Irene Baker

On the calculation of sugar concentration in flower nectar

SummaryThere are several sources of potential error in calculating the concentration or energy value of floral nectar. Errors resulting from confusing data become substantial with increasing concentration. The different methods of expressing sugar concentration are here clarified, and the correct methods of converting from one to the other are provided. Refractometers in use in field studies usually read on a weight per total weight basis; this is recommended as the mode of statement. The perils of oversimplifying conversions from this mode, as is often done, are pointed out.

Ecological and evolutionary implications of digestive processes: Bird preferences and the sugar constituents of floral nectar and fruit pulp

Plants pollinated and dispersed by different groups of birds offer different kinds of sugars in nectar and fruit pulp. The preferences and physiological traits of avian pollinators and seed dispersers are broadly correlated with the sugar composition of the nectar and fruit that they feed on and appear to have influenced the evolution of the sugar composition of the rewards that plants offer. Hummingbirds prefer sucrose whereas many nectar- and fruit-eating passerines prefer glucose and fructose. Preference for hexoses in passerines seems to be associated with poor sucrose assimilation resulting from two physiological mechanisms: lack of intestinal sucrase activity and fast passage rates. Sucrase activity absence appears to be restricted to a single phylogenetic group (the sturnid-muscicapid lineage). Fast passage rates seem to be characteristic of many small frugivores and to hinder the assimilation of complex nutrients that require hydrolysis before absorption. Hummingbirds have extremely specialized digestive traits that allow them to assimilate sucrose at high rates and with extremely high efficiency. These specialized digestive traits appear not to be present in many nectar-feeding passerines.

The predictive value of nectar chemistry to the recognition of pollinator types.

ABSTRACT The analysis of “nectar sugar ratios” shows a close relationship between the sucrose/(glucose + fructose) ratio and the type of pollinator that visits the flowers. When taken in conjunction with the morphology of the flowers and inflorescence, these ratios can be good predictors of the pollinators. Predictions are made for Erythrina crista-galli (Fabaceae), where its hexose-dominated nectar suggests passerine-bird pollination in contrast to species which are hummingbird-pollinated. Similarly, the genus Puya (Bromeliaceae) contains species with abundant sucrose in the nectar (hummingbird-pollinated species) and species with abundant hexose belonging to the massive monocarpie subgenus Puya. In P. raimondii, the hexose-dominated nectar and the inflorescence morphology suggest that it will be passerine-bird pollinated in the high Andes where it is native. Luehea speciosa (Tiliaceae), in the American tropics, has hexose-rich nectar and we predicted that it would be bat-pollinated. No bats were seen to...

The Maintenance of the Gynodioecious Breeding System in Plantago lanceolata L.

Plantago lanceolata is a weedy species in California, known for its gyno- dioecious breeding system. Because male-sterile plants do not contribute genes via pollen, some mechanism must compensate for their evolutionary disadvantage compared to hermaphrodites. The proximate and ultimate factors responsible for the evolution and maintenance of the gynodioecious breeding system in this species in Northern California were investigated. High frequencies of male-steriles (i.e., females) were found most commonly in mesic coastal sites. Studies of the relative fecundities of females and her- maphrodites revealed that females produce 1.77 times the number of seeds produced by hermaphrodites over the flowering season. The reproductive superiority of females is not, however, manifest in lst-year plants. Thus, it is hypothesized that high frequencies of females are found in mesic sites where plants can exhibit the perennial habit and females can manifest their reproductive advantage. Electrophoretic comparisons revealed that females and hermaphrodites are genetically very similar and remarkably uniform. The relationship of this genetic uniformity to the role of gynodioecy as an outcrossing mechanism is discussed.

Avian pollination and nectar use in Combretum fruticosum (Loefl.)

In a tropical dry forest of southwestern Mexico, the flowers of the liana Combretum fruticosum (Loefl.) were visited by a diverse assemblage of birds (31 spp.), dominated by passerines but including hummingbirds. Nearctic migrants constituted a high percentage of the individuals (42%i) and species (40%) visiting the plant. The flowers produced abundant, hexose-dominated dilute nectar, and varied among plants in both the total amount of nectar produced and in the temporal pattern of secretion. The morphology of inflorescences and flowers, nectar chemistry and the species of birds identified as visitors of C. fruticosum indicate that this plant is pollinated by passerine birds rather than hummingbirds.

Starch in angiosperm pollen grains and its anthecological significance: Extended Abstract

Pollen grains of almost 1000 species were examined and measured to test four predictions: (1) “Primitive” angiosperms will have pollen that is starch-containing at the time of anther-dehiscence; more advanced families will have starchless pollen. (2) Where Hymenoptera and Diptera use pollen nutritionally, there will be selection of starchless (oil-rich) pollen, particularly where pollen is the only reward for visitors. (3) Conversely, in autogamous species, anemophilous species, and those pollinated by Lepidoptera or birds who make no nutritional use of pollen, the energetically more economical accumulation of starch will be seen. (4) Small pollen grains will be more likely to be starchless (oil-rich); starch-containing grains will tend to be larger.