Vachellia sieberiana var. woodii stem tip

This leafy stem tip of Vachellia sieberiana var. woodii shows the colour change upon the leaves as they mature.

Even the spines at stem tips start off with some green when they are, like the stems, still soft. They can’t delay hardening unduly, because it is exactly the soft and juicy stem tip foliage, invitingly coloured that is likely to be browsed first.

What else, apart from (some) protection could have caused the evolution of thorns? Although a stem tip of a big Vachellia tree is only occasionally within reach of the average sized antelope (and photographer), there used to be giraffes around these trees in habitat in “the good old days”.

In the photo the alternate young leaves are angled upwards, while the spines that flank them point outwards. Definition of leaflet shape happens quickly at stem tips (Coates Palgrave, 2002).