The structure and cytochemistry of the pistil of Hypericum calycinum: the style

Summary A typical style of Hypericum calycinum is solid with a core of transmitting tissue traversing the whole length of the style. This transmitting tissue consists of loosely arranged cells and large intercellular spaces filled with a secretion product. The secretion product is rich in lipids, but poor in proteins and polysaccharides. The intercellular spaces of the transmitting tissue originate partly by a separation of cells as a result of the decomposition of the middle lamella and partly by degeneration of some of the cells of the transmitting tissue. H. calycinum is self-compatible. Both self- and cross-pollinations result in profuse pollen germination on the stigma and pollen tube growth through the style. The data on Hypericum is discussed in relation to information available on other solidstyled systems.     

Scales and costs of habitat selection in heterogeneous landscapes

Summary Two scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.     

Endoparasitic flies,pollen-collection by bumblebees and a potential host-parasite conflict

Summary Conopid flies (Conopidae, Diptera) are common larval parasites of bumblebees. The larva develops inside the abdomen of workers, queens and males. Development is completed within 10–12 days after oviposition when the host is killed and the parasite pupates in situ. Development results in parasitised bees becoming unable to carry large loads of nectar, as the conopid larvae reside where the honey crop is normally located. Furthermore, an addition to the bee's unloaden body mass is likely (average larval weight reached at pupation by the common parasite species Sicus ferrugineus: ±SD 36.3±12.3 mg, n=59; by Physocephala rufipes: 55.8±16.9 mg, n=108). We here asked whether the propensity of workers of the bumblebee Bombus pascuorum to collect nectar rather than pollen is related to the presence of conopid larvae. For samples of bees (n=2254 workers) collected over 3 years of field studies in northwestern Switzerland, there was no difference in the frequency of bees caught as pollen collectors among parasitised (38.1% of cases, n=210) as compared to non-parastised bees (43.9%, n=360) ( ²=1.83, n.s.). However, compared to the non-parasitised bees (n=360), those hosts containing a third (last) instar larva (n=9) were less likely to collect pollen than expected by chance ²=6.91, P=0.003. Similarly, hosts with short survival time between capture and being killed by the developing larva (which hence must have harboured a late instar parasite at time of capture) were less likely to collect pollen (8%, n=25) than those found not parasitised (37.6%, n=891 ²=9.16, P<0.001). Late instar larvae grow so big that they fill the entire abdomen. Although there was also a tendency for presumably older bees to collect less pollen, this is unlikely to explain the observations. We also discuss whether these changes in foraging behaviour of bumblebees may reflect a host-parasite conflict over the type of resource to be collected.     

Effect of electrical fields and external ionic currents on pollen-tube orientation

Summary Agapanthus umbelatus pollen tubes (PTs) display a number of different growth patterns when germinated in an electric field of 750 mV· mm^–1. When pollen is germinated near the cathode (82.44% of orientation to the cathode side) or near the anode (55.35% of orientation to the anode), growth is oriented parallel to the applied field but when germinated at an intermediate position, there is random growth. An increase and decrease in the orientation rates as well as reversion of the polarized growth were observed when the growth conditions were systematically altered. These findings reflect the influence of different ionic currents present in the germination medium. These ionic currents induce the formation of ionic gradients, which were monitored by ion-HPLC. The individual omission of Ca²⁺, K⁺, Mg²⁺ and Cl^– suppresses or alters the oriented growth pattern. The presence of ionic gradients is not by itself suficient to trigger the polarization of tube growth as the presence of an electric field which drives the ionic currents is essential for this to occur.Abbreviations PT Pollen tube - DNS 3,5-dinitro salycilic acid; - TP transient polarization - HPLC high precision liquid chroma tography - DC direct current     

Anatomical observations on the nucellar apex of Wellwitschia mirabilis and the chemical composition of the micropylar drop

Summary In the young ovule of Welwitschia mirabilis the nucellar apex is dome shaped and starch begins to accumulate near the female gametophyte. With the degeneration of the cells of the nucellar apex, a pollen chamber is formed, which contains the micropylar fluid. Starch storage increases considerably in the upper part of the nucellus. Pollen drop emission is not a rhythmic process, and pollination does not produce the rapid withdrawal of droplets. The micropylar drop consists almost entirely of sugars, uronic acids and a very small amount of free amino acids and enzymes. The mechanism of micropylar drop secretion and its probable role in the process of pollination is discussed.This work was supported by a grant from MURST 40%     

A unique anther-mucilage in the pollination biology of Tylosema esculentum

Summary Tylosema esculentum, a perennial geophyte bearing yellow distylous flowers in racemes, maintains a high degree of outbreeding through reciprocal herkogamy. In addition, a viscous liquid, the anther-mucilage, is produced by the anther connective tissue and released concurrently with the pollen. The polysaccharide- and lipid-rich mucilage, which is functional in the shedding and transfer of pollen, is available for more than 1 day due to the gradual solidification of the mucilage. The assimilation of the pollen with the liquid substance significantly affects the pollination biology of T. esculentum. This is the first report on the unique phenomenon of wet pollen in the Caesalpiniaceae.     

Characteristics of dispersal in sexually mature dragonflies

Abstract.

• 1 The population dynamics of a natural population of Sympetrum danae (Sulzer) were studied, by means of a mark—release—recapture method, to find indications of dispersal by mature adults. Additional information on orientation and the influence of phenotype (including age), density and weather was obtained by observations under natural conditions and in a large outdoor field cage.
• 2 80% of all mature adults caught around the natural pond were estimated to be immigrants. Considerable immigration was confirmed by the abdomen length of unmarked matures which did not accord with that expected from the locally emerged population. Total numbers captured and the minimum number known to be alive fell sharply at the time when most individuals were mature. At the same time, new matures continued to enter and disappear from the study area at an increasing rate.
• 3 Dispersive behaviour (escape flights) in the field enclosure was primarily age dependent in both sexes: it occurred from an age of 25–30 days onwards. This coincides with the time-lag between the emergence peak and increased dispersal in the field. Secondary effects on escape flights suggested that males tended to abandon low density areas.
• 4 Males flew relatively more southward than females.