全文获取类型
收费全文 | 544篇 |
免费 | 30篇 |
专业分类
林业 | 49篇 |
农学 | 15篇 |
基础科学 | 6篇 |
168篇 | |
综合类 | 48篇 |
农作物 | 36篇 |
水产渔业 | 26篇 |
畜牧兽医 | 159篇 |
园艺 | 18篇 |
植物保护 | 49篇 |
出版年
2023年 | 6篇 |
2022年 | 5篇 |
2021年 | 19篇 |
2020年 | 16篇 |
2019年 | 17篇 |
2018年 | 20篇 |
2017年 | 17篇 |
2016年 | 16篇 |
2015年 | 14篇 |
2014年 | 18篇 |
2013年 | 27篇 |
2012年 | 45篇 |
2011年 | 53篇 |
2010年 | 20篇 |
2009年 | 25篇 |
2008年 | 26篇 |
2007年 | 36篇 |
2006年 | 28篇 |
2005年 | 26篇 |
2004年 | 17篇 |
2003年 | 17篇 |
2002年 | 16篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 3篇 |
1993年 | 4篇 |
1990年 | 2篇 |
1988年 | 3篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1972年 | 2篇 |
1966年 | 2篇 |
1963年 | 3篇 |
1959年 | 1篇 |
1956年 | 2篇 |
1955年 | 2篇 |
1954年 | 1篇 |
1936年 | 2篇 |
1934年 | 1篇 |
1932年 | 1篇 |
1931年 | 1篇 |
1925年 | 1篇 |
排序方式: 共有574条查询结果,搜索用时 31 毫秒
31.
One year old golden ide (Leuciscus idus melanotus) were starved for 2 weeks at 20°C (time zero) and then they were fed either freeze-dried mosquito larvae (natural diet) or a commercial fish chow (artificial diet) at an ambient temperature of 14°C and 20°C, respectively. Growth parameters and biochemical data in the liver were measured at time zero and after 3 and 7 weeks of specific regimen. If compared to natural food, the artificial diet caused transient increase in anabolic activity of the liver, but prevented long-lasting hepatocyte proliferation (increase of total liver DNA). After 3 weeks on artificial diet, the body weight was significantly higher and the liver mass doubled compared to ide kept on the natural regimen; tissue DNA content indicated that the hepatocyte volume increased mainly by increased protein content and corresponding uptake of water; lipid and glycogen contents were increased by a factor of 3–6, and the RNADNA ratio was increased accordingly. The growth of the ide, as reflected by the condition factor and hepatosomatic index (HSI), was virtually stopped when the artificial regimen was maintained for 7 weeks, whereas on natural food the condition factor was increased and liver weight and DNA were doubled. Growth of the fish maintained at 14°C was significantly lower than that of fish maintained at 20°C, though the total liver DNA was still increased on the natural regimen during the experiment. Less lipid was stored by fish maintained at 14°C, whereas protein and glycogen deposits were enlarged, if compared to fish maintained at 20°C. The specific activity of cytochrome oxidase in liver mitochondria and of NADPH cytochrome c reductase in the microsomal fraction were found independent of diet and ambient temperature. Oxidative capacity of hepatocytes (mitochondrial protein/mg DNA) remained unchanged, and microsomal protein/cell appeared reduced in response to the artificial food. However, reduction of cell number on this diet resulted in less total mitochondrial and microsomal protein in the organ. Golden ide liver is found to be a sensitive experimental model to characterize the mutual influence of diet and temperature on fish; the results are discussed with regard to the usage of golden ide as test fish for water pollution. 相似文献
32.
Soil pH is one of the most influential variables in soil, and is a powerful factor in influencing the size, activity and community structure of the soil microbial community. It was previously shown in a century old artificial pH gradient in an arable soil (pH 4.0-8.3) that bacterial growth is positively related to pH, while fungal growth increases with decreasing pH. In an attempt to elucidate some of the mechanisms for this, plant material that especially promotes fungal growth (straw) or bacterial growth (alfalfa) was added to soil samples of the pH gradient in 5-day laboratory incubation experiments. Also, bacterial growth was specifically inhibited by applying a selective bacterial growth inhibitor (bronopol) along the entire pH gradient to investigate if competitive interaction caused the shift in the decomposer community along the gradient. Straw benefited fungal growth relatively more than bacterial, and vice versa for alfalfa. The general pattern of a shift in fungal:bacterial growth with pH was, however, unaffected by substrate additions, indicating that lack of a suitable substrate was not the cause of the pH effect on the microbial community. In response to the bacterial growth inhibition by bronopol, there was stimulation of fungal growth up to pH 7, but not beyond, both for alfalfa and straw addition. However, the accumulation of ergosterol (an indicator of fungal biomass) during the incubation period after adding alfalfa increased at all pHs, indicating that fungal growth had been high at some time during the 5-day incubation following joint addition of alfalfa and bronopol. This was corroborated in a time-series experiment. In conclusion, the low fungal growth at high pH in an arable soil was caused to a large extent by bacterial competition, and not substrate limitation. 相似文献
33.
