Page:Popular Science Monthly Volume 80.djvu/207

Rh primitive stock the other tribes separated later, and the pine tribe itself became more modern.

The history of Gymnosperms is not complete without mention of its seventh great group, the Gnetales. They comprise three genera, extremely unlike in habit, habitat and geographical distribution, but held together by certain important characters. They are so different from other Gymnosperms that no phylogenetic connection is clear, and there is no sure record of them as fossils. This seems to indicate that they are very modern, but their dissimilarity and their wide separation from one another geographically make it certain that if they are genetically related they must have a history that remains to be discovered. Such suggestions of connection as can be obtained indicate as a possibility that Gnetales are derivatives from the prolific Coniferales stock.