THE EVOLUTION OF THE GENUS MONARDA (LABIATAE).

Modern biosystematic research is increasingly concerned with evolutionary relationships of plant groups. Phylogenetic relationships are often expressed in diagrams as made for the genus Clarkia by Lewis and Lewis (1955), as schematic representations as used for the species of Hymenopappus by Turner (1956), or in phylogenetic trees such as the "Koenigsberger Stammbaum" by Mez and Ziegenspeck (1926) including in part even serological data. One of the latest methods of presenting evolutionary relationships is by th'e "Divergence Index Method" (Hardin, 1957). The following paper deals with the evolutionary relationships of all the species of the genus Monarda L. as diagrammed by the Divergence Index Method. Using this method, however, we must make two assumptions. The first is that, since we do not have a way of measuring objectively the value of a character, we treat all those which show an evolutionary tendency as of equal value, in Adansonian fashion. After all, which is evolutionally more significant, the lobe of a leaf margin or a fused upper corolla lip? Unless methods are developed to use values in a logically defensive manner, they must be weighed equally. Presumably, if enough characters are used, the reliability of the Divergence Index Method will increase. The second assumption is that a character, if it occurs in most taxa of a given group, is produced by the basic gene pool and therefore unspecialized. The norm is usually unspecialized. If th'e character occurs in only a few taxa, it diverges from the "norm," i.e., it has evolved. But one must be careful not to confuse the remnants (as judged by studies of related genera) of a prior widespread and primitive condition as beinL2 sDecialized. A survey of herbarium and living material of the following genera was conducted: Blephilia Raf., Hyssopus L., Lamium L., Leonotis L., Lepechina Willd., Marrubium L., Mellissa L., Melittis L., Mentha L., Meriandra Benth., Nepeta L., Perilla L., Pogostemon Desf., Salvia L., Sphacele Benth., and Thymus L. Some characters, which were found to form patterns in one subgenus or section of Monarda only, turned out to fluctuate in another, and were therefore not considered as usable data. Of 118 characters tested, 31 were accepted as significant indicators of evolutionary relationships. The most probable specialized or divergent character states were each given a value of one, the primitive ones zero, and the intermediate conditions 0.5. The total for each species shows its estimated divergences which are plotted upon concentric circles (Fig. 1). The letters designate the character states used in the evolutionary evaluation. Letters with subscript mark those with uncertain evolutionary meaning. These are therefore included in the formulae only to distinguish the taxa, not to measure their specialization. Full circles denote taxa existing today, open circles probable ancestors. The evidence used in the evaluation of evolutionary tendencies is enumerated as follows: A. Number of glomerules.-Several verticillastrate glomerules (i.e., several "layers" of verticels) appear in both sections of the subgenus Cheilyctis and are sometimes found as teratologies in the following taxa of subgenus Monarda: M. fistulosa and all of its varieties except var. brevis, as well as in M. lindheimeri, M. media, M. clinopodia, and M. didyma. Solitary glomerules are actually not truly terminal, they are derived from the verticillastrate condition.