首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   112288篇
  免费   2041篇
  国内免费   2574篇
  116903篇
  2024年   57篇
  2023年   299篇
  2022年   602篇
  2021年   1034篇
  2020年   747篇
  2019年   855篇
  2018年   12538篇
  2017年   11148篇
  2016年   8238篇
  2015年   1848篇
  2014年   1845篇
  2013年   1918篇
  2012年   6047篇
  2011年   14298篇
  2010年   12981篇
  2009年   8963篇
  2008年   10699篇
  2007年   12222篇
  2006年   1038篇
  2005年   1203篇
  2004年   1594篇
  2003年   1581篇
  2002年   1248篇
  2001年   607篇
  2000年   457篇
  1999年   345篇
  1998年   171篇
  1997年   155篇
  1996年   136篇
  1995年   118篇
  1994年   129篇
  1993年   122篇
  1992年   154篇
  1991年   147篇
  1990年   78篇
  1989年   90篇
  1988年   80篇
  1987年   63篇
  1986年   50篇
  1985年   48篇
  1984年   40篇
  1983年   43篇
  1982年   19篇
  1979年   18篇
  1975年   16篇
  1973年   21篇
  1972年   258篇
  1971年   282篇
  1965年   20篇
  1962年   31篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
During the past two decades, the use of probiotics as an alternative to the use of antibiotics has shown to be promising in aquaculture, particularly in fish and shellfish larviculture. This article reviews the studies on probiotics in larviculture, focusing on the current knowledge of their in vivo mechanisms of action. The article highlights that the in vivo mechanisms of action largely remain to be unravelled. Several methodologies are suggested for further in vivo research, including studies on gut microbiota composition, the use of gnotobiotic animals as test models, and the application of molecular techniques to study host–microbe and microbe–microbe interactions.  相似文献   
972.
The Houses That Matsutake Built. In the mountains of northwest Yunnan, China, a valuable mushroom, matsutake or song rong (Tricholoma matsutake) was commodified in the 1980s. Since that time, it has been exported in large quantities to Japan. The sale of matsutake now contributes more to the income of Shangri-la County (Diqing Autonomous Tibetan Prefecture) than any other crop, including timber and livestock. During the 1980s and 1990s, villagers in this remote region used their mushroom earnings to build spacious, beautiful new houses in the traditional local (Kham) style, and in some cases to buy motor vehicles or open businesses. In villages with access to productive matsutake habitat, virtually every household was able to build a new house; entire villages were transformed. During the 1990s, several villages developed locally based management regimes to enhance production and to address the problems and conflicts that arose from the harvest of such a valuable product. More recently, government agencies and NGOs have played a highly visible role in promoting “sustainable” harvest policies. The implications of their involvement are briefly examined and the future of matsutake harvest in Yunnan is discussed.  相似文献   
973.

Background

Limited health facilities and malnutrition are major problems in the Karakorum Range of Northern Pakistan, often resulting in various human disorders. Since centuries, however, local communities in these areas have developed traditional methods for treating various ailments and local foods capes that can be significant for devising public health and nutritional policies. This study was intended to document the ethnobotanical knowledge of the local peoples in the Tormik Valley, especially in the medical and food domains.

Methods

Field trips were undertaken in 14 different villages of the study area from 2010 to 2012. Ethnobotanical data were gathered using semi-structured interviews and group conversation with 69 informants. Details about local uses of plant species were recorded along with demographic characteristics of the visited communities. Relative frequency citation index (RFCi) and preference ranking index (PRi) tools were applied to determine the cultural significance of the reported species.

Results

Sixty-three plant species, with a predominance of Asteraceae and Fabaceae family members, as well as their detailed folk uses were documented. Forty-three percent of the species were used to treat various diseases, 21 % were consumed as wild fruits and vegetables and 53 % of the species had multipurpose applications. Thymus linearis Benth, Hippophae rhamnoides ssp. turkestanica L. and Convolvulus arvensis L. were found to be the most utilized medicinal plant species, i.e. those with significant RFCi values (0.54, 0.51 and 0.48, respectively). Betula utilis D. Don was the most versatile taxon (seven different ways of utilization); being this species a common and easily accessible subalpine tree and then under anthropogenic pressure, the implementation of concrete strategies aimed at its in-situ and ex-situ conservation is strongly recommended.

