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   3508篇
  免费   241篇
  国内免费   367篇
  2024年   9篇
  2023年   92篇
  2022年   103篇
  2021年   136篇
  2020年   157篇
  2019年   188篇
  2018年   153篇
  2017年   188篇
  2016年   149篇
  2015年   140篇
  2014年   217篇
  2013年   345篇
  2012年   128篇
  2011年   217篇
  2010年   159篇
  2009年   192篇
  2008年   175篇
  2007年   193篇
  2006年   157篇
  2005年   144篇
  2004年   135篇
  2003年   115篇
  2002年   104篇
  2001年   55篇
  2000年   46篇
  1999年   45篇
  1998年   35篇
  1997年   47篇
  1996年   42篇
  1995年   30篇
  1994年   23篇
  1993年   26篇
  1992年   29篇
  1991年   17篇
  1990年   21篇
  1989年   10篇
  1988年   7篇
  1987年   9篇
  1986年   10篇
  1985年   17篇
  1984年   12篇
  1983年   10篇
  1982年   5篇
  1981年   5篇
  1980年   3篇
  1979年   3篇
  1978年   3篇
  1977年   4篇
  1976年   2篇
  1973年   2篇
排序方式: 共有4116条查询结果,搜索用时 62 毫秒
11.
For quality assurance purposes, the results of the 1990's obtained by the National Cervical Cancer Screening Programme (NCCSP) launched in 1962 were reviewed. The positive cytodiagnosis, the histologically verified in situ and invasive cervical cancers and the mortality rates were reported.  相似文献   
12.
Fourteen laboratories participated in a national slide exchange study to investigate whether borderline nuclear changes (BNC) constitute a reliable reporting category. Slides were submitted by participating laboratories, having achieved a 100% intralaboratory consensus at the primary screener, checker, and medical levels. Sets of seven slides were examined in laboratories for 1 week, and exchanges were undertaken over a 6-month period. Each laboratory was requested to submit three consensus opinions on each slide at the primary screener, checker, and medical levels.
Response patterns for submitted slides achieving a reporting category consensus at the 50 and 80% consensus levels indicated that negative, BNC, and mild dyskaryosis are distinct and comparable categories. Similarly, the two subcategories of BNC with or without human papillomavirus (HPV) are nearly as distinct as the overall BNC category.
The percentage of submitted slides achieving consensus at consensus levels between 50 and 80% produced variable findings with regard to the practical success of the main reporting categories. The negative category was reasonably successful, whereas mild dyskaryosis was consistently poor. Borderline nuclear changes were successful at the 50% consensus level but showed a rapid decline by the 65% consensus level. The reason(s) for this remains speculative but indicates a possible potential of BNC to work successfully with additional training and education.
Reporting practices were not consistent among the laboratories and differences were identified between medical and nonmedical staff. A high use of the BNC category was noted in slides that failed to achieve consensus. A national study assessing all grades of abnormalities would appear essential.  相似文献   
13.
This paper describes a study of cervical screening in 50 general practices in the East Berkshire Health District over a period of 2 1/2 years. Six practices organized their own practice based screening scheme. Thirty-nine participated in a district organized scheme and five did not actively participate in any scheme. The proportion of women screened was highest among the practices which organized their own scheme. These practices were large, in non-urban locations and employed a practice nurse. In contrast, practices which had a poor record of screening were small, single handed, in an urban location and were unwilling to participate in a district call scheme or organize their own scheme. It is recommended that the practices which are prepared to organize their own screening programmes should be encouraged to do so. The resources which are saved could then be more usefully spent on providing assistance to the practices which do not offer a cervical screening service to women on their practice lists.  相似文献   
14.
Summary Tarantula heart cDNA libraries were screened with synthetic oligonucleotide probes deduced from the highly conserved amino acid sequences of the two copper-binding sites, copper A and copper B, found in chelicerate hemocyanins. Positive cDNA clones could be obtained and four different cDNA types were characterized.  相似文献   
15.
