全文获取类型
收费全文 | 1579篇 |
免费 | 22篇 |
国内免费 | 52篇 |
出版年
2023年 | 2篇 |
2022年 | 12篇 |
2021年 | 16篇 |
2020年 | 83篇 |
2019年 | 22篇 |
2018年 | 20篇 |
2017年 | 50篇 |
2016年 | 52篇 |
2015年 | 35篇 |
2014年 | 68篇 |
2013年 | 96篇 |
2012年 | 73篇 |
2011年 | 95篇 |
2010年 | 69篇 |
2009年 | 125篇 |
2008年 | 110篇 |
2007年 | 88篇 |
2006年 | 95篇 |
2005年 | 61篇 |
2004年 | 47篇 |
2003年 | 50篇 |
2002年 | 35篇 |
2001年 | 25篇 |
2000年 | 19篇 |
1999年 | 33篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1996年 | 27篇 |
1995年 | 16篇 |
1994年 | 14篇 |
1993年 | 20篇 |
1992年 | 12篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 12篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 18篇 |
1981年 | 8篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 9篇 |
1977年 | 2篇 |
1976年 | 6篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有1653条查询结果,搜索用时 328 毫秒
781.
To investigate the relationship between fish parasite communities and water quality level, metazoan parasites were examined in 157 specimens of chub (Leuciscus cephalus L.), sampled in four lowland water courses (northern Italy): Ticino river (unpolluted), Naviglio Pavese Canal (slightly polluted), Lambro river near the Merone village (polluted) and near Monza (severely polluted). Dactylogyrus vistulae, Paradiplozoon ergensi, Bucephalus polymorphus, Acanthocephalus anguillae and larval stages of Tylodelphys clavata were found in all the sampled sites. The distribution of Lamproglena pulchella and Pomphorhynchus laevis was limited to the unpolluted and slightly polluted river sectors, while Asymphylodora tincae, glochidia, along with larval stages of Diplostomum spathaceum, were absent in the severely polluted site. The variability of the calculated infection indices (prevalence, mean abundance and mean intensity), the degree of interactivity among parasites, as well as parameters of species richness and diversity suggest that the structure of parasite communities are affected by the water contamination level. 相似文献
782.
The mycorrhizal status of Phragmites australis in several polluted soils and sediments of an industrialised region of Northern Portugal 总被引:4,自引:0,他引:4
Roots of Phragmites australis from three polluted soils and sediments (a periodically flooded stream bank containing organic pollutants, a high-pH drying
sedimentation pond and an acidic, periodically flooded sand polluted by industrial effluents) were sampled over a 1-year cycle
of plant growth to assess the degree of colonisation by arbuscular mycorrhizal fungi (AMF). At the dry sedimentation pond,
root samples of Juncus effusus and Salix atrocinerea were also taken to assess the presence of AMF throughout the year. Root colonisation was low (<5% root length colonised)
but arbuscule presence peaked in P. australis during the spring and autumn prior to flowering. These changes in arbuscule abundance were also seen in a parallel greenhouse
trial using seed taken from one of the sites. Roots of J. effusus contained mainly vesicular colonisation but arbuscule activity peaked during the winter months (December–March). S. atrocinerea roots were found to be ectomycorrhizal throughout the year but the fine feeder roots were colonised by AMF. The results confirm
that semi-aquatics, like P. australis, can become arbuscular mycorrhizal but that this status changes during the year depending on soil moisture content and plant
phenology. The influence of AMF in these polluted soils is uncertain but the potential exists to establish a more diverse
plant ecosystem during the landscaping of these areas (phytostabilisation) by management of adapted plant and AMF ecotypes.
Accepted: 6 November 2000 相似文献
783.
George S. Bakken Andrew F. Boysen Carl E. Korschgen Kevin P. Kenow Steven L. Lima 《Journal of thermal biology》2001,26(6):297-604
The lack of a truly satisfactory sensor which can characterize the thermal environment at the spatial scale experienced by small endotherms has hindered study of their thermoregulatory behavior. We describe a general design for a rugged, easily constructed sensor to measure standard operative temperature, Tes. We present specific designs for adult dark-eyed juncos (Junco hyemalis) and hatchling mallards (Anas platyrhynchos). Sensor response was stable and repeatable (±1.4%) over the course of several months. Over the range of conditions for which validation data were available (variable air temperature and wind with negligible net radiation), sensors predicted the mean net heat production of live animals to within ±0.023 W (equivalent to ±1°C at Tes=15°C). The main limit on accuracy was scatter in the data on metabolism and evaporative water loss in live animals. These sensors are far more rugged and easily constructed than the heated taxidermic mounts previously used to measure Tes. These characteristics facilitate the use of significant numbers of sensors in thermal mapping studies of endotherms. 相似文献
784.
Thermal preference of the salamander Desmognathus fuscus was measured in a linear thermal gradient with floor temperatures ranging from 10 to 30°C. Salamanders were acclimated to 21±1°C and a 12 : 12 photoperiod with photophase centered at 1200 h for 8 weeks prior to being placed in the gradient. Substrate temperatures were measured under the salamanders’ stomachs from 1200 to 2400 h at 2 h intervals immediately after feeding and after seven days fasting. We found no selection for temperature in fasting or postprandial D. fuscus. We compared the rate at which D. fuscus cooled and heated with that of a control and found no significant difference. We determined the desiccation rate of D. fuscus at 16 and 26°C and found a significantly more rapid desiccation at 26°C. 相似文献
785.
