The effects of temperature on the web-building behaviour of the common house spider, Achaearanea tepidariorum

1. Because spiders are ectothermic animals, the temperature regime of the microhabitat in which an individual finds itself may affect important performance traits of that individual. The present study examined the effects of temperature on attributes of webs spun by Achaearanea tepidariorum (C. L. Koch), as well as testing temperature preference in this species. The effects of temperature on the amount of silk per web produced by Achaearanea tepidariorum and the prey-capture efficiency of webs produced at different temperatures were determined by using webs constructed at 5, 10, 15, 20, 25 and 30°C. The temperature preferences of A. tepidariorum within a thermal gradient were also determined.
2. Web mass was related to temperature, exhibiting a quadratic relation with a maximum web mass occurring at approximately 20°C.
3. Number of strands per cm³ of webs varied directly with web mass; webs with greater strand densities were more efficient at capturing flies.
4. The number of spiders observed in each temperature range in the thermal gradient indicated a non-uniform distribution, with the spiders avoiding temperatures in the highest range (27·3±2·0°C).
5. These data suggest an optimal temperature for web construction at which webs produced are more efficient at capturing prey. The data also suggest that this species may avoid sites that do not provide an adequate thermal environment.     

Effects of the Environment on the Symptom Pattern of Nickel Toxicity in the Oat Plant

Under conditions of nickel toxicity in oats (Avena byzantina),a predisposing condition for the development of chlorosis isinduced in areas of the leaf before emergence from the coleoptileor the enclosing leaf sheath. These areas give rise to chloroticbands which develop on the emerged leaf a little over 24 h afteremergence of the tissue. Alternating light and dark are essentialfor the development of chlorotic bands. The evidence indicatesthat the potentially green tissue is that which is developingunder the coleoptile during the day and emerges later in theday or early in the night, while the potentially chlorotic tissueis that which develops under the coleoptile during the nightand emerges from the top of the coleoptile later in the nightor during the following daylight hours before midday. Plants containing high levels of nickel contain higher levelsof protochlorophyll and lower levels of of chlorophyll thancontrol plants. The visual symptom of nickel toxicity is influenced by the lengthof the light and dark periods. The nature of these effects isdiscussed. Avena byzantina, oat, nickel toxicity symptoms, chlorosis     

Spermatozoa and spermiogenesis in XyIopIax (Class Concentricycloidea): a new type of spermatozoon in the Echinodermata

In Xylopax turnerae Rowe et al ., 1988 (Class Concentricycloidea, Subphylum Asterozoa) from the Caribbean spermiogenesis culminates in the formation of a complex spermatozoon unlike that of any other echinoderm. Mature testicular spermatozoa are composed of: (1) an elongate, tapered acrosome, segmented internally; (2) a nucleus, finely tapered anteriorly, extremely elongate and rod-shaped posteriorly; (3) a single flagellum attached via a centriolar rootlet to the anterior portion of the nucleus; (4) a single elongate mitochondrion located posterior to the nucleus. Spermatozoa deposited in the ovary have acrosomes which are distinctly granular internally: otherwise these cells resemble testicular spermatozoa. Spermiogenesis differs substantially from the pattern observed in other echinoderms, especially in relation to acrosome development. Examination of alcohol-fixed testes from the New Zealand species Xyloplax medusiforrnis confirmed the presence of filiform spermatozoa, but no details could be established. In contrast to Xyloplax , spermatozoa of Caymanostella sp. (Asteroidea) from a similar habitat to Xyloplax are not modified from those of externally fertilizing Asteroidea, Ophiuroidea, Crinoidea and Holothuroidea. Results lend further support to the existence of a separate class of extant echinoderms (Concentricycloidea) for Xyloplax spp.     

