Gnetophytes

The development of 'vessel elements' allows you to grow longer and longer.  You don't need to have as much wood now, as you can lean on other tall plants and still pump water up.  Slowly, most varieties  lose the wood and become creepers.

In the Cretaceous era, the Gnetophytes were a wide ranging and varied group of plants which must have been worthy of several entries on their own to describe their speciation.  However, since then, they have slowly declined until now there are only a few varieties remaining.  Most are creepers in the arid parts of the Americas.  One is a tree growing in the tropics.  One is a desert plant growing in Central Africa - which only ever grows two leaves, but they are metres long.

Who can guess how they came to be so distinct?

If you can, please let me know...