 The old spines remain in place for decades as new spines are created at the continually growing apex.The result is that along each external "rib" of the cactus is a series of spines arranged in the order they grew in – the oldest spines are at the bottom and the youngest spines are at the top.These spines can be dated using bomb-spike Carbon-14 and isotopes of carbon (Carbon-13) and oxygen (Oxygen-18) may be used to infer past climate (e.g.