Q.Opuntia has spines but Carissa has thorns. What is the difference?

The stem develops underground, sub-aerial and aerial modifications for storage, perennation, propagation and photosynthesis, all recognisable by the presence of nodes/internodes and buds (unlike modified roots). Underground stems: the stem tuber (Potato) swells at its tip and bears axillary-bud 'eyes' at nodal scale leaves; the rhizome (Ginger, Turmeric) is a horizontal/oblique stem with nodes, scale leaves and buds, spreading by sympodial growth via lateral buds or monopodial growth via the terminal bud (Nymphaea, Lotus, fern); the bulb is a reduced discoid stem wrapped in concentric fleshy storage leaves, either tunicated/layered (Onion) or scaly/compound tunicated (Garlic); the corm (Colocasia, Amorphophallus) is a swollen vertical stem-internode with ring-like nodes and adventitious buds/roots. Sub-aerial stems: the trailer spreads flat over the ground (Euphorbia); the runner sends out horizontal green branches from a crown (Cynodon); the stolon arches over and roots where it touches ground (Mentha); the sucker grows underground before emerging as a new shoot (Chrysanthemum, Banana); the offset is a one-internode rosette runner (Pistia, Eichhornia). Aerial modifications: the thorn is a hard modified bud (apical in Car …