Conclusion

The valleys in the Karakorum Mountains in the Northern Pakistan host significant Traditional Knowledge on local food and medicinal plant species, which need to be reconsidered and cautiously re-evaluated by ethnopharmacologists, and public health/nutrition actors. Furthermore, germane trans-disciplinary investigations are suggested to ensure the dynamic conservation of precious local knowledge systems, as well as plant diversity in Pakistani mountain regions.
  相似文献   
974.
为了综合评价枝条覆盖对不同立地(平地、丘顶、丘间低地)平茬花棒(Hedysarum scoparium)生长特征和浅层土壤水分的影响,以吉兰泰荒漠-绿洲过渡区平茬花棒为研究对象,于2017年4—10月对覆盖与无覆盖平茬花棒物候期、土壤容积含水量、土壤储水量和生长指标等相关指标进行测定。实验结果表明:(1)枝条覆盖有利于平茬花棒提前萌芽,较无覆盖提前5 d以上。(2)枝条覆盖使不同立地平茬花棒生长初期和生长末期0—100 cm土层平均土壤容积含水量增加20.12%以上,且土壤含水量变异系数随土层深度增加变化减缓;土壤储水量在生长初期和生长末期枝条覆盖较无覆盖增加27%以上。(3)枝条覆盖使平茬花棒株高、新生枝数、基径、冠幅较无覆盖增加10.91%以上,同时其生物量鲜重和干重较无覆盖分别增加4.45%—24.17%和3.78%—18.93%。尤其丘间低地平茬花棒生物量增加效果更加明显(P>0.05)。但枝条覆盖不能改变平茬花棒生物量分配格局;(4)枝条覆盖和平茬对花棒生长均具有积极作用,枝条覆盖对丘间低地平茬花棒生长促进作用最好。  相似文献   
975.
Dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS), is a very powerful tool for studying the diffusion behaviour of macromolecules in solution. The diffusion coefficient, and hence the hydrodynamic radii calculated from it, depends on the size and shape of macromolecules. In this review, we provide evidence of the usefulness of DLS to study the homogeneity of proteins, nucleic acids, and complexes of protein–protein or protein–nucleic acid preparations, as well as to study protein–small molecule interactions. Further, we provide examples of DLS’s application both as a complementary method to analytical ultracentrifugation studies and as a screening tool to validate solution scattering models using determined hydrodynamic radii.  相似文献   
976.
It has been suggested that the allele frequency of thrombophilic mutations is affected by glucose-6-phosphate dehydrogenase (G6PD) deficiency. The prevalence of thrombophilic mutations were studied in sixty G6PD deficient individuals including 57 males and three females with the mean age of 15 ± 3.08 and 110 age and sex matched healthy individuals consisted of 95 males and 15 females with the mean age of 16.19 ± 2.17 from the Kermanshah Province of Iran. Using a combination of PCR-RFLP technique, single strand conformation polymorphism (SSCP) analysis and DNA sequencing polymorphic G6PD mutations were identified. The factor V Leiden, prothrombin G20210A and methylenetetrahydrofolate reductase (MTHFR) C677T were detected by PCR-RFLP method using MnlI, HindIII and HinfI restriction enzymes, respectively. Three mutations, G6PD Mediterranean, G6PD Chatham and G6PD Cosenza were identified in 60 G6PD deficient individuals with highest prevalence of G6PD Mediterranean (91.6%). In G6PD deficient individuals the prevalence of factor V Leiden tended to be higher (5%) compared to healthy individuals (2.7%). The prevalence of prothrombin G20210A mutation in G6PD deficient individuals was 1.7%. However, in normal subjects the prevalence of this mutation was 2.7%. The frequency of T allele in G6PD deficient individuals were insignificantly higher (29.16%) than those in healthy individuals (26.8%). Our finding indicates that the prevalence of factor V Leiden, prothrombin G20210A and MTHFR C677T in G6PD deficient individuals is not statistically different compared to normal subjects and G6PD deficiency is not associated with these thrombophilic mutations in Western Iran.  相似文献   
977.
In this study, we characterized a putative peroxidase Prx1 of Candida albicans by: 1) demonstrating the thioredoxin-linked peroxidase activity with purified proteins, 2) examining the sensitivity to several oxidants and the accumulation of intracellular reactive oxygen species with a null mutant (prx1Δ), a mutant (C69S) with a point mutation at Cys69, and a revertant, and 3) subcelluar localization. Enzymatic assays showed that Prx1 is a thioredoxin-linked peroxidase which reduces both hydrogen peroxide (H2O2) and tert-butyl hydroperoxide (t-BOOH). Compared with two other strong H2O2 scavenger mutants for TSA1 and CAT1, prx1Δ and C69S were less sensitive to H2O2, menadione and diamide at all concentrations tested, but were more sensitive to low concentration of t-BOOH. Intracellular reactive oxygen species accumulated in prx1Δ and C69S cells treated with t-BOOH but not H2O2. These results suggest that peroxidase activity of Prx1 is specified to t-BOOH in cells. In both biochemical and physiological cases, the evolutionarily conserved Cys69 was found to be essential for the function. Immunocytochemical staining revealed Prx1 is localized in the cytosol of yeast cells, but is translocated to the nucleus during the hyphal transition, though the significances of this observation are unclear. Our data suggest that PRX1 has a thioredoxin peroxidase activity reducing both t-BOOH and H2O2, but its cellular function is specified to t-BOOH.  相似文献   
