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   44070篇
  免费   119篇
  国内免费   149篇
系统科学   232篇
丛书文集   852篇
教育与普及   131篇
理论与方法论   243篇
现状及发展   19769篇
研究方法   1659篇
综合类   20934篇
自然研究   518篇
  2013年   292篇
  2012年   589篇
  2011年   1105篇
  2010年   304篇
  2008年   764篇
  2007年   767篇
  2006年   804篇
  2005年   819篇
  2004年   785篇
  2003年   777篇
  2002年   769篇
  2001年   1246篇
  2000年   1172篇
  1999年   802篇
  1994年   381篇
  1992年   737篇
  1991年   615篇
  1990年   669篇
  1989年   641篇
  1988年   650篇
  1987年   688篇
  1986年   702篇
  1985年   836篇
  1984年   652篇
  1983年   580篇
  1982年   501篇
  1981年   491篇
  1980年   617篇
  1979年   1333篇
  1978年   1156篇
  1977年   1117篇
  1976年   815篇
  1975年   927篇
  1974年   1289篇
  1973年   1107篇
  1972年   1150篇
  1971年   1447篇
  1970年   1798篇
  1969年   1416篇
  1968年   1332篇
  1967年   1326篇
  1966年   1147篇
  1965年   835篇
  1959年   482篇
  1958年   787篇
  1957年   623篇
  1956年   529篇
  1955年   484篇
  1954年   517篇
  1948年   333篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
281.
Summary 7 monoterpenes, -pinene, -pinene, myrcene, p-cymene, limonene, neral and geranial, were identified together with n-undecane in the scent scales ofP. melete, and also, together with considerable amounts of linalool, in the scent scales ofP. napi. Content ratio of neral to geranial inP. melete ranged from 0.77 to 1.04, and that inP. napi from 1.84 to 2.43. The present chemotaxonomic approach shows the 2 species ofPieris to be distinct.The authors thank Dr T. Yushima (National Institute of Agricultural Science, Japan) for supplying the references 3 and 4.  相似文献   
282.
The effect ofγ-irradiation on soil enzyme stability   总被引:1,自引:0,他引:1  
Summary Arylsulphatase, -1,3 glucanase, phosphatase and urease responded differently to -irradiation (5–50 Mrad) in air-dried and moist soils. In all instances phosphatase was the most stable. The variability between enzymes may be due to inherent biochemical and stuctural characteristics or to their location within the soil microenvironment.Supported by a grant from the NERC.  相似文献   
283.
Summary Large-scale purification of hepatitis B surface antigen, applicable to the preparation of potential vaccines for prevention of hepatitis B, is described. The method involves the following steps: precipitation of serum with polyethylene glycol 6000, affinity chromatography on concanavalin A-Sepharose and on -aminononyl-Sepharose, and isopycnic centrifugation.This study was supported by grant 09011 from the National Heart, Lung and Blood Institute. We thank Dr. B. Hollinger for testing HBsAg samples by a complement fixation test.  相似文献   
284.
The behavioural study, in particular rearing environmental conditions, of the mutant mouse staggerer has shown that such animals may live more than 90 days. (he behavioural diagnosis of this mutation has been possible from the second week of life, using specific tests. A typical "bat posture" permits one to recognize the mutant from the normal Mouse. Locomotory and feeding behaviours also present late and various qualitatige particularities.  相似文献   
285.
CBA Mice were immunized by two intraperitoneal injections of 30 X 10(6) DBA/2 or C57BL/6 spleen cells at days--12 and--2. Peritoneal cell population was obtained at day zero by washing the peritoneal cavity of Mice. Adherent cells were then separated using a 2 hrs. incubation in "Falcon" plates followed by washing. This macrophage-rich peritoneal cell population was found nonspecifically cytotoxic against 51Cr labeled tumoral target cells: P815 X DBA/2 mastocytoma cells, EL4 X C57BL/L lymphoma cells and spontaneous lymphoma AKR cells (same H--2k as CBA). This adherent peritoneal cell cytoxicity was demonstrated after 24 hrs. incubation with the target cells. It was found in nonspecific combination as well as when using target cells syngeneic to the donor. These findings suggest that adherent peritoneal cell cytotoxicity could be at least partly due to macrophages and result from factor (s) released by sensitized lymphocytes in vivo in the same way as has been previously demonstrated in vitro.  相似文献   
286.
A strain of Aspergillus fumigatus Fres., isolated from sugar-beet draffs, synthesizes in vitro four toxic metabolites which have not yet been described in these fungal species. Toxic effects of the most abundant of these metabolites "fumitoxin A" have been studed on chick embryo. Artemia salina larvae, cell cultures, and on mice.  相似文献   
287.
Triiodothyronine injected daily to pregnant rats for the last week of gestation (50 microgram/100 g b.wt) increased the specific activities of 5 acid glycosidases in the fetal forebrain and cerebellum. Cortisone (50 mg/100 g b.wt) administered in the same period had no effect on cerebellum acid hydrolases, but decreased their activity in the forebrain.  相似文献   
288.
Raloff J 《Science news》1978,114(6):92-3, 95
  相似文献   
289.
The construction of molecular models for the human serotransferrin glycans shows that they present one compact section linked to the protein and constituted by the pentasaccharide alpha-Man-(1 leads to 3)-[alpha-Man-(1 leads to 6)]-beta-Man-(1 leads to 4)-beta-GlcNAc-(1 leads to 4)-beta-GlcNAc-(1 leads to)-Asn to which are attached two "antennae" consisting of the trisaccharide alpha-NANA-(2 leads to 6)-beta-Gal-(1 leads to 4)-beta-GlcNAc. The trisaccharide sequence beta-Man-(1 leads to 4)-beta-GlcNAc-(1 leads to 4)-beta-GlcNAc adopts a flat and rigid conformation, stabilised by hydrogen bonds. In contrast, the sequence alpha-NANA-(1 leads to 6)-beta-Gal-(1 leads to 4)-beta-GlcNAc-(1 leads to 2)-alpha-Man takes up a helical configuration. The two "antennae" can be disposed on the pentasaccharide core to give two possible configurations, one Y-shaped and the other T-shaped. In both cases, the general conformation of the glycans is perfectly compatible with their postulated role as a recognition signal.  相似文献   
290.
The glycoprotein CEA (carcinoembryonic antigen) carries carbohydrate groups, which react with the plant lectins from Agaricus bisporus, Arachis hypogaea (peanut), with Tridacnin from invertebrate clams and with the anti-A lectins from snails. Accordingly, it has cryptantigenic structures, which correspond to the T or T-like antigen, the Tridacnin receptor and to the so called A-like antigen.  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