全文获取类型
收费全文 | 70870篇 |
免费 | 3965篇 |
国内免费 | 34篇 |
专业分类
林业 | 3224篇 |
农学 | 2532篇 |
基础科学 | 477篇 |
8696篇 | |
综合类 | 11415篇 |
农作物 | 2606篇 |
水产渔业 | 3640篇 |
畜牧兽医 | 36607篇 |
园艺 | 907篇 |
植物保护 | 4765篇 |
出版年
2018年 | 1053篇 |
2017年 | 1129篇 |
2016年 | 1056篇 |
2015年 | 907篇 |
2014年 | 1136篇 |
2013年 | 2628篇 |
2012年 | 2058篇 |
2011年 | 2461篇 |
2010年 | 1683篇 |
2009年 | 1751篇 |
2008年 | 2592篇 |
2007年 | 2362篇 |
2006年 | 2326篇 |
2005年 | 2124篇 |
2004年 | 2193篇 |
2003年 | 2165篇 |
2002年 | 2036篇 |
2001年 | 2366篇 |
2000年 | 2375篇 |
1999年 | 1925篇 |
1998年 | 912篇 |
1997年 | 821篇 |
1996年 | 782篇 |
1995年 | 861篇 |
1994年 | 843篇 |
1993年 | 767篇 |
1992年 | 1523篇 |
1991年 | 1542篇 |
1990年 | 1662篇 |
1989年 | 1507篇 |
1988年 | 1450篇 |
1987年 | 1340篇 |
1986年 | 1384篇 |
1985年 | 1336篇 |
1984年 | 1128篇 |
1983年 | 1025篇 |
1982年 | 628篇 |
1979年 | 1004篇 |
1978年 | 795篇 |
1977年 | 717篇 |
1976年 | 676篇 |
1975年 | 719篇 |
1974年 | 766篇 |
1973年 | 830篇 |
1972年 | 803篇 |
1971年 | 748篇 |
1970年 | 729篇 |
1969年 | 744篇 |
1967年 | 658篇 |
1966年 | 631篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
Hekinian R Fevrier M Bischoff JL Picot P Shanks WC 《Science (New York, N.Y.)》1980,207(4438):1433-1444
Massive sulfide deposits were discovered from the diving saucer Cyana on the accreting plate boundary region of the East Pacific Rise near 21 degrees N. The deposits form conical and tubular structures lying on a basaltic basement. Mineralogical and geochemical analyses showed two main types of intimately associated products: a polymetallic sulfide-rich material composed of pyrite and marcasite in association, zinc-rich phases, and copper-rich compounds, and an iron-rich oxide and hydroxide material (also called gossan) composed largely of goethite and limonite. Silicate phases such as opaline, silica, iron-silicon clay, and trace amounts of mica and zeolite are encountered in both types of material. Possible mechanisms for the formation of the sulfide deposits on the East Pacific Rise are discussed. 相似文献
993.
994.
995.
薛培元 《中国农业大学学报》1957,(1)
宋代興修農田水利主要方面可分為四大項:一、利用河北淀泊開闢稻田,二、利用東南窪地修築圩田,三、引濁放淤,改良鹽鹼土地,四、修復古渠舊堰,以利灌溉。這些都是具有一定的基礎和原因,也都在農業生產上獲得成績,因而促進了經濟的發展。現在我們正在執行發展國民經濟的第一個五年計劃,其中有"積極興修農田水利"的規定。古人說"温故而知新",為由歷史上找出發展的道路起見,特寫此短篇,聊供設計時的參考。 相似文献
996.
Unwinding of duplex DNA from the SV40 origin of replication by T antigen 总被引:49,自引:0,他引:49
The T antigen specified by SV40 virus is the only viral-encoded protein required for replication of SV40 DNA. T antigen has two activities that appear to be essential for viral DNA replication: specific binding to duplex DNA at the origin of replication and helicase activity that unwinds the two DNA strands. As judged by electron microscopy, DNA unwinding is initiated at the origin of replication and proceeds bidirectionally. Either linear or circular DNA molecules containing the origin of replication are effective substrates; with closed circular DNA, a topoisomerase capable of removing positive superhelical turns is required for an efficient reaction. Presence of an origin sequence on duplex DNA and a single-strand DNA-binding protein appear to be the only requirements for T antigen to catalyze unwinding. This reaction mediated by T antigen defines a likely pathway to precise initiation of DNA replication: (i) the sequence-specific binding activity locates the origin sequence, (ii) the duplex DNA is unwound at this site, and (iii) the DNA polymerase and primase begin DNA replication. A similar pathway has been inferred for the localized initiation of DNA replication by bacteriophage lambda and by Escherichia coli in which a sequence-specific binding protein locates the origin and directs the DnaB helicase to this site. Observations with the SV40 system indicate that localized initiation of duplex DNA replication may be similar for prokaryotes and eukaryotes. 相似文献
997.
Similarity of synthetic peptide from human tumor to parathyroid hormone in vivo and in vitro 总被引:18,自引:0,他引:18
N Horiuchi M P Caulfield J E Fisher M E Goldman R L McKee J E Reagan J J Levy R F Nutt S B Rodan T L Schofield 《Science (New York, N.Y.)》1987,238(4833):1566-1568
One mechanism considered responsible for the hypercalcemia that frequently accompanies malignancy is secretion by the tumor of a circulating factor that alters calcium metabolism. The structure of a tumor-secreted peptide was recently determined and found to be partially homologous to parathyroid hormone (PTH). The amino-terminal 1-34 region of the factor was synthesized and evaluated biologically. In vivo it produced hypercalcemia, acted on bone and kidney, and stimulated 1,25-dihydroxy-vitamin D3 formation. In vitro it interacted with PTH receptors and, in some systems, was more potent than PTH. These studies support a long-standing hypothesis regarding pathogenesis of malignancy-associated hypercalcemia. 相似文献
998.
999.
Dickerson RR Huffman GJ Luke WT Nunnermacker LJ Pickering KE Leslie AC Lindsey CG Slinn WG Kelly TJ Daum PH Delany AC Greenberg JP Zimmerman PR Boatman JF Ray JD Stedman DH 《Science (New York, N.Y.)》1987,235(4787):460-465
Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems. 相似文献
1000.
Unit responses to photic stimuli were studied in cat visual cortex. After the baseline response pattern of a cell was determined, conditioning trials were given during which reinforcement was contingent upon increased firing during a selected segment of the poststimulus interval. Density of reinforcement increased substantially in about half the cells studied; significant increases in firing occurred within, but not outside, the criterion segment. 相似文献