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   34757篇
  免费   3291篇
  国内免费   13篇
  2022年   181篇
  2021年   494篇
  2020年   280篇
  2019年   392篇
  2018年   458篇
  2017年   415篇
  2016年   684篇
  2015年   1068篇
  2014年   1254篇
  2013年   1648篇
  2012年   1956篇
  2011年   2055篇
  2010年   1279篇
  2009年   1216篇
  2008年   1811篇
  2007年   1871篇
  2006年   1793篇
  2005年   1852篇
  2004年   1764篇
  2003年   1763篇
  2002年   1683篇
  2001年   409篇
  2000年   236篇
  1999年   385篇
  1998年   484篇
  1997年   349篇
  1996年   356篇
  1995年   285篇
  1994年   329篇
  1993年   309篇
  1992年   291篇
  1991年   264篇
  1990年   265篇
  1989年   233篇
  1988年   255篇
  1987年   240篇
  1986年   221篇
  1985年   304篇
  1984年   390篇
  1983年   316篇
  1982年   373篇
  1981年   436篇
  1980年   417篇
  1979年   254篇
  1978年   261篇
  1977年   276篇
  1976年   263篇
  1975年   183篇
  1974年   233篇
  1973年   195篇
排序方式: 共有10000条查询结果,搜索用时 453 毫秒
991.
Synopsis This paper reports on the incorporation of acid phosphatase histochemistry with a quantitative technique designed to measure the percentage of histochemically-localized enzyme-reactive cells found in adjuvant arthritic articular cartilage, synovial membrane and bone marrow. With the data presented and the comparisons made, this incorporation is validated and adjuvant arthritic acid phosphatase lesions established. In addition to discussing the importance of the histochemical data and its relationship with intra-articular lysosomes, the applicability of the reactive cell values to drug inhibition studies is discussed.  相似文献   
992.
The biological oxygen demand (BOD) of filtered water from Lake Wingra, Wisconsin is significantly higher in the littoral zone than in the pelagial zone. Laboratory experiments indicate that BOD is not influenced by water temperature at the time of sampling or by enrichment with nitrate or ammonia. Rather, enrichment with macrophyte leachate sharply increases BOD, and enrichment with phosphate produces a small but significant increase in BOD. We conclude that high BOD in littoral waters of the lake is an indication of production of labile organic matter in the water by dense beds of the macrophyte Myriophyllum spicatum.  相似文献   
993.
Recovery of ATP by boiling tris extraction was 90–95 percent greater in 1 liter grab samples than in concentrated net samples. ATP losses were attributed to insulating effects promoted by accumulation of detritus on filters. A series of extractions over a concentration range of whole or size-segregated plankters and cultured algae was made to determine volume of water to be filtered for optimum extraction efficiency. Accuracy of ATP assays was optimized by: (i) using large diameter (i.e. 47 mm) acetate filters; (2) limiting sample volume filtered to 50 ml when particulate organic carbon (POC) exceeded 0.4 mg l–1; and (3) performing extractions in boiling tris maintained initially on a laboratory hot plate at 400°C as opposed to hot water bath at 100°C.Additional problems were encountered in using published cellular carbon: ATP ratios for conversion of ATP data to biomass as carbon. Ratios of POC: ATP in cultures of sheathed blue-green algae reached 550 : i, while non-sheathed forms yielded ratios near values previously reported for plankton communities. Difficulties in applying a uniform conversion factor may be expected in plankton communities containing significant volumes of sheathed blue-greens.  相似文献   
994.
Goldfish (Carassius auratus) and bluegill sunfish (Lepomis macrochirus) were placed in aquaria where their locomotor activity was monitored by photocells, and tested at various acclimation temperatures over a range encompassing their final thermal preferenda. Activity was pooled over 24-hour periods to eliminate any circadian rhythm effects. Both species exhibited an activity well of reduced locomotor activity in the region of the final preferendum. Goldfish, tested either singly or in groups of 2–5 individuals, exhibited a social-interaction effect which became more pronounced at higher temperatures. These results are discussed in relation to a thermokinetic interpretation of thermo-regulatory behavior in fishes, and to the correspondence between thermal preferenda and thermal optima.  相似文献   
995.
Twenty bluespotted sunfish, Enneacanthus gloriosus, were individually tested for thermoregulatory behavior in an electronic shuttlebox. The first fishes of this genus to be tested for thermoregulatory behavior, these individuals were captured near the northern limit of their zoogeographic range, in northeastern Pennsylvania. The mean final preferendum from pooled data of all 20 fish was 28.5°C; there were no significant differences between day and night.  相似文献   
996.