Maintenance of sympatric floral tube‐length variation in a Cape irid

Transition zones between morphologically different individuals of the same species provide insights into the evolution and maintenance of gene‐flow barriers. Here we investigate Tritoniopsis revoluta, which has geographically variable tube lengths, thought to be adaptations to insects with different proboscis lengths. We found a narrow transition zone between plants differing by three‐fold in perianth tube length. We determined whether strong gene‐flow barriers result from assortative mating arising from different pollinators, a high prevalence of selfing, or post‐pollination incompatibilities between plants with different tube lengths. We found that there was little evidence to support assortative mating through different pollinators. Both short‐ and long‐tubed plants were mainly visited by bees with short proboscides. Selfing is unlikely to contribute significantly to seed set, plants with different tube lengths were interfertile and hybrid plants were fertile. We conclude that the contact zone is unstable because these ecotypes have not accrued enough allopatric differences to translate into strong gene‐flow barriers, or, alternatively, bimodality is not a consequence of secondary contact but the result of a novel mutation for short tubes spreading through a long‐tubed population. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 129–137.     

Root responses along a subambient to elevated CO2 gradient in a C3–C4 grassland

Atmospheric CO₂ (C_a) concentration has increased significantly during the last 20 000 years, and is projected to double this century. Despite the importance of belowground processes in the global carbon cycle, community‐level and single species root responses to rising C_a are not well understood. We measured net community root biomass over 3 years using ingrowth cores in a natural C₃–C₄ grassland exposed to a gradient of C_a from preglacial to future levels (230–550 μmol mol^?1). Root windows and minirhizotron tubes were installed below naturally occurring stands of the C₄ perennial grass Bothriochloa ischaemum and its roots were measured for respiration, carbohydrate concentration, specific root length (SRL), production, and lifespan over 2 years. Community root biomass increased significantly (P<0.05) with C_a over initial conditions, with linear or curvilinear responses depending on sample date. In contrast, B. ischaemum produced significantly more roots at subambient than elevated C_a in minirhizotrons. The lifespan of roots with five or more neighboring roots in minirhizotron windows decreased significantly at high C_a, suggesting that after dense root growth depletes soil resource patches, plants with carbon surpluses readily shed these roots. Root respiration in B. ischaemum showed a curvilinear response to C_a under moist conditions in June 2000, with the lowest rates at C_a<300 μmol mol^?1 and peak activity at 450 μmol mol^?1 in a quadratic model. B. ischaemum roots at subambient C_a had higher SRLs and slightly higher carbohydrate concentrations than those at higher C_a, which may be related to drier soils at low C_a. Our data emphasize that belowground responses of plant communities to C_a can be quite different from those of the individual species, and suggest that complex interactions between and among roots and their immediate soil environment influence the responses of root physiology and lifespan to changing C_a.     

Ecophysiological studies on Sonoran Desert plants,VI. Seasonal photosynthesis and production of Machaeranthera gracilis,a winter ephemeral*

Abstract. A comparative study of two chromosomal races of the winter-active ephemeral Machaeranthera gracilis showed that the seasonal magnitudes of photosynthesis were only slightly greater for a progeny desert race than for an ancestral foothills race. Maximum observed photosynthetic capacity and the seasonal reduction in foliar photosynthesis occurred earlier in the year for the desert race. The relative growth rate was higher in this race up until the time of its peak seasonal biomass. A ratio of harvested net production to estimated gross primary production decreased until anthesis. Photosynthesis contributing to net growth continued into periods with moderate environmental stress. The continuation of growth by the desert race was enhanced by maintenance of a higher root-shoot ratio, as well as greater relative stem growth. During reproduction, foliar CO₂ assimilation could not solely provide the measured dry matter accumulation, suggesting the importance of assimilate contribution by photosynthetic stems. Seasonal increases in the enthalpy content of whole plants and plant organs occurred for both races, indicating the absence of significant translocation during reproduction and the potential for stem photosynthesis.     

Characterization of Exudation from Excised Roots of Onion, Allium cepa: II. CHEMICAL COMPOSITION OF EXUDATE

The exudate of excised onion roots bathed in simple KCl/CaCl₂solutions contains a variety of inorganic cations and anionstogether with quantities of organic molecules. This complexityof composition contrasts with the simplicity of the exudatesof maize roots under identical conditions and presumably reflectstransfer of these materials from the onion bulb to the rootsduring growth.