Fine Structure of Sieve Tubes Prepared Mainly for Observation in the Electron Microscope by a Cryogenic Method

Using a cryogenic method of fixation, electron microscopic evidencewas obtained of structures in the sieve element lumina and sieveplate pores of Cucurbita pepo which can be interpreted as constituentsof the transcellular strands repeatedly seen in the light microscope.In longitudinal sections these appeared as more or less hollowtubes with clearly defined filamentous walls which showed someorganized substructure. Transverse and oblique sections revealedcircular or elliptical profiles of boundaries surrounding largelyempty central areas. Owing to the still inadequate preservationof specimens, the centres of the strands contained only sparsecytoplasmic material with an occasional indication of fibrilsin parallel arrangement. Improvement of fixation techniques,with rapid freezing as the preferential method, is seen as themain task in any effort to preserve the in vivo state of thesestructures.     

Culturally Situated Design Tools: Ethnocomputing from Field Site to Classroom

Ethnomathematics is the study of mathematical ideas and practices situated in their cultural context. Culturally Situated Design Tools (CSDTs) are web-based software applications that allow students to create simulations of cultural arts—Native American beadwork, African American cornrow hairstyles, urban graffiti, and so forth—using these underlying mathematical principles. This article is a review of the anthropological issues raised in the CSDT project: negotiating the representations of cultural knowledge during the design process with community members, negotiating pedagogical features with math teachers and their students, and reflecting on the software development itself as a cultural construction. The move from ethnomathematics to ethnocomputing results in an expressive computational medium that affords new opportunities to explore the relationships between youth identity and culture, the cultural construction of mathematics and computing, and the formation of cultural and technological hybridity.     

The importance of bacterivorous protists in the decomposition of stream leaf litter

1. Allochthonous organic matter, in the form of senesced leaves, is a major source of carbon supporting detrital food webs. While studies have documented the role of bacteria and fungi in the decomposition of leaf litter, little information is available regarding the role of protists in the decomposition process. 2. We tested the hypothesis that the presence of stream‐dwelling bacterivorous protists leads to an increased rate of leaf decomposition through grazing pressure on bacteria. We isolated live protists from decomposing leaves collected in a stream in Northern Virginia, U.S.A. (Goose Creek) and established laboratory cultures of common bacterivorous protists. 3. Recently senesced leaves from the field were used in laboratory microcosm experiments to determine if the rate of litter decomposition differed between four treatments: bacteria only, bacteria + flagellates, bacteria + flagellates + ciliates, autoclaved stream water (control). We determined the dry weight of leaf remaining, bacterial abundance, flagellate abundance and ciliate abundance for each replicate on days 0, 7, 14, 30, 60 and 120. 4. The rate of leaf decomposition was significantly higher in treatments with protists than without and bacterial abundance declined in protist treatments compared with bacteria only treatment. Weight loss in the presence of flagellates was three to four times higher when protists were present compared with treatments with bacteria alone. These results provide experimental evidence that protists could play a significant role in the detrital processes of streams.     

Macroinvertebrate community structure,secondary production and trophic‐level dynamics in urban streams affected by non‐point‐source pollution

1. Despite non‐point‐source (NPS) pollution being perhaps the most ubiquitous stressor affecting urban streams, there is a lack of research assessing how urban NPS pollution affects stream ecosystems. We used a natural experimental design approach to assess how stream macroinvertebrate community structure, secondary production and trophic structure are influenced by urban NPS pollution in six streams. 2. Differences in macroinvertebrate community structure and secondary production among sites were highly correlated with stream‐water specific conductivity and dissolved inorganic phosphorus (DIP) concentrations. Macroinvertebrate richness, the Shannon diversity index and the Shannon evenness index were all negatively correlated with specific conductivity. These patterns were driven by differences in the richness and production of EPT and other intolerant taxa. Production of the five most productive taxa, tolerant taxa, non‐insect taxa and primary consumers were all positively correlated with stream‐water DIP. 3. Despite the positive correlation between primary consumer production and DIP, there was no correlation between macroinvertebrate predator production and either total or primary consumer macroinvertebrate production. This was observed because DIP was positively correlated with the production of non‐insect macroinvertebrate taxa assumed to be relatively unavailable for macroinvertebrate predator consumption. After removing production of these taxa, we observed a strong positive correlation between macroinvertebrate predator production and production of available prey. 4. Our results suggest that urban NPS pollution not only affects macroinvertebrate community structure, but also alters secondary production and trophic‐level dynamics. Differences in taxon production in our study indicate the potential for altered energy flow through stream food webs and potential effects on subsidies of aquatic insect prey to riparian food webs.