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   101642篇
  免费   1133篇
  国内免费   948篇
  103723篇
  2023年   63篇
  2022年   121篇
  2021年   218篇
  2020年   150篇
  2019年   196篇
  2018年   11998篇
  2017年   10834篇
  2016年   7692篇
  2015年   1034篇
  2014年   748篇
  2013年   893篇
  2012年   4886篇
  2011年   13429篇
  2010年   12408篇
  2009年   8593篇
  2008年   10234篇
  2007年   11754篇
  2006年   674篇
  2005年   851篇
  2004年   1330篇
  2003年   1295篇
  2002年   1071篇
  2001年   413篇
  2000年   263篇
  1999年   141篇
  1998年   86篇
  1997年   92篇
  1996年   77篇
  1995年   56篇
  1994年   65篇
  1993年   86篇
  1992年   96篇
  1991年   117篇
  1990年   97篇
  1989年   92篇
  1988年   68篇
  1987年   65篇
  1986年   57篇
  1985年   65篇
  1984年   61篇
  1983年   64篇
  1982年   51篇
  1981年   36篇
  1979年   46篇
  1977年   30篇
  1976年   31篇
  1975年   36篇
  1974年   36篇
  1972年   265篇
  1971年   299篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
It has been shown that both IAA and ethylene application inhibit flower induction in the short-day plant Pharbitis nil. However application of IAA has elevated ethylene production in this plant, as well. Strong enhancement of ethylene production is also correlated with the night-break effect, which completely inhibits flowering. In order to determine what the role of IAA and ethylene is in the photoperiodic flower induction in Pharbitis nil, we measured changes in their levels during inductive and non-inductive photoperiods, and the effects of ethylene biosynthesis and action inhibitors on inhibition of flowering by IAA. Our results have shown that the inhibitory effect of IAA on Pharbitis nil flowering is not physiological but is connected with its effect on ethylene biosynthesis.  相似文献   
62.
63.
Associated with the neuronal plasma membrane are cytoskeletal proteins which probably control the specialization of the membrane into axonal and dendritic domains. Specialized isoforms of the proteins spectrin and ankyrin are located in each region and provide molecular mechanisms for locating specific transmembrane proteins at required points. However, spectrin and ankyrin were defined by extensions of the model for the erythrocyte membrane, an analogy unlikely to provide a complete account of the neuronal membrane skeleton. We have defined two new proteins of the neuronal membrane skeleton, designated p103 and A60. p103 is enriched in post-synaptic densities and binds with high affinity to integral membrane proteins--we suggest that it may have a role in linking the cytoskeleton to synaptic glycoproteins. A60 is a 60 kDa axonal protein, which appears to form a lining to the axolemma. It is almost exclusively axonal, although some neurons (such as Purkinje cells) appear to contain it in the cell body and initial dendrite segment. A60 binds both ankyrin and neurofilaments, and may have a role in transmitting information critical to axonal morphology to the membrane.  相似文献   
64.
65.
Systematics and taxonomy of hares of the genus Lepus (Lagomorpha) are under contentious debate, and phylogenetic relationships among many taxa are not well understood. Here we study genetic differentiation and evolutionary relationships among North African hares, currently considered subspecies of Lepus capensis , cape hares ( L. capensis ) from the Cape province in South Africa, and brown hares ( L. europeaus ) from Europe and Anatolia, using maternally (mtDNA) and biparentally (allozymes) inherited markers. A polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) analysis of a c. 1.8 kb long segment of the mitochondrial control region using eight hexanucleotide-recognizing restriction endonucleases yielded 28 haplotypes, and horizontal starch gel electrophoresis of proteins encoded by 25 structural gene loci revealed 52 alleles at 18 polymorphic loci. Diverse phylogenetic analyses (neighbor joining dendrogram, median joining network, multidimensional scaling of pairwise distances, AMOVA, F -statistics, hierarchical F -statistics) of genetic variants revealed marked substructuring of mtDNA into three phylogeographic groups, namely an African, a central European, and an Anatolian, but a somewhat less pronounced overall differentiation of the nuclear genome, despite a relatively high number of population-specific (private) alleles. However, all our results are not incongruent with Petter's (1959: Mammalia 23 , 41; 1961: Z. f. Säugetierkunde 26 , 30; 1972 : Société Des Sciences Naturelles et Physiques du Maroc 52 , 122) hypothesis that North African hares generally belong to L. capensis and that brown hares should be included in this species as well.  相似文献   
66.
67.
68.
Hematological and serum chemistry values, as well as rectal temperatures, were obtained from greater galagos (Galago garnettii and G. crassicaudatus), in order to establish normative values. No species or sex differences were found for four hematological parameters and 15 serum chemistry parameters. Species differences were seen in phosphate, magnesium, cholesterol, alkaline phosphate, G-glutamyl transferase, mean corpuscular volume and leucocyte, neutrophil, and lymphocyte number. Significant sex differences were observed in glucose, hemoglobin, and hematocrit values. Species and sex differences were seen in chloride and erythrocyte number.  相似文献   
69.
Summary We have mapped and sequenced the globin gene and seven surrounding Alu repeat sequences in the orangutan globin gene cluster and have compared these and other orangutan sequences to orthologously related human sequences. Noncoding flanking and intron sequences, synonymous sites of , , and globin coding regions, and Alu sequences in human and orangutan diverge by 3.2%, 2.7%, and 3.7%, respectively. These values compare to 3.6% from DNA hybridizations and 3.4% from the globin gene region. If as suggested by fossil evidence and molecular clock calculations, human and orangutan lineages diverged about 10–15 MYA, the rate of noncoding DNA evolution in the two species is 1.0–1.5×10–9 substitutions per site per year. We found no evidence for either the addition or deletion of Alu sequences from the globin gene cluster nor is there any evidence for recent concerted evolution among the Alu sequences examined. Both phylogenetic and phenetic distance analyses suggest that Alu sequences within the and globin gene clusters arose close to the time of simian and prosimian primate divergence (about 50–60 MYA). We conclude that Alu sequences have been evolving at the rate typical of noncoding DNA for the majority of primate history.Presented at the FEBS Symposium on Genome Organization and Evolution, held in Crete, Greece, September 1–5, 1986  相似文献   
70.
Detection of MR signals with surface coils provides increased signal-to-noise ratio for superficial structures relative to detection by circumferential coils, permitting improved spatial resolution. Different geometries of surface coils can be used for different regions. Coils that are flat or curved to fit body contours are good for general imaging, with a range of coil sizes useful for structures of different size or depth. Solenoidal coils are useful for imaging protruding structures such as breasts, while smaller versions of conventional circumferential coils that can be slipped over limbs are useful for imaging extremities.  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