Shoot apices ofCarica papaya were multiplied in vitro on solidified nutrient media supplemented with -naphthyl-acetic acid and 6-benzylaminopurine. The micropropagated shoots were inoculated in vitro, through a stem wound, with a sporangial suspension (1.2×104 sporangia ml-1) ofPhytophthora palmivora. The symptoms exhibited by the shoots in vitro were similar to those described previously for infection of the whole plant in the field. The time taken for the host tissue to become brown and to wilt and the time to sporulation of the pathogen were all recorded for each shoot of four varieties of papaya challenged with each of ten isolates ofP. palmivora. Significant differences were observed between host-pathogen combinations for these variables and host-specificity was detected amongst the isolates ofP. palmivora. The time taken for the shoot to wilt was positively correlated with the time to sporulation of the isolated but both these variables were negatively correlated with the time to browning of the shoot. In vitro selection for disease resistance will be useful during breeding programmes involving papaya genotypes which are maintained through clonal propagation.  相似文献   
16.
Radiolabelled calmodulin has previously been used to screen cDNA expression libraries to isolate calmodulin-binding proteins. We have modified this technique for the isolation of plant calmodulin-binding proteins. [35S]-methionine was used instead of the inorganic [35S]-sulfate, or125I used in previous methods. In addition, theE. coli pET expression system was chosen to obtain high levels of recombinant calmodulin at the time of labelling. The procedure thus takes into account both the specific activity of the probe and the amount of protein necessary for screening a large number of filters. Here we describe in detail a procedure for the production and purification of [35S]-recombinant calmodulin and the use of the radiolabelled protein as a probe to screen plant cDNA expression libraries. The [35S]-labeled calmodulin probe easily detects the λICM-1 phage encoding a partial mouse calmodulin-dependent protein kinase II that was previously isolated using a [125I]-calmodulin probe (Sikela and Hahn, 1987). Subsequently, a tobacco root cDNA expression library was screened and a positive clone encoding a calcium-dependent calmodulin-binding protein was isolated.  相似文献   
17.
Five strains of Bradyrhizobium japonicum (USDA 6, 110, 122, 138, and 143) were screened in cell culture for tolerance to acidity (pH 4.2, 4.4, and 4.6) and Al (0, 3, 4, 5, and 6 mg L–1) under low P conditions. Each strain was later grown in association with seven soybean [Glycine max. (L) Merr.] cultivars which were also screened for tolerance to the same stresses in nutrient culture to determine which soybean-Bradyrhizobium combinations would establish the most effective symbiotic N2 fixing relationships. Results indicated that strains USDA 110 and 6 were more tolerant than USDA 122, 138 and 143 with USDA 110 being the most tolerant. Acidity appeared to be the more severe stress; but even when strains showed tolerance to the stresses, cell numbers were significantly reduced. This suggests that colonization of soils and soybean roots can be adversely affected under similar conditions in the field which may result in reduced nodulation. The strains found to be more tolerant to the stresses were more effective N2 fixers in symbiosis with all soybean cultivars, with USDA 110 being definitely superior. The association between the more tolerant strains and cultivars had the largest nitrogenase activity. Further studies on the inclusion of tolerant Bradyrhizobium strains in inoculum used on tolerant soybean cultivars in the field are warranted.  相似文献   
18.
Summary Bamboo species are difficult to identify because flowering material is seldom available and taxonomy is of necessity based on vegetative characters. To evaluate the utility of restriction fragment length polymorphism (RFLP) analysis in bamboo systematics and germplasm screening, a library of random genomic probes from a Phyllostachys nigra PstI library was constructed. Probes from the library were used to screen bamboo germplasm consisting mostly of temperate bamboos of the genus Phyllostachys. RFLP variation was abundant, and species-specific patterns were readily obtained. Chloroplast DNA showed little variation among the bamboo accessions analyzed.  相似文献   
19.
20.
Summary Our field surveys conducted in 10 Hungarian grape producing areas have revealed that the nutrient contents of vine leaves of different grape varieties were closely correlated with the EUF-nutrient contents of different soil types with different contents of clay. Resulting from these relationships the following EUF-nutrient values are considered as required for optimal nutrition of the vine-stock to attain grape yields of 10–12 t/ha in the areas under investigation:  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