978.
This paper seeks to understand why multinationals prefer to launch a label specific to their own product and examines how reliable these product-specific eco-labels are. A new methodology is applied to assess the extent to which eco-labels live up to claims about their contribution to conservation and the sustainable use of agricultural biodiversity. Product-specific eco-labels are considered as industry self-regulation and all three regulatory stages are studied: the planning, implementation and outcome stage. There are major differences between the product specific eco-labels in the degree in which agrobiodiversity management is part of the normative labeling schemes. Although there are some problems of reliability, such as transparency in the implementation stage and the monitoring in the outcome stage, the degree of reliability of product-specific labels is comparable with eco-labels of international labeling families. The conclusion is that only one of the product-specific eco-labels examined here is reliable when examined in the light of all three stages. The main reason why multinationals establish a product-specific eco-label instead of adopting one from an existing labeling family is that they want to profile themselves as distinct from other companies. The unique character of a product-specific label creates a market opportunity for them.  相似文献   
979.
The attractiveness of herbivore-induced plant volatiles (HIPVs) from a specific plant species to natural enemies has been well established. However, under natural conditions and polycultural agriculture systems, the interactions among trophic levels are thought to be more complex. For instance, complex mixtures of volatiles emitted from diverse host plant species infested by polyphagous herbivores might affect responses of natural enemies. In this study, we investigated whether a mixture of HIPVs emitted from herbivore-damaged multiple host plant species affect responses of a predatory bug. Therefore, we report (1) olfactory responses of the predatory bug (Orius strigicollis) to volatiles emitted from cotton bollworm (Helicoverpa armigera) first instar larvae-damaged multiple plant species (tomato, French bean and sweet corn), (2) chemical analyses of volatiles emitted from the three plant species exposed to different treatments and (3) olfactory responses of the predators to a reconstituted HIPV blend from multiple plant species based on chemical analyses. O. strigicollis significantly preferred volatiles emanating from H. armigera-damaged multiple plant species to volatiles emanating from a single plant species. In all the three plant species, H. armigera-damaged seedlings emitted significantly a greater amount of volatiles as well as a larger number of volatile compounds than an undamaged or a mechanically injured seedling. The predators preferred the reconstituted HIPVs from multiple plant species to the reconstituted HIPVs from a single plant species. Thus, the mixture of HIPVs from multiple plant species enhanced the attractiveness to the predators.  相似文献   
980.

Background  

Mucolipidosis Type IV is currently characterized as a lysosomal storage disorder with defects that include corneal clouding, achlorhydria and psychomotor retardation. MCOLN1, the gene responsible for this disease, encodes the protein mucolipin-1 that belongs to the "Transient Receptor Potential" family of proteins and has been shown to function as a non-selective cation channel whose activity is modulated by pH. Two cell biological defects that have been described in MLIV fibroblasts are a hyperacidification of lysosomes and a delay in the exit of lipids from lysosomes.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号