全文获取类型
收费全文 | 174篇 |
免费 | 12篇 |
国内免费 | 9篇 |
专业分类
195篇 |
出版年
2022年 | 3篇 |
2021年 | 2篇 |
2019年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2015年 | 13篇 |
2014年 | 9篇 |
2013年 | 11篇 |
2012年 | 7篇 |
2011年 | 14篇 |
2010年 | 11篇 |
2009年 | 7篇 |
2008年 | 4篇 |
2007年 | 7篇 |
2006年 | 8篇 |
2005年 | 2篇 |
2004年 | 6篇 |
2003年 | 5篇 |
2002年 | 2篇 |
2001年 | 4篇 |
2000年 | 5篇 |
1999年 | 4篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 6篇 |
1993年 | 4篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 1篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1955年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有195条查询结果,搜索用时 0 毫秒
61.
Both metabolic syndrome (MS) and polycystic ovary syndrome (PCOS) are common among women. The exact prevalence of MS in women with PCOS is dependent upon the diagnostic criteria used for each. However, the frequent co-occurrence of both MS and PCOS in women is suggestive of a common aetiology. In this short review article we argue that insulin resistance, as a consequence of abdominal obesity, may represent such a common aetiology. We also review the literature on the prevalence of MS in women with PCOS and consider the impact that the particular criteria used to diagnose both MS and PCOS may have had on these estimates of prevalence. 相似文献
62.
Mortality of newborn red deer (Cervus elaphus) calves is a major concern on New Zealand farms, as perinatal losses average approximately 10% of calves born. Primiparous red deer (calving as yearlings) lose more calves than multiparous hinds (adults). We performed a study on yearling and adult red deer hinds in order to improve knowledge of their calving behaviour and determine any apparent reasons for calf mortality. Pacing along fence lines was observed frequently during the 24 h period before birth (individuals were pacing in 43% of observations during this period). Adult hinds had a significantly earlier onset and longer duration of pacing (P<0.05). Hinds were often observed isolated (>20 m) from the rest of the herd during the 2 days prior to parturition, and this behaviour was also observed earlier in the adults than in the yearlings (P<0.05). Forty-four percent of adult hinds and 60% of yearlings experienced some form of interference from other hinds during parturition. Among hinds that were interfered with, yearlings had a higher average number of interferences from other hinds during parturition than adults (P<0.05). Calves born to yearling hinds took significantly longer to suckle for the first time (mean=44 min) from their dams than calves born to adult hinds (33 min; P<0.05). We concluded that an inability of hinds to express natural isolation behaviour was likely to contribute to calf mortality due to increased anxiety (indicated by fence line pacing) and encroachment on the birth areas of others, and that calves of yearlings were at an increased risk of mortality due to the higher number of interferences taking place during parturition and the longer time interval between birth and suckling. 相似文献
63.
Robert L. Knight William E. Walton George F. OMeara William K. Reisen Roland Wass 《Ecological Engineering》2003,21(4-5):211-232
Constructed wetlands hold considerable promise for providing water quality and wildlife habitat benefits. At the same time, constructed wetlands have been described as “mosquito-friendly habitats” and may raise potential conflicts with neighboring human populations. Conflicts arise because some design features, such as shallow water and emergent vegetation that are essential for optimizing water quality polishing, can result in undesirable increases in mosquito production. The attraction of large numbers of birds to constructed wetlands could also increase the risk of transmission of mosquito-borne viral infections to humans in the vicinity of the wetland. The potential for conflict is typically highest in arid regions where natural mosquito populations have limited abundance and are found near newly urbanizing areas.The creation of wildlife habitat is a significant goal of many treatment wetlands. Humans are also welcome in many treatment wetlands for recreational and educational activities. Risks of disease transmission to humans and livestock as well as the inconvenience of mosquitoes as pests must be offset by the economic savings of inexpensive water quality enhancement and the resulting reduction in pollution that also poses a risk to society’s health and well-being. Ecological risks associated with the use of mosquito control chemicals must be offset by the increased habitat benefits provided by these constructed wetlands. The right balance between these competing goals can be recognized by the design that provides the greatest net environmental and societal benefit. This paper describes these tradeoffs between mosquito control and the constructed wetland technology and provides a synthesis of information that can be used to optimize the benefits of these wetland systems. Basic research is recommended to better define the cost-effectiveness of the various design and management options. 相似文献
64.
65.
66.
67.
Neal Gutterson 《Acta Botanica Sinica》2011,(3)
Selection of energy crops is the first priority for large-scale biofuel production in China. As a major topic, it was extensively discussed in the Second International Symposium on Bioen- 相似文献
68.
