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   14291篇
  免费   1122篇
  2022年   73篇
  2021年   177篇
  2020年   122篇
  2019年   157篇
  2018年   222篇
  2017年   166篇
  2016年   341篇
  2015年   562篇
  2014年   557篇
  2013年   819篇
  2012年   1003篇
  2011年   952篇
  2010年   666篇
  2009年   524篇
  2008年   787篇
  2007年   859篇
  2006年   853篇
  2005年   824篇
  2004年   800篇
  2003年   735篇
  2002年   747篇
  2001年   168篇
  2000年   122篇
  1999年   165篇
  1998年   221篇
  1997年   155篇
  1996年   133篇
  1995年   142篇
  1994年   123篇
  1993年   118篇
  1992年   142篇
  1991年   97篇
  1990年   82篇
  1989年   98篇
  1988年   90篇
  1987年   83篇
  1986年   86篇
  1985年   97篇
  1984年   130篇
  1983年   104篇
  1982年   118篇
  1981年   112篇
  1980年   99篇
  1979年   65篇
  1978年   73篇
  1977年   63篇
  1976年   59篇
  1975年   50篇
  1974年   58篇
  1972年   46篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
81.
A new species of freshwater dinoflagellates, Ceratium rhomvoides nov. sp., is described from the Plußsee, northern West Germany, where it occurs regularly. C. rhomvoides nov. sp. is distinguished from closely related taxa by various aspects of the morphology of the vegetative cell and the cyst and by its sexual reproduction and seasonal distribution.  相似文献   
82.
The life cycle of Ceratium furcoides (Levander) Langhans which features sexual reproduction is described for the first time in situ from the lake Plußsee (northern West Germany). Anisogamic copulation occurs during the bloom period in July–August by incorporation of the male gamete by the female gamete, resulting in an overwintering planozygote. The planozygote develops into a benthic hypnozygote which germinates in spring to establish a planomeiocyte. The morphology and structure of the stages of the life cycle are described using light and scanning electron microscopy.  相似文献   
83.
Germinating spores of Bacillus subtilis mutants which lack small, acid-soluble spore proteins α and β did not exhibit the transient elevated UV resistance seen during germination of wild-type spores.  相似文献   
84.
Seasonal variations in levels of serum testosterone, dihydrotestosterone (DHT), reproductive behavior, and social behavior were investigated in 12 adult males (5 to 20 + years of age) of the Oregon troop of Japanese macaques (Macaca fuscata). Blood samples were collected at 2- to 4-month intervals, and behaviors were monitored twice weekly over a 15-month period. Significant seasonal variations in levels of testosterone and DHT, and in frequencies of mount series, ejaculations, number of female partners, displays, courtship, and aggression were observed. Seasonal variations in reproductive and social behaviors did not correlate with seasonal variations in androgen levels because seasonal increases in these behaviors followed seasonal increases in the androgens with a 1- to 2-month delay. However, significant correlations between increased androgen levels and the onset of mating activity occur when mean monthly frequencies of mount series are shifted 1 to 2 months earlier to coincide with the rise in serum androgen levels. The frequency of adult male play and male-male mounting increased significantly when androgen levels were low. We suggest that photoperiod changes may function as a proximate cue in male Japanese macaques which induces a state of biological readiness for mating, and the behavioral consequences (i.e., mating) are then dependent upon the presence of receptive females.  相似文献   
85.
Binding of Germanium to Pseudomonas putida Cells   总被引:1,自引:1,他引:0       下载免费PDF全文
The binding of germanium to Pseudomonas putida ATCC 33015 was investigated by using whole intact cells grown in a medium supplemented with GeO2 and catechol or acetate. Electron-microscopic examination of the control and metal-loaded samples revealed that germanium was bound within the cell envelope. A certain number of small electron-dense deposits of the bound element were found in the cytoplasm when the cells were grown in the presence of GeO2 and catechol. The study of germanium distribution in cellular fractions revealed that catechol facilitated the intracellular accumulation of this element.  相似文献   
86.
DNA sequence evidence for polymorphic forms of human serum amyloid A (SAA)   总被引:8,自引:0,他引:8  
Serum amyloid A (SAA) is an acute-phase reactant and precursor to amyloid A protein, the major constituent of the fibril deposits of reactive amyloidosis. The factors determining whether the 104-amino acid SAA molecule is converted into the 76-amino acid amyloid A protein and deposited as fibrils are not known. As an initial step toward investigating the possibility that a particular primary structure of SAA is involved in amyloid formation, we have cloned and determined the nucleotide sequence of human SAA-specific cDNAs. The first clone, selected using an oligonucleotide probe, was shown to encode the signal peptide and amino-terminal region of SAA. The cDNA of this clone served as probe in the selection of two distinct, full-length SAA cDNAs, initially differentiated by the presence (pSAA21) or absence (pSAA82) of a PstI site in the coding sequence. The complete nucleotide sequence of pSAA82 cDNA was determined. Since there appear to be multiple human SAA alleles, it is conceivable that their differential expression is important to amyloid formation.  相似文献   
87.
Summary Six independently isolated mutants of Chlamydomonas reinhardtii that require elevated CO2 for photoautotrophic growth were tested by complementation analysis. These mutants are likely to be defective in some aspect of the algal concentrating mechanism for inorganic carbon as they exhibit CO2 fixation and inorganic carbon accumulation properties different from the wild-type. Four of the six mutants defined a single complementation group and appear to be defective in an intracellular carbonic anhydrase. The other two mutations represent two additional complementation groups.Abbreviations HS high salt medium which has 13 mM phosphate at pH 6.8 - HSA high salt plus 36 mM acetate medium - YA high salt medium with 4 g yeast extract per L and 36mM acetate - Arg arginine - cia- CO2 accumulation mutants that cannot grow on low CO2 - Ci inorganic carbon (CO2+HCO - 3 ) - CA carbonic anhydrase - mt mating type Supported in part by the McKnight Foundation and by NSF grant PCM 8005917 and published as journal article 11924 from the Michigan State Agriculatural Experiment Station  相似文献   
88.
Oxygen-evolving photosystem II-phycobilisome particles froma red alga were inhibited 50–80% by aging, dilution, lowpH and salt-washing. Bovine serum albumin, and dithiothreitolwere found to stimulate activity in all but salt-washed particles.CaCl2 and MnCl2 partially restored activity lost after agingor dilution. 1Current address: Waksman Institute of Microbiology, RutgersUniversity, Piscataway, New Jersey 08854-0759, U.S.A. (Received October 5, 1985; Accepted March 31, 1986)  相似文献   
89.
We developed an aqueous spreading procedure that permits simultaneous analysis of human chromosomes by Q-banding and indirect immunofluorescence. Using this methodology and anticentromere antibodies from an autoimmune patient we compared the active and inactive centromeres of an isodicentric X chromosome. We show that a family of structurally related human centromere proteins (CENP-A, CENP-B, and CENP-C) is detectable only at the active centromere. These antigens therefore may be regarded both as morphological and functional markers for active centromeres.  相似文献   
90.
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