澳洲杨(大戟科)胚珠及其种子附属结构的个体发育研究

以澳洲杨的胚珠及其种子为材料,运用光学显微镜和电镜扫描的方法对其从胚珠发生直到种子成熟的个体发育过程和结构进行观察,同时与鸭脚西番莲的种子附属结构的发育过程进行对比研究。结果表明:(1)澳洲杨的珠孔类型为外珠孔类型,种子附属结构起源于珠孔而非珠柄,其为种阜,而非假种皮。(2)鸭脚西番莲的珠孔类型为内珠孔类型,种子附属结构起源于珠柄,并且最终将珠孔包被,其为真正的假种皮结构。通过种阜与假种皮的不同个体发育过程,建立了大戟科种阜与假种皮的不同发育模式,并对种子附属结构的生物学功能及其暗示的不同植物进化路径进行了讨论。     

Quantifying soil organic carbon in complex landscapes: an example of grassland undergoing encroachment of woody plants

The invasion of woody plants into grass‐dominated ecosystems has occurred worldwide during the past century with potentially significant impacts on soil organic carbon (SOC) storage, ecosystem carbon sequestration and global climate warming. To date, most studies of tree and shrub encroachment impacts on SOC have been conducted at small scales and results are equivocal. To quantify the effects of woody plant proliferation on SOC at broad spatial scales and to potentially resolve inconsistencies reported from studies conducted at fine spatial scales, information regarding spatial variability and uncertainty of SOC is essential. We used sequential indicator simulation (SIS) to quantify spatial uncertainty of SOC in a grassland undergoing shrub encroachment in the Southern Great Plains, USA. Results showed that both SOC pool size and its spatial uncertainty increased with the development of woody communities in grasslands. Higher uncertainty of SOC in new shrub‐dominated communities may be the result of their relatively recent development, their more complex above‐ and belowground architecture, stronger within‐community gradients, and a greater degree of faunal disturbance. Simulations of alternative sampling designs demonstrated the effects of spatial uncertainty on the accuracy of SOC estimates and enabled us to evaluate the efficiency of sampling strategies aimed at quantifying landscape‐scale SOC pools. An approach combining stratified random sampling with unequal point densities and transect sampling of landscape elements exhibiting strong internal gradients yielded the best estimates. Complete random sampling was less effective and required much higher sampling densities. Results provide novel insights into spatial uncertainty of SOC and its effects on estimates of carbon sequestration in terrestrial ecosystem and suggest effective protocol for the estimating of soil attributes in landscapes with complex vegetation patterns.     

Flow- and substratum-mediated movement by a stream insect

1. In streams subject to frequent hydrologic disturbance, the ability of benthic invertebrates to disperse within the channel is key to understanding the mechanisms of flow refugium use and population persistence. This study focuses on crawling invertebrates, the effects on movement of abiotic factors (specifically, flow near the stream bed and bed micro‐topography) and the consequences for dispersal. 2. In a large flume, we observed individual cased caddisflies, Potamophylax latipennis, moving in fully turbulent flows over a precise replica of a water‐worked surface. From maps of movement paths, we quantified crawling behaviour and entrainment, and the influence of bed micro‐topography. We manipulated discharge and tested its effect on movement, linear displacement and areal dispersal. The highest discharge treatment was a disturbance to the caddis; the lowest discharge was not. Crawling behaviours were used to parameterise random walk models and estimate population dispersal, and to test the effects of abiotic factors on movement. 3. Bed micro‐topography influenced crawling in several ways. Caddis spent most of their time at the junctions between proud particles and the adjacent plane bed. The frequency distribution of turn angles was bimodal, with modal values approximating the angle required to travel around median‐sized particles. Larvae generally crawled downstream, but crawling direction relative to the flow was skewed by bed micro‐topography and was not directly downstream, unlike drift. 4. Caddis crawled for most of the time and discharge affected almost every aspect of their movement. As discharge increased, caddis crawled less often, more slowly and over shorter distances; they also became entrained more frequently and over greater distances. With increased discharge, caddis spent proportionately less time at the junctions between proud particles and the adjacent plane bed, and more time on the tops and sides of proud clasts. This is curious as most entrainment occurred from the tops and sides of clasts and entrainment is generally considered to be disadvantageous during disturbances. 5. Linear displacement (drift and entrainment combined) was downstream, but the relation between total displacement and discharge was complex. Total displacement decreased at intermediate discharge as crawling decreased, but increased at high discharge as entrainment and drift played a greater role in movement. 6. Within‐stream dispersal via crawling contained elements of both a correlated random walk (we observed directional persistence in turn angles) and a biased random walk (we observed downstream bias in move direction angles) and was best described as a biased correlated random walk. Dispersal was inversely related to discharge, suggesting that the ability of P. latipennis to crawl into flow refugia on the streambed is reduced at high flow.     

