全文获取类型
收费全文 | 44899篇 |
免费 | 138篇 |
国内免费 | 155篇 |
专业分类
系统科学 | 392篇 |
丛书文集 | 879篇 |
教育与普及 | 84篇 |
理论与方法论 | 147篇 |
现状及发展 | 20110篇 |
研究方法 | 1667篇 |
综合类 | 21135篇 |
自然研究 | 778篇 |
出版年
2013年 | 309篇 |
2012年 | 593篇 |
2011年 | 1363篇 |
2010年 | 252篇 |
2008年 | 717篇 |
2007年 | 841篇 |
2006年 | 799篇 |
2005年 | 840篇 |
2004年 | 901篇 |
2003年 | 792篇 |
2002年 | 768篇 |
2001年 | 1292篇 |
2000年 | 1243篇 |
1999年 | 814篇 |
1992年 | 843篇 |
1991年 | 653篇 |
1990年 | 693篇 |
1989年 | 632篇 |
1988年 | 630篇 |
1987年 | 663篇 |
1986年 | 676篇 |
1985年 | 905篇 |
1984年 | 672篇 |
1983年 | 550篇 |
1982年 | 481篇 |
1981年 | 514篇 |
1980年 | 644篇 |
1979年 | 1424篇 |
1978年 | 1111篇 |
1977年 | 1073篇 |
1976年 | 881篇 |
1975年 | 947篇 |
1974年 | 1318篇 |
1973年 | 1119篇 |
1972年 | 1188篇 |
1971年 | 1351篇 |
1970年 | 1842篇 |
1969年 | 1440篇 |
1968年 | 1271篇 |
1967年 | 1307篇 |
1966年 | 1207篇 |
1965年 | 904篇 |
1964年 | 281篇 |
1959年 | 506篇 |
1958年 | 877篇 |
1957年 | 634篇 |
1956年 | 526篇 |
1955年 | 471篇 |
1954年 | 508篇 |
1948年 | 359篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
22.
Humoral immune reactions to heat shock proteins (hsp) from microorganisms are one aspect of microbial infections in humans. The production of antibodies which are specific to epitopes present on procaryotic hsp leads also to the appearance of cross-reactive serum antibodies in the host organism that react with human hsp. This article discusses the consequences of such autoreactive antibodies for the host in context with the development of immune tolerance and autoimmune diseases, especially rheumatoid arthritis (RA), and in experimental animal models for arthritis such as adjuvant arthritis in rats. On the basis of epitope cross-reactivity between hsp and other host proteins, a hypothesis is presented for the development of autoimmune disease following the production of hsp-specific antibodies. 相似文献
23.
Astacin, a digestive zinc-endopeptidase from the crayfish Astacus astacus L., is the prototype for the 'astacin family', which includes mammalian metallo-endopeptidases and developmentally regulated proteins of man, fruitfly, frog and sea urchin. Here we report the X-ray crystal structure of astacin, which reveals a deep active-site cleft, with the zinc at its bottom ligated by three histidines, a water molecule and a more remote tyrosine. The third histidine (His 102) forms part of a consensus sequence, shared not only by the members of the astacin family, but also by otherwise sequentially unrelated proteinases, such as vertebrate collagenases. It may therefore represent the elusive 'third' zinc ligand in these enzymes. The amino terminus of astacin is buried forming an internal salt-bridge with Glu 103, adjacent to His 102. Astacin pro-forms extended at the N terminus, as observed for some 'latent' mammalian astacin homologues, did not exhibit this 'active' conformation, indicating an activation mechanism reminiscent of trypsin-like serine proteinases. 相似文献
24.
It was established that there is little diversity of bryophytes in the derived savanna. Mosses were found in the sampling sites, whereas liverworts were rarely observed. The reproductive methods of four dominant sexually reproducing savanna mosses —Archidium ohioense, Bryum coronatum, Fissidens minutifolius andTrachycarpidium tisserantii were monitored over two consecutive rainy seasons. Protonemal and gametophyte production were noticed in the field in March/April, and capsule dehiscence and spore dispersal occurred in September/October. The sequential stages of development, starting with gametangial production and ending with the falling of the dehisced capsules, occurred within the rainy season. However,A. ohioense andT. tisserantii did not discharge their spores easily (cleistocarpous), unlike the stegocarpous speciesB. coronatum andF. minutifolius. Water availability and possibly high humidity may have contributed to growth. The short period between sex organ formation and dehiscence of capsule seen in these studies, compared with the longer period in some temperate mosses, may be an advantage for bryophytes in a savanna environment. 相似文献
25.
大型仪器设备是高校开展教学、科研的物质保证,是高校实验室的主要技术装备,衡量一所高校办学实力和水平的重要指标之一.…… 相似文献
26.
27.
28.
29.
30.
R. Poggioli A. V. Vergoni A. Bertolini 《Cellular and molecular life sciences : CMLS》1985,41(2):265-266
Summary Hydrochlorothiazide, acutely injected in rats, has a weak analgesic activity per se and potentiates and prolongs the antinociceptive effect of morphine.This work was supported in part by grants from Consiglio Nazionale delle Ricerche, and by Ministero della Pubblica Istruzione, Roma. 相似文献