Morphology and Dispersal of One- and Two-Seeded Diaspores of Cryptantha flava

The wind-dispersed diaspore of the herbaceous perennial Cryptantha flava (Boraginaceae) consists of one, or occasionally two, nutlets (seeds) enclosed in a pubescent calyx. Physical characteristics of oneand two-seeded diaspores and their rates of descent in still air were compared. Natural dispersal distances in the field of calyces containing 0, 1, or 2 seeds were also measured. Two-seeded diaspores are wider at the ovary and have a greater total mass, a greater mass of the ovary, and a higher rate of descent in still air than one-seeded diaspores. Under field conditions, however, there is no difference in how far calyces containing 0, 1, or 2 seeds disperse. Thus rate of descent is not a good predictor of relative dispersal distances in this species. Much of the dispersal in the field must occur as movement along the surface of the ground, and during such secondary dispersal, the greater width of two-seeded diaspores may compensate for the potentially opposing effect of their greater mass. ALTHOUGH ALL FOUR OVULES within flowers of Cryptantha flava (A. Nels.) Payson (Boraginaceae) may be fertilized, usually only one (sometimes two) matures (Casper and Wiens, 1981). The nutlets (hereafter, seeds) of congeneric species that mature all four ovules are individually deciduous, but in C. flava the mature seed(s) and aborted ovules remain attached to the gynobasic style. The large pubescent calyx and the enclosed ovary abscise from the plant as a unit. Wind is the main dispersal agent (Casper, 1987), and Casper and Wiens (1981) suggested that by decreasing the mass of the diaspore, the reduction in seed number results in increased dispersal distance of propagules. A similar pattern of seed reduction occurs in several species of tropical trees in the Leguminosae (Augspurger and Hogan, 1983). Although flowers have more than one ovule, they produce indehiscent, wind-dispersed fruits that usually contain a single seed. Augspurger and Hogan (1983) and Augspurger (1986) have shown that in Lonchocarpus pentaphyllus and Platypodium elegans the few multiseeded fruits disperse shorter distances than single-seeded fruits. In this study, the size, mass, and dispersal characteristics of oneand two-seeded diaspores of C. flava are compared. Their rate of descent in still air (indoors) and the distances I Received for publication 20 October 1986; revision accepted 26 August 1987. We thank J. Haynes and W. Moise for help in dropping seeds down the stairwell and two anonymous reviewers for their comments on an earlier version of the manuscript. The study was supported by NSF Grant BSR 83-07888. they disperse naturally in the field are measured. Natural dispersal distances of calcyes that contain no mature seeds are also determined. STUDY SPECIES-Cryptanthaflava is a perennial herbaceous species that grows in sandy soil throughout the semiarid Colorado Plateau in communities dominated by sagebrush and juniper. Typically, plants are 10 to 40 cm tall and produce from one to more than 40 flowering stalks, each with at least 25 flowers. Inflorescences are made up of several cymules (lateral branches) and flowering progresses sequentially along the cymules beginning with positions proximal to the main stalk. Fruit ripening also occurs sequentially by position, but mature diaspores may be retained on the plant for several days if there is not sufficient wind to cause their abscission. In Uintah Co. in northern Utah where the two sites for this study are located, seed dispersal begins in late June or early July. In one ofthe populations, the mean percentage of twoseeded diaspores per plant was 10.4 (N = 19) and 4.6 (N= 17) in 1979 and 1980, respectively (Casper, 1984). Both sites have extensive areas of bare ground between shrubs; they are described more fully in Casper (1987). METHODS-Features that might affect dispersal potential were measured for 21 oneseeded and 19 two-seeded diaspores collected from the same field-grown plants: the mass of the entire diaspore, its length, the width of the calyx at the widest point (the tip), and the width of the calyx at the ovary (Fig. 1). Because mea-