DYNAMIC PHYTOGEOGRAPHY OF DORYOPTERIS

Abstract

STATIC PHYTOGEOGRAPHY is a study of plant distribution at a given point in time. Dynamic or historical phytogeography takes into consideration the element of time, past as well as present ranges being mapped, compared and analyzed. Several types of evidence are available upon which one may project the present range of a plant backward in time. Fossils, including pollens and spores in peat, furnish the most trustworthy lines of evidence. Reliet colonies and a comparison of groups with similar ranges may also give a clue to past distribution. In the case of the fern genus Doryopteris it is not possible to determine the previous range by these methods. However, this may be done by the use of another method-namely, the study of phylogeny. This paper is an illustration of the use of phylogenetic data in phytogeography. THE PHYLOGENY OF DORYOPTERIS.-A phylogenetic chart of the genus is presented on plate 2. This has been constructed from the evidence of comparative morphology, assembled during a recent taxonomic study of the genus (Tryon, 1942). While the conclusions in regard to the phylogeny on the whole seem to be sound, many details are quite uncertain and since the phytogeographic conclusions are based on phylogenetic ones, and their validity is dependent upon them, I have not relied upon details but only upon the more general phylogenetic relations in the development of the phytogeographic theory. There have been two main lines of evolution in the genus, stemming from a single source. These are represented by the sections Eudoryopteris and Lytoneuron, which comprise exclusively New World species. A third section, comprising species of the Old World and Hawaiian Islands, is a miscellaneous assortment with few clear relations. None of the species in this section are closely related to any of the New World species. Although D. concolor of this third section is pantropical, its relations are with D. decora of the Hawaiian Islands and probably D. Kitchingii of Madagascar. In the following discussion species of the third section are not considered. They are either not monophyletically related to the New World species or else the relation is considerably older than my starting point, which comprises the basic New World species D. triphylla, D. crenulans and D. Lorentzii. 1 Received for publication April 27. 1944. Section Eudoryopteris is more advanced than section Lytoneuron. It has areolate venation and a specialized type of rhizome scale while Lytoneuron has free venation and an unspecialized type of rhizome scale. In both sections there is a general tendency toward broader leaf surfaces and a reduction in the number of segments in the advanced species. Palmate division of the blade occurs in a single highly advanced species in each section. In Lytoneuron a single vascular bundle is considered more advanced than two bundles, a sclerotic border on the blade advanced over a cartilaginous one and a pubescent stipe advanced over a naked or scaly one. In Eudoryopteris partially areolate venation is, of course, primitive; the presence of proliferous buds is an advanced character and a wingangled or sulcate stipe is advanced over a terete or plane one. These are the more important trends in the genus; many minor characters and tendencies have naturally entered into consideration in the construction of the chart. PHYTOGEOGRAPHIC THEORY.-To ascertain the probable point of origin of the genus, ranges of the most primitive species in each section and of the most highly evolved ones are plotted on map 12 (The species plotted are underlined on the phylogenetic chart.) The ranges of the two groups are adjacent in Paraguay but do not overlap. Assuming that the primitive species have not migrated extensively since they evolved, the point of origin of the genus is taken to be within, or very near, their present range. The area can be further localized by other considerations. Since the genus is predominantly one of rocky places above 1,000 feet elevation, the Argentine Pampas and lowlands may be eliminated as a possible point of origin; and since all of the twenty species occur in eastern Brazil and only six in the Andes (there is evidence that three more once grew there, compare maps 9, 10, and 13 with 6), it would also seem justifiable to eliminate the central Andean Region. I would pick the southern end of the Brazilian Highlands, continuously available to plant occupation since the Mesozoic, as the point of origin of the genus. 2 The map of South America is from Goode's Series of Base Maps, Henry M. Leppard, Editor, copyright by the University of Chicago Press. The map of South America, West Indies and southern North America has been kindly furnished by Dr. L. B. Smith.