MORPHOLOGY, EVOLUTION, AND TAXONOMIC SIGNIFICANCE OF THE INFLORESCENCE IN CORDYLANTHUS (SCROPHULARIACEAE)

The genus Cordylanthus has considerable diversity in its inflorescences while the other genera of tribe Rhinantheae (Scrophulariaceae) uniformly have racemes or spikes. Four distinct inflorescence types are recognized and their homologies and evolutionary history are postulated. Thus it is suggested that the basic florescence type, the elongated spike (Type I), has undergone evolutionary reduction to a few-flowered spike and ultimately to a single-flowered florescence (Type II). Further evolution involving processes of compaction and clustering of the single-flowered florescences has resulted in glomerulate clusters (Type III) and spiciform clusters (Type IV). Knowledge of inflorescence homologies and distribution of the four inflorescence types in the genus has been of considerable aid in formulating a new infrageneric classification. Using evidence primarily from inflorescence, floral, and seed morphology, as well as from geographical distribution and ecology, a classification is proposed establishing three subgenera, namely subg. Dicranostegia, subg. Hemistegia, and subg. Cordylanthus, the last with three sections, sect. Cordylanthus, sect. Anisocheila, and sect. Ramosi. CORDYLANTHUS, a small genus of the hemi-parasitic tribe Rhinantheae (Scrophulariaceae), is unusual in having several distinct inflorescence types ranging from a spike to cymose clusters of many to few flowers, and even to unifloral shoots. We know of no other genus of Scrophulariaceae that exhibits such diversity-of inflorescence types. Indeed, Cordylanthus is unique in tribe Rhinantheae, a large group of some 25 genera, in being the only one in which diversification of the inflorescence has proceeded beyond a spike or a raceme. As Weberling (1965) emphasized, it is necessary in systematics to use homologous structures in comparative studies, and the traditional descriptive terminology of inflorescences does not allow legitimate comparisons. This study therefore attempts to determine the homology of inflorescence structures in Cordylanthus, to trace their possible evolutionary development, and to utilize the comparative data in formulating a more useful and meaningful infrageneric classification. Members of the genus Cordylanthus are annual species restricted to western North America. Pennell (1947, 1951), author of the last taxonomic revision of the group, recognized about 35 species, but we would suggest that 18 is a more realistic number. The plants are root hemi-parasites, oc'Received for publication 10 January 1975. This study was supported in part by funds from National Science Foundation grants GB-12434 and GB35202, and the Illinois State University. We are grateful to Professor Focko Weberling for helpful discussions, to Professors Lincoln Constance and Donald Kaplan for critical reading of the manuscript, and to Ms. Charlotte Mentges for the illustrations. curring mostly on various trees and shrubs but also on herbaceous perennials (Chuang and Heckard, 1971), and are remarkable for their ability to survive hot summer months in arid habitats. Less well known than the related genera Castilleia and Orthocarpus of subtribe Castilleiinae, the genus stands somewhat apart from them in possessing a unique spathe-like calyx as well as a distinctive corolla. Earlier workers (Ferris, 1918; Morton, 1932; Pennell, 1947, 1951) in this genus did not explore diversity of inflorescences as to either morphological interpretation or possible evolutionary significance. Pennell (1951), in his identification keys to the species, described the inflorescence as consisting of either a spike or "head-like clusters of one to many flowers." We found the types of inflorescences in Cordylanthus considerably more complex than this, offering a striking example within a single genus of evolution of a morphological feature that is evolutionarily more stable in related groups. MATERIALS AND METHODS-Inflorescence materials used for this study were obtained from field-collected specimens taken during the summers of 1969-1972 and from plants raised from seed in the greenhouse at the University of California, Berkeley. Studies were made on living plants, on materials fixed in 70 % alcohol, and on herbarium specimens. Our specimens are deposited in the Jepson Herbarium, University of California (JEPS) and Illinois State University Herbarium, Normal (ISU). Otther herbarium material examined is being cited as representative