Mesarchy

There is no doubt, however, that if endarchy is primitive in the protostelic and immediately derived types, it very rapidly gives place to mesarchy and exarchy in response to various demands, and it is not difficult to show how this may have occurred.

Fossil evidence supports neither the view that exarchy is more primitive than mesarchy, nor the view that mesarchy is more primitive than exarchy.

Exarchy, endarchy, and mesarchy are all found within comparatively small groups, and the actual course of evolution seems to affect the position of the protoxylems much more freely and rapidly than in the other great groups of vascular plants.