Deposition of faeces by black swans (Cygnus atratus Latham) feeding on benthic algae in a shallow New Zealand lake was determined by collection of faeces from the lake bottom and from the shore. The two methods showed good agreement after adjustment for the weight loss on immersion. The mean daily faecal output per swan was 52 g dry weight. The nitrogen content of the faeces averaged 2.3% of dry weight, and was dominated by soluble organic nitrogen (59% of total N). Phosphorus averaged 0.44% of dry weight, with 66% of it being particulate, and 30% soluble reactive phosphorus. Although faecal inputs of total phosphorus were sufficient to generate concentrations of 15–30 mg m–3, the faecal contributions of both N and P were only a minor component of the fluctuations observed in the lake, and were also small in relation to the total nutrient pool in the water and benthic algae. Waterfowl faeces appear to have low ratios of N to P, which will favour dominance of the phytoplankton by cyanobacteria in lakes where the faecal component of nutrient loads is large. The few data available suggest that the nitrogen content of waterfowl faeces is largely independent of that in their food. Food consumption, calculated by using cellulose as an indigestible faecal marker, was 104 g dry weight swan–1 d–1, a figure that appears low in relation to those for other swan species. Even the highest published figure for food intake by a swan is only about one half of the corresponding average metabolically-adjusted figures for geese, and we caution against the uncritical use of bioenergetic models for determining rates of food consumption and defaecation. 相似文献
69.
AA Bozzo CA Soñez I Monedero Cobeta A Rolando MC Romanini D Cots 《Biotechnic & histochemistry》2014,89(4):296-303
The model of chronic intermittent stress by immobilization during pregnancy may produce alterations in the mechanisms that maintain adrenal gland homeostasis. In earlier investigations using this model, significant variations in plasma prolactin and corticosterone levels, and adrenal gland weights were observed. We hypothesized that chronic stress causes changes in apoptosis in the adrenal glands of pregnant rats. We identified and quantified apoptotic cells in the adrenal cortex and examined their ultrastructural characteristics using transmission electron microscopy. Adrenal glands of pregnant rats at gestation days 12, 17 and 21 were studied for control and experimental (stressed) rats. Immunolabelling techniques, stereological analysis and image quantification of adrenal gland sections were combined to determine differences in apoptosis in the different cell populations of the adrenal cortex. The apoptotic index of the experimental rats showed a significant reduction at gestation day 17, while at days 12 and 21 there were no differences from controls. Moreover, the apoptotic index of the reticular zones in control and experimental animals showed a significant increase compared to the glomerular and fascicular zones at the three gestation times studied. Chronic stress by immobilization reduced the caspase-dependent apoptotic index at gestation day 17, which may be related to variations in plasma concentrations of estrogens and prolactin. 相似文献
70.
Substitution bias, rapid saturation, and the use of mtDNA for nematode systematics 总被引:13,自引:0,他引:13
Only relatively recently have researchers turned to molecular methods for
nematode phylogeny reconstruction. Thus, we lack the extensive literature
on evolutionary patterns and phylogenetic usefulness of different DNA
regions for nematodes that exists for other taxa. Here, we examine the
usefulness of mtDNA for nematode phylogeny reconstruction and provide data
that can be used for a priori character weighting or for parameter
specification in models of sequence evolution. We estimated the
substitution pattern for the mitochondrial ND4 gene from intraspecific
comparisons in four species of parasitic nematodes from the family
Trichostrongylidae (38-50 sequences per species). The resulting pattern
suggests a strong mutational bias toward A and T, and a lower
transition/transversion ratio than is typically observed in other taxa. We
also present information on the relative rates of substitution at first,
second, and third codon positions and on relative rates of saturation of
different types of substitutions in comparisons ranging from intraspecific
to interordinal. Silent sites saturate extremely quickly, presumably owing
to the substitution bias and, perhaps, to an accelerated mutation rate.
Results emphasize the importance of using only the most closely related
sequences in order to infer patterns of substitution accurately for
nematodes or for other taxa having strongly composition-biased DNA. ND4
also shows high amino acid polymorphism at both the intra- and
interspecific levels, and in higher level comparisons, there is evidence of
saturation at variable amino acid sites. In general, we recommend using
mtDNA coding genes only for phylogenetics of relatively closely related
nematode species and, even then, using only nonsynonymous substitutions and
the more conserved mitochondrial genes (e.g., cytochrome oxidases). On the
other hand, the high substitution rate in genes such as ND4 should make
them excellent for population genetics studies, identifying cryptic
species, and resolving relationships among closely related congeners when
other markers show insufficient variation.
相似文献