Geographic Distribution of the Species of Equisetum in Relation to Their Phylogeny

The genus Equisetum is the surviving remnant of a once extensive phylum of plants. Representatives of the Calamophyta, to which Equisetum belongs, were important and characteristic plants of the Carboniferous period and fossils have also been found in the Devonian. Besides what were undoubtedly true Equisetales, directly related to our modern plants, the Sphenophyllales and Calamariales flourished in the Carboniferous and extended somewhat beyond this period. The Sphenophyllales were apparently a more primitive group than the Equisetales, while the Calamariales represented a more advanced group which had not only attained the heterosporous condition but also a very efficient physiological reaction system, giving rise to trees over a foot in diameter and often over 60 ft. high. In thinking of the most primitive vascular forms we must think of small, slow-growing sporophytes in which there was no sex expression and in which the functional system was still very inefficient as compared with what evolved later in the great trees of all the higher phyla.