When bakers' yeast cells were grown anaerobically in a medium supplemented with Tween 80 and ergosterol, exposure during aeration to the fatty acid synthesis inhibitor, cerulenin, had little effect upon respiratory adaptation, the induction of enzymes of electron transport, or the in vivo incorporation of [(14)C]leucine into mitochondrial membranes. These lipid-supplemented cells were apparently able to undergo normal respiratory adaptation utilizing endogenous lipids alone. The level of cerulenin used (2 mug/ml) inhibited the in vivo incorporation of [(14)C]acetate into mitochondrial membrane lipids by 96%. If, however, the cells were deprived of exogenous lipid during anaerobic growth, subsequent exposure to cerulenin severely reduced their capacity to undergo respiratory adaptation, to form enzymes of electron transport, and to incorporate amino acid into both total cell and mitochondrial membrane proteins. This cerulenin-mediated inhibition of enzyme formation and of protein synthesis was nearly completely reversed by the addition of exogenous lipid during the aeration of the cells. In lipid-limited cells, chloramphenicol also had dramatic inhibitory effects, both alone (75%) and together with cerulenin (85%), upon total cell and mitochondrial membrane [(14)C]leucine incorporation. This marked chloramphenicol-mediated inhibition was also largely reversed by exogenous lipid. It is concluded that, in lipid-limited cells, either cerulenin or chloramphenicol may prevent the emergence of a pattern of lipids required for normal levels of protein synthetic activity. The effect of cerulenin upon the formation of mitochondrial inner membrane enzymes thus appears to reflect a nonspecific effect of this antilipogenic antibiotic upon total cell protein synthesis.  相似文献   
997.
The plastochron index provides a morphological time scale which has proved more reliable than chronological age in studies relating morphological and physiological development of a whole plant or plant organ. Since its inception in 1957, the index has been utilized in a variety of investigations from leaf ontogeny in cottonwood trees to rhizoid cluster initiation in algae. The plastochron index has been extensively used in studies involving source and sink relationships, leaf anatomy, cell differentiation, and primary vascularization. It has been used in investigations of hormonal regulation of plant growth and in studies of the effects of various environmental factors on developmental processes in crops. This paper reviews some of the literature from 1957 to present concerning the development and use of the plastochron index.  相似文献   
998.
Estrogen receptor (ER) assays in human breast cancer tissue have proved useful in selecting patients for endocrine therapies. The absence of ER indicates hormone independent tumors and precludes the use of endocrine therapy. Patients with positive tumor ER respond to endocrine therapy at nearly twice the rate of those patients chosen by clinical criteria, although about a third of ER positive tumors in patients still do not respond. Recently, research has been directed toward increasing the accuracy of the ER assay in the ER positive group. The absolute tumor ER value and the presence of progesterone receptor appear promising in this regard. The significance of nuclear estrogen receptor is being studied. Finally, the ER status of a primary breast tumor appears to be a marker for the length of time until recurrence after mastectomy, and for survival. The ER assay may prove valuable in planning new adjuvants in the treatment of breast cancer.  相似文献   
999.
1000.
Rat [(59)Fe]haem-(125)I-labelled haemopexin complexes (700pmol/rat) associate rapidly and exclusively with the liver after intravenous injection into anaesthetized rats. The two isotopes exhibit different patterns of accumulation. Liver (125)I-labelled haemopexin is maximum 10min after injection (20+/-4.9pmol/g of liver) and then declines by 2h to the low values (about 3pmol/g of liver) seen after injection of the apoprotein. In contrast, [(59)Fe]haem accumulates in the liver for at least 2h. Haemopexin undergoes no extensive proteolysis during 2h of haem transport as shown by precipitation with acid (98%) and specific antiserum (92%) and by electrophoresis. Moreover, only 1-2% of the dose is located in extrahepatic tissues, and there is no significant urinary excretion of either (125)I or (59)Fe. Hepatic uptake at 10min is saturable, reaching 200pmol of haemopexin/g of liver and 350pmol of haem/g of liver at a dose of 9nmol/rat, whereas uptake of the apoprotein is 3-5% of the dose. This suggests that the interaction of haem-haemopexin with the liver is a specific receptor-mediated process. The complex probably interacts via the protein moiety, since the haem analogues mesohaem and deuterohaem do not affect association of the protein with the liver but the species of haemopexin does. Increasing amounts of protein are associated with the liver 5min after injection in the order: human>rabbit>rat, and haem uptake is consistently increased. For both rat and rabbit haemopexin saturation is reached at the same concentration of protein, i.e. 180-200pmol/g of liver, indicating that the different protein species bind to a common receptor. We propose that haemopexin transports haem to the liver by a specific receptor-mediated process and then returns to the circulation.  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