaf or stipule. Thorns are actually modified axillary buds or terminal buds possessing vascular supply.

Usually thorns are present in plants which live in desert areas. In desert areas the availability of water is very low. Only in night time the place will be cool in deserts so the plants should absorb only the moisture and grow. in order to avoid water loss that is transpiration, the stems are modified into thorns in some plants which live in desert areas and that's because to avoid water loss through transpiration.

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