Divergent evolution/Homologous organs. Similar in origin but perform different function.
Detailed Answer:
The thorn of Bougainvillea and tendril of cucurbits are homologous structures as both are similar in structure and origin but have different functions. Both of them arise from the axil of leaf and have anatomical similarity. Thorn provide protection and tendril gives support. Divergent evolutionary relationship has brought such similarity between them.