Soil extracts are routinely used to quantify dissolved organic nutrient concentrations in soil. Here we studied the loss and transformation of low molecular weight (LMW) components of DOC (14C-glucose, 1 and 100 μM) and DON (14C-amino acid mixture, 1 and 100 μM) during extraction of soil (0-6 h) with either distilled water or 0.5 M K2SO4. The extractions were performed at 20 °C, at 4 °C, or in the presence of an inhibitor of microbial activity (HgCl2 and Na-azide). We showed that both glucose and amino acids became progressively lost from solution with increasing shaking time. The greatest loss was observed in H2O extracts at 1 μM for both substances (>90% loss after 15 min). Lower temperature (4 °C) and presence of K2SO4 both resulted in reduced loss rates. The presence of microbial inhibitors effectively eliminated the loss of glucose and amino acids. We conclude that microbial transformation of LMW-DOC and DON during H2O or K2SO4 extraction of soil may affect the estimation of their concentrations in soil. This finding has significant implications for methods that rely on chemical extractions to estimate LMW-C components of DOC and DON. 相似文献
34.
35.
36.
Ilse K. van Vonderen Hein P. Meyer Johannes S. Kraus Hans S. Kooistra 《Journal of veterinary internal medicine / American College of Veterinary Internal Medicine》1997,11(5):300-303
In dogs, secondary polycythemia (SP) may be associated with polyuria and polydipsia (PU/PD). The pathogenesis of this PU/PD has not yet been explained. We hypothesized that hyperviscosity and increased blood volume in SP might affect vasopressin (VP) release, resulting in PU/PD. This hypothesis was tested in 2 dogs with SP caused by renal neo-plasia and PU/PD. Osmoregulation of VP release was studied by a modified water deprivation test and by investigating the VP response to hypertonic saline infusion.
Water deprivation test results were consistent with an inability to produce concentrated urine despite increasing plasma osmolality. During hypertonic saline infusion, the osmotic threshold of VP release was markedly increased in both dogs, resulting in a delayed VP response to increasing plasma osmolality. The sensitivity of VP release was low normal in both dogs. We conclude that blood hyperviscosity and increased blood volume led to impaired VP release and polyuria. 相似文献
Water deprivation test results were consistent with an inability to produce concentrated urine despite increasing plasma osmolality. During hypertonic saline infusion, the osmotic threshold of VP release was markedly increased in both dogs, resulting in a delayed VP response to increasing plasma osmolality. The sensitivity of VP release was low normal in both dogs. We conclude that blood hyperviscosity and increased blood volume led to impaired VP release and polyuria. 相似文献
37.
38.
39.
Summary Gelprotein or SDS-insoluble gel-forming glutenin was isolated from wheat flour by extraction with an aqueous 1.5% SDS solution. Remarkable intervarietal differences were observed both in amount and subunit composition of these proteins.The amount of gelprotein and the SDS-sedimentation volume both proved to be good parameters for the bread-making quality of wheat cultivars. A high correlation was observed between amount of gelprotein and SDS-sedimentation volume. The amount of gelprotein was therefore tentatively assumed to be the essential basis of the SDS-sedimentation test.The subunit composition of the gelprotein was studied by SDS-PAGE after reduction of SS bonds by mercaptoethanol. It was found that the average bread-making quality of wheat cultivars and progeny of the cross Atlas 66 x Atys which possessed subunits 3 and 10, coded for by chromosome 1D, was significantly higher than that of wheat samples possessing subunit 2 and 11, their allelic counterparts. 相似文献
40.
Sebastian Klimek Anne Richter gen. Kemmermann Johannes Isselstein 《Biological conservation》2007,134(4):559-570
Decline of grassland diversity throughout Europe within the last decades is threatening biological diversity and is a major conservation problem. There is an urgent need to determine the underlying factors that control vascular plant species richness and composition in managed grasslands. In this study, 117 grasslands were sampled using standardised methods. Explanatory variables were recorded for each grassland site, reflecting the local field management, site-specific environmental conditions and large-scale spatial trends. Using variation partitioning methods, we determined the pure and shared effects of these three sets of explanatory variables on the plant species richness and composition in grasslands. Most of the explained variation in plant species richness was related to the joint effect of local field management and environmental variables. However, the applied variation partitioning approach revealed that the pure effect of spatial variables contributed relatively little to explaining variation in both the plant species richness and species composition. The largest fractions of explained variation in plant species composition were accounted for by the pure effects of environmental and local field management variables. Moreover, the results revealed that the main mechanisms by which these sets of explanatory variables affect plant species vary according to the type of management regime under study. From our findings we could conclude that particularly a reduction of nitrogen fertilisation on meadows and grazing at a low stocking rate on pastures can help to conserve biodiversity. 相似文献