The progymnosperms are an extinct group of woody, spore-bearing plants that is presumed to have evolved from the "trimerophytes", and eventually gave rise to the gymnosperms. They have been treated formally at the rank of division Progymnospermophyta or class Progymnospermopsida (as opposite). The stratigraphically oldest known examples belong to the Middle Devonian order the Aneurophytales, with forms such as Protopteridium, in which the vegetative organs consisted of relatively loose clusters of axes. Tetraxylopteris

Tetraxylopteris

Tetraxylopteris is a genus of extinct vascular plants of the Middle to Upper Devonian . Fossils were first found in New York State, USA. A second species was later found in Venezuela.-Description:...

is another example of a genus lacking leaves. In more advanced aneurophytaleans such as Aneurophyton these vegetative organs started to look rather more like fronds, and eventually during Late Devonian times the anuerophytaleans are presumed to have given rise to the pteridosperm order, the Lyginopteridales

Lyginopteridales

The Lyginopteridales were the archetypal pteridosperms: They were the first plant fossils to be described as pteridosperms and, thus, the group on which the concept of pteridosperms was first developed; they are the stratigraphically oldest-known pteridosperms, occurring first in late Devonian...

. In Late Devonian times, another group of progymnosperms gave rise to the first really large trees known as Archaeopteris

Archaeopteris

Archaeopteris is an extinct genus of tree-like plants with fern-like leaves. A useful index fossil, this tree is found in strata dating from the Upper Devonian to Lower Carboniferous , and has a global distribution....

.

Other characteristics:

• Vascular cambium
  Vascular cambium
  The vascular cambium is a part of the morphology of plants. It consists of cells that are partly specialized, for the tissues that transport water solutions, but have not reached any of the final forms that occur in their branch of the specialization graph...
  
   with unlimited growth potential is present as well as xylem
  Xylem
  Xylem is one of the two types of transport tissue in vascular plants. . The word xylem is derived from the Classical Greek word ξυλον , meaning "wood"; the best-known xylem tissue is wood, though it is found throughout the plant...
  
   and phloem
  Phloem
  In vascular plants, phloem is the living tissue that carries organic nutrients , in particular, glucose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word "bark"...
  
  .
• Ancestors of the earliest seed plants as well as the first true trees.

• Strong monopodial growth is exhibited.

• Some were heterosporous but others were homosporous.

External links

• Progymnospermophyta
• http://books.google.com/books?id=0DfYJsVRmUcC&pg=PA723&lpg=PA723&dq=Progymnospermophyta&source=bl&ots=8gLDGKNZ3U&sig=K86Hb_ITkTlATOtMQpJrByaMx3w&hl=en&sa=X&oi=book_result&resnum=7&ct=result