全文获取类型
收费全文 | 14046篇 |
免费 | 931篇 |
国内免费 | 1篇 |
专业分类
林业 | 1379篇 |
农学 | 550篇 |
基础科学 | 80篇 |
3258篇 | |
综合类 | 820篇 |
农作物 | 616篇 |
水产渔业 | 1135篇 |
畜牧兽医 | 5498篇 |
园艺 | 365篇 |
植物保护 | 1277篇 |
出版年
2023年 | 117篇 |
2022年 | 180篇 |
2021年 | 312篇 |
2020年 | 313篇 |
2019年 | 268篇 |
2018年 | 678篇 |
2017年 | 616篇 |
2016年 | 564篇 |
2015年 | 457篇 |
2014年 | 509篇 |
2013年 | 801篇 |
2012年 | 1068篇 |
2011年 | 993篇 |
2010年 | 575篇 |
2009年 | 500篇 |
2008年 | 859篇 |
2007年 | 797篇 |
2006年 | 632篇 |
2005年 | 688篇 |
2004年 | 625篇 |
2003年 | 549篇 |
2002年 | 474篇 |
2001年 | 317篇 |
2000年 | 295篇 |
1999年 | 251篇 |
1998年 | 81篇 |
1997年 | 70篇 |
1996年 | 54篇 |
1995年 | 81篇 |
1994年 | 45篇 |
1993年 | 43篇 |
1992年 | 92篇 |
1991年 | 79篇 |
1990年 | 79篇 |
1989年 | 71篇 |
1988年 | 61篇 |
1987年 | 61篇 |
1986年 | 56篇 |
1985年 | 52篇 |
1984年 | 48篇 |
1983年 | 46篇 |
1980年 | 26篇 |
1979年 | 41篇 |
1974年 | 29篇 |
1972年 | 25篇 |
1971年 | 26篇 |
1970年 | 29篇 |
1969年 | 32篇 |
1968年 | 27篇 |
1967年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
Plant circadian clocks increase photosynthesis, growth, survival, and competitive advantage 总被引:3,自引:0,他引:3
Dodd AN Salathia N Hall A Kévei E Tóth R Nagy F Hibberd JM Millar AJ Webb AA 《Science (New York, N.Y.)》2005,309(5734):630-633
Circadian clocks are believed to confer an advantage to plants, but the nature of that advantage has been unknown. We show that a substantial photosynthetic advantage is conferred by correct matching of the circadian clock period with that of the external light-dark cycle. In wild type and in long- and short-circadian period mutants of Arabidopsis thaliana, plants with a clock period matched to the environment contain more chlorophyll, fix more carbon, grow faster, and survive better than plants with circadian periods differing from their environment. This explains why plants gain advantage from circadian control. 相似文献
992.
NSP1 of the GRAS protein family is essential for rhizobial Nod factor-induced transcription 总被引:1,自引:0,他引:1
Smit P Raedts J Portyanko V Debellé F Gough C Bisseling T Geurts R 《Science (New York, N.Y.)》2005,308(5729):1789-1791
993.
994.
Pimenta BV Haddad CF Nascimento LB Cruz CA Pombal JP 《Science (New York, N.Y.)》2005,309(5743):1999; author reply 1999
995.
Brédas JL 《Science (New York, N.Y.)》1994,263(5146):487-488
996.
Reidar Almås 《Agriculture and Human Values》1989,6(1-2):127-136
This article raises the issue of the extent to which a single nation can develop a “national agricultural policy,” pursuing internal goals in agrarian development, goals that vary significantly from those of other industrialized countries. What are the conflicts arising from such a policy and how do these conflicts interfere with the general agricultural crisis of these countries? The Norwegian case is explored as an example of a blend of social-democratic and center-populist agricultural policies. The decision in 1975 by the Norwegian Parliament to establish the income level of farmers at the same level as workers is seen as an experiment. As agricultural policy, this new departure represented a deviation from the earlier policies of rationalization. The Social Democrats developed their new position as a result of internal and external pressure. A series of reforms financed by the new state incomes from oil production were undertaken, of which income equalization and a farm relief service were the most important. Most Norwegian farmers speak of the reform years as “the good years of escalation.” Equity was reached in 1982, but nevertheless several problems emerged: 1) the escalation of subsidized incomes led to overproduction and decreasing incomes from the market (after 1982), 2) the large budgetary transfers to agriculture caused a legitimation crisis, fueling the constituency of the right-wing Progress Party, 3) the large subsidies to agriculture contributed to the fiscal crisis of the state, 4) the problems mentioned above, contributed to the general crisis of the Norwegian Social Democracy which finds itself with dwindling support. The likely outcome of the present Norwegian farm crisis is the implementation of an agricultural policy that will protect the most vulnerable sector and open the more competitive sector to market forces. 相似文献
997.
A major evolutionary divide occurs in the animal kingdom between the so-called radially symmetric animals, which includes the cnidarians, and the bilaterally symmetric animals, which includes all worm phyla. Buddenbrockia plumatellae is an active, muscular, parasitic worm that belongs to the phylum Myxozoa, a group of morphologically simplified microscopic endoparasites that has proved difficult to place phylogenetically. Phylogenetic analyses of multiple protein-coding genes demonstrate that Buddenbrockia is a cnidarian. This active muscular worm increases the known diversity in cnidarian body plans and demonstrates that a muscular, wormlike form can evolve in the absence of overt bilateral symmetry. 相似文献
998.
Darwin KH Ehrt S Gutierrez-Ramos JC Weich N Nathan CF 《Science (New York, N.Y.)》2003,302(5652):1963-1966
The production of nitric oxide and other reactive nitrogen intermediates (RNI) by macrophages helps to control infection by Mycobacterium tuberculosis (Mtb). However, the protection is imperfect and infection persists. To identify genes that Mtb requires to resist RNI, we screened 10,100 Mtb transposon mutants for hypersusceptibility to acidified nitrite. We found 12 mutants with insertions in seven genes representing six pathways, including the repair of DNA (uvrB) and the synthesis of a flavin cofactor (fbiC). Five mutants had insertions in proteasome-associated genes. An Mtb mutant deficient in a presumptive proteasomal adenosine triphosphatase was attenuated in mice, and exposure to proteasomal protease inhibitors markedly sensitized wild-type Mtb to RNI. Thus, the mycobacterial proteasome serves as a defense against oxidative or nitrosative stress. 相似文献
999.
1000.
Phoberomys is reported to be the largest rodent that ever existed, although it has been known only from isolated teeth and fragmentary postcranial bones. An exceptionally complete skeleton of Phoberomys pattersoni was discovered in a rich locality of fossil vertebrates in the Upper Miocene of Venezuela. Reliable body mass estimates yield approximately 700 kilograms, more than 10 times the mass of the largest living rodent, the capybara. With Phoberomys, Rodentia becomes one of the mammalian orders with the largest size range, second only to diprotodontian marsupials. Several postcranial features support an evolutionary relationship of Phoberomys with pakaranas from the South American rodent radiation. The associated fossil fauna is diverse and suggests that Phoberomys lived in marginal lagoons and wetlands. 相似文献