Imagine a world without trees, and then try to think about the changes that would need to happen for these trees to evolve from the small primitive plants that came before them.
When paleobotanist think about this possibility, it usually results in a really weird looking fossil (paleontologists spend a lot of time thinking about fossils…). It has a tapering truck, at least up to eight meters high, with distinctive short branches attached around the top to form a crown. From a distance, the trees would have looked like palms, with bases up to a meter in diameter. There were no leaves as such, just branched twig-like appendages which presumably had a photosynthetic function in the carbon dioxide rich atmosphere.
Most trees today have a solid trunk, which gets bigger through the formation of a new ring of woody tissues – made up of xylem cells – under the bark each year.
However, this primitive older tree, cladoxylopsids, the xylem grew in a ring of individual parallel strands around the outside of the trunk. Inside this zone, more xylem strands formed a complex network with many interconnections both to each other and to the outer parallel strands. The majority of the inside of the trunk was completely hollow.
But if they are hollow, then how did they grow so big? Read more here!