Phylogeny of families in the Pectinoidea (Mollusca: Bivalvia): importance of the fossil record

The Triassic fossil record points to the monophyly of the Pectinoidea (scallops), all members of which have a triangular resilium with a nonmineralized medial core that functions below the hinge line. The elastic properties of this resilium in extant taxa predict that the initial adaptation of the Pectinoidea was to swimming. This is indeed corroborated by the shell form of Pernopecten , the earliest known pectinoidean genus, which ranged from late Devonian to earliest Triassic. The new family Entolioididae, a largely Triassic group, provides the missing link between the Pernopectinidae and the families Propeamussiidae, Entoliidae, and Pectinidae, all of which originated by the Middle Triassic and survive to the present day. A new Triassic genus Filamussium shows that the Propeamussiidae originated from the Entolioididae, not directly from the Pernopectinidae as previously supposed. Evidence from morphology, the fossil record, and molecular genetics indicates that the family Spondylidae originated in the Middle Jurassic from an ancestor within the Pectinidae, possibly the genus Spondylopecten , which was already present in the Late Triassic. Journal compilation © 2006 The Linnean Society of London, Zoological Journal of the Linnean Society , 2006, 148 , 313–342. No claim to original US government works     

Production of the chironomid Procladius bellus in an annual drawdown reservoir

SUMMARY. 1. In Laurel Creek Reservoir. Ontario, a small, eutrophic annual drawdown reservoir, some individuals of the chironomid Procladius bellus survive the winter drawdown.
2. With refilling of the reservoir in spring, colonization by P. bellus is continuous and there were three generations a year.
3. Production estimated by the size-frequency method was 117.12 kg (175 mg m⁻²) in 1980 and 70.14 kg (105 mg ^m-2) in 1981. Annual P:B was about 13 in both years.
4. Substantial slation-to-station variations in production occurred and overall production was lower than in shallow habitats.     

Oxygen stress and reduced growth of Lobelia dortmanna in sandy lake sediments subject to organic enrichment

1. Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation in nutrient‐poor lakes in Europe and North America. Because of large permeable root surfaces and continuous air lacunae Lobelia exchanges the majority of O₂ and CO₂ during photosynthesis across the roots. This leads to profound diel pulses of O₂ and CO₂ in sandy sediments with low microbial O₂ consumption rates. The ready radial root loss of O₂ may, however, make Lobelia very susceptible to more reducing sediments. Therefore, we grew Lobelia for 6 months on natural and organically enriched sandy sediments to test how: (i) root oxygenation influenced degradation of organic matter and depth profiles of N and C; (ii) Lobelia and microbial O₂ consumption rates influenced pool size and depth penetration of O₂ in the sediments; and (iii) sediment enrichment influenced growth and mineral nutrition of Lobelia. 2. Naturally low‐organic sediments (0.32% DW) accumulated organic C and N during the experiment as a result of growth of Lobelia and surface micro‐algae. In contrast, surface layers of enriched sediments (0.58, 0.87 and 2.46% DW) lost organic C and N because of enhanced mineralisation rates because of oxygen availability. In deeper layers of enriched sediments no significant differences in organic C and N pools were found between plant‐covered and plant‐free sediments probably because faster organic degradation because of root oxygenation was balanced by release of organic matter from the plants and because short roots with dense Fe‐Mn coatings in the most enriched sediments constrained O₂ release. 3. Depth‐integrated O₂ pools were much higher in light than darkness, higher in plant‐covered than plant‐free sediments and higher in sandy than in organically enriched sediments. All sediments had a primary O₂ maximum 1–2 mm below the sediment surface in light because of photosynthesis of micro‐algae. Plant‐covered sediments of low organic content (0.32 and 0.58% DW) also had a secondary deep maximum (2–4 cm) because of higher O₂ release from Lobelia roots than microbial O₂ consumption. Nitrification occurred here resulting in depletion of NH and accumulation of NO. In low organic sediments, oxygen pools increased with higher plant biomass both in light and darkness. The deep O₂ and NO₃ maxima disappeared in high organic sediments of greater O₂ consumption rates and smaller O₂ release rates. 4. Lobelia was stressed by increasing O₂ consumption rate of the sediments. Plant weight and leaf number declined twofold and maximum root length declined fourfold suggesting severe problems maintaining sufficient axial O₂ transport to the root tips because of rapid radial O₂ loss. Despite markedly higher nutrient concentrations in the enriched sediments, leaf‐N declined twofold and leaf‐P declined fourfold to growth‐limiting levels. These responses can be explained by constrains on mycorrhisal activity, root metabolism and vascular transport because of O₂ depletion. Management efforts to stop the decline and ensure the recovery of the isoetid vegetation should therefore focus on improving water quality as well as sediment suitability for growth.