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Thorns are modified stem in Bougainvillea,  actually stem of plants like Citrus and Bougainvillea get  modified into thorns and these thorns  protect the plants from the browsing animals.

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Modification of thorns in Bougainvillea is Stem. So thorn is the Modification of axillary bud or terminal bud of the stem. It may bear leaves, flowers & Endogenous in origin. Ex: Catissa (Karonda), pomegranate, citrus, Bougainvillae. You can get such more examples once you finish reading your ncert textbook and then solve some mcqs.

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