PHLOX AND COLIAS: THE EFFICIENCY OF A POLLINATION SYSTEM.

Abstract

Pollination in hundreds of self-incompatible plant species is accomplished by insects and other animals which use these species as food sources. Although pollinators are adapted for exploiting these resources, they are not adapted for pollination per se. Pollination is a fortuitous by-product of food gathering, and in no way maximizes the result of this activity. Accordingly, there is no a priori reason to assume that the transfer of pollen from one plant to another will be performed with a high degree of efficiency. Very little is known about the efficiency of pollination systems, although anecdotal information suggests extensive pollen wastage. This investigation was undertaken to shed light on the efficiency of intraand interspecific pollen transfer in Phlox. The species under consideration are P. pilosa L. and P. glaberrima L. They display the floral syndrome of butterfly-pollinated plants (cf. Faegri and van der Pijl, 1966), and are pollinated almost exclusively by lepidopterans (Grant and Grant, 1965; Levin and Kerster, 1968). One pollinator which services both species is Colias eurytheme Boisduval. Two sets of questions were posed in this investigation. The first questions relate to C. eurytheme as studied in the laboratory and are as follows. (1) What is the size of the pollen load on the proboscis of C. eurytheme after it is withdrawn from the corolla tubes of the 2 phloxes? (2) What fraction of the pollen load is lost on the palps as a consequence of proboscis coiling? (3) What fraction of the pollen remaining on the proboscis is deposited on the first conspecific and heterospecific stigmas encountered? (4) How rapidly is the outcross pollen load dissipated as the pollinator moves from one flower to another on a recipient plant? Other questions deal with phenomena in natural populations. What is the size of the outcross pollen loads on stigmas of P. pilosa and P. glaberrima? What is the differential between outcross conspecific and heterospecific pollen loads in a mixed population of the aforementioned species? A discussion of the floral design of the phloxes is a necessary prelude to what follows. The corollas of both species are salverform with relatively narrow and flat rims. They are also colored pink and emit a weak fragrance. The nectaries are located at the base of a narrow corolla tube which averages 14 mm and 19 mm in length in P. pilosa and P. glaberrima, respectively. The species differ in characters of the gynoecium and androecium (Fig. 1). In P. pilosa, the style is approximately 3 mm long, and is buried deep within the corolla tube. The stamens are inserted near the middle of the tube. Pollen grain diameters average ca. 30 tu. In P. glaberrima, the style is nearly as long as the corolla tube, and carries the stigma to the orifice of the tube. Stamen insertion is just below the tube orifice. Pollen grain diameters average ca. 55 tu. Both species produce an average of 15,000 pollen grains per flower. In spite of differences in the position of anthers and stigmas, the floral design of the two species is such that most pollinators of one can, and often do, service the other. Both species are self-incompatible.