Natural selection may have chosen vascular systems because the rigidity they provide allows plants to grow more upright and photosynthesize better. All vascular plants have true stems and most have true roots and leaves. Common ancestral groups include zosterophyllophytes, rhyniophytes, and trimerophytes [Speer 1995]. Vascular plants have a conspicuous sporophyte generation. A possible reason for this evolution is that radiation from the sun constantly hits larger plants. In hitting a diploid, one copy of the chromosome may be salvaged. Seedless tracheophytes appear first in plant phylogeny.

Next:
   Zosterophyllophytes:
       Lycophyta
   Rhyniophytes:
       Psilophyta
       Trimerophytes:
           Pterophyta
           Sphenophyta
           Progymnosperms
Back: Charophytes
Geologic Timeline