The Reproductive Biology of Agavaceae: I. Pollen and Nectar Production in Four Arizona Agaves

Abstract

The rates and timing of nectar and pollen production in four species of agaves were studied. Two species ( Agave palmeri and A. schottii) were found to be primarily nocturnal producers and additionally were shown to secrete relatively unconcentrated nectars. The remaining species (A. toumeyana and A. parviflora) produce nectar and pollen primarily during the daytime. The adaptive significance of these and related variations is discussed. Despite the fact that agaves are widely planted as ornamentals, and have recently been the subject of taxonomic inquiry (Pompa 1963; Gentry 1972), very little is known about their reproductive biology under natural conditions. The present paper is concerned with the timing of nectar and pollen production in four Arizona species. Such knowledge is valuable since it helps identify the pollen vectors to which these plants became adapted prior to the onset of human interference in the ecology of the deserts of North America. The subject is particularly interesting because several authors (van der Pijl 1936; Porsch 1936; Howell 1972) have suggested that some agaves are pollinated by nectar-feeding bats. In this regard, Howell (1972) has hypothesized that the paniculate agaves (subgenus Agave) show a greater degree of specialization in the direction of chiropterophily than do species belonging to the subgenus Littaea (spicate inflorescences). The results of the present study tend to support this conjecture. However, before the general validity of this statement can be properly assessed, much additional evidence is needed, particularly from the Mexican forms which comprise the overwhelming bulk of the genus. MATERIALS AND METHODS. Populations of four species (Table 1) were studied. Three of these, A. schottii, A. parviflora, and A. toumeyana, possess spikelike inflorescences. The fourth, A. palmeri, has a paniculate flower stalk. Entire