Fluorescent and haptenized, monofunctionally binding platinum compounds are increasingly used for chemically labeling nucleic acids for hybridization detection purposes. Here we analyze in detail the effect of labeling density of the cyanin-3 and -5 platinum DNA adducts on fluorescence and thermal stability. We also analyzed the kinetics of the reaction of the cyanin platinum compounds with DNA. The data provided are important for the design of optimal platinum DNA labeling and hybridization conditions for fluorescence hybridization applications. 相似文献
786.
Computer prediction of human thermoregulatory and temperature responses to a wide range of environmental conditions 总被引:1,自引:0,他引:1
A mathematical model for predicting human thermal and regulatory responses in cold, cool, neutral, warm, and hot environments
has been developed and validated. The multi-segmental passive system, which models the dynamic heat transport within the body
and the heat exchange between body parts and the environment, is discussed elsewhere. This paper is concerned with the development
of the active system, which simulates the regulatory responses of shivering, sweating, and peripheral vasomotion of unacclimatised
subjects. Following a comprehensive literature review, 26 independent experiments were selected that were designed to provoke
each of these responses in different circumstances. Regression analysis revealed that skin and head core temperature affect
regulatory responses in a non-linear fashion. A further signal, i.e. the rate of change of the mean skin temperature weighted
by the skin temperature error signal, was identified as governing the dynamics of thermoregulatory processes in the cold.
Verification and validation work was carried out using experimental data obtained from 90 exposures covering a range of steady
and transient ambient temperatures between 5°C and 50°C and exercise intensities between 46 W/m2 and 600 W/m2. Good general agreement with measured data was obtained for regulatory responses, internal temperatures, and the mean and
local skin temperatures of unacclimatised humans for the whole spectrum of climatic conditions and for different activity
levels.
Received: 20 November 2000 / Revised: 24 April 2001 / Accepted: 14 May 2001 相似文献
787.
788.
Redman P Selman C Speakman JR 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1999,169(8):581-587
Nest construction is an extremely widespread behaviour. In small endotherms the nest serves primarily to provide insulation,
and thereby retard heat loss of the constructor, or its offspring. In arctic and temperate regions many small mammals build
nests to protect themselves from low ambient temperatures. We measured the physical properties of nests built by short-tailed
field voles Microtus agrestis that were kept in captivity under cold conditions. The most important factor influencing nest insulation was nest wall thickness;
however, nests with thick walls also contained more nesting material. Insulative capacity of the nest did not reach an asymptote
up to nests containing 20 g of material. Nest insulation was not correlated with resting metabolic rate, body mass or body
composition of the vole that constructed the nest. However, nests built by males had greater insulation than those made by
females; males also had significantly lower food intake rates when compared to females with nests. No significant difference
was observed in either fat mass or whole animal thermal conductance between males and females. Thermal conductance did increase
significantly with increasing body mass, although not with resting metabolic rate. Voles with nests for prolonged periods
had lower food intakes than voles without nests. The absolute saving averaged 1.9 g and was independent of body mass. This
was a 28% saving on intake for a 22-g vole but only an 18% saving for a 40-g individual. When voles had nests for short periods
they used the energy they saved to reduce food intake and increase body mass.
Accepted: 2 September 1999 相似文献
789.
Hot weather challenges livestock production but technology exists to offset the challenge if producers have made appropriate
strategic decisions. Key issues include understanding the hazards of heat stress, being prepared to offer relief from the
heat, recognizing when an animal is in danger, and taking appropriate action. This paper describes our efforts to develop
biological response functions; assesses climatic probabilities and performs associated risk analyses; provides inputs for
computer models used to make environmental management decisions; and evaluates threshold temperatures as estimates of critical
temperature limits for swine, cattle and sheep.
Received: 3 September 1998/Accepted: 5 October 1998 相似文献
790.
Jefimow M Wojciechowski M Tegowska E 《Comparative biochemistry and physiology. Part A, Molecular & integrative physiology》2003,134(4):717-726
Daily variations in sensitivity to noradrenaline (NA) and the activation of nonshivering thermogenesis (NST) are important for survival under a potentially wide range of environmental conditions. However, little is known regarding the ability of the Siberian hamster and other species to activate NST in the day and night when they may be subjected to marked variations in environmental temperature. In this study, the effects of acclimation temperature and time of day on the behavioral thermoregulatory response to NA injections in Siberian hamsters (Phodopus sungorus) was investigated. Hamsters were acclimated for 4 weeks to 23 degrees C and a L:D 12:12 h photoperiod. After acclimation, preferred ambient temperatures (PT(a)) in saline- and NA-injected animals were measured continuously in the temperature gradient system. NA (0.6 mg/kg; s.c.) was given every 4 h while PT(a) was monitored. After NA injections there was a rapid drop in PT(a), decreasing to approximately 15 degrees C within 10-20 min after each NA injection. Following 4 weeks of acclimation to 10 degrees C and a L:D 8:16 h photoperiod, the same hamsters were re-tested in the temperature gradient system. Cold acclimation led to an accentuation in the behavioral response with a decrease in PT(a) of approximately 10 degrees C. The maximal decrease in preferred ambient temperatures was recorded during the light phase of the day and during the second part of the night. Lowering of PT(a) after NA allows for rapid dissipation of the heat from NST. Overall, the behavioral response reflects the daily changes in brown adipose tissue sensitivity to NA and thus capacity for NST. 相似文献