全文获取类型
收费全文 | 665篇 |
免费 | 6篇 |
专业分类
林业 | 17篇 |
94篇 | |
综合类 | 230篇 |
农作物 | 14篇 |
畜牧兽医 | 302篇 |
园艺 | 1篇 |
植物保护 | 13篇 |
出版年
2019年 | 5篇 |
2014年 | 3篇 |
2013年 | 4篇 |
2012年 | 48篇 |
2011年 | 55篇 |
2010年 | 11篇 |
2009年 | 7篇 |
2008年 | 41篇 |
2007年 | 53篇 |
2006年 | 34篇 |
2005年 | 39篇 |
2004年 | 31篇 |
2003年 | 33篇 |
2002年 | 36篇 |
2001年 | 20篇 |
2000年 | 28篇 |
1999年 | 14篇 |
1998年 | 3篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 5篇 |
1993年 | 3篇 |
1992年 | 4篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1987年 | 5篇 |
1979年 | 4篇 |
1976年 | 5篇 |
1972年 | 3篇 |
1970年 | 5篇 |
1967年 | 8篇 |
1966年 | 9篇 |
1963年 | 4篇 |
1959年 | 4篇 |
1958年 | 3篇 |
1957年 | 6篇 |
1956年 | 3篇 |
1954年 | 4篇 |
1948年 | 7篇 |
1944年 | 3篇 |
1943年 | 3篇 |
1941年 | 3篇 |
1940年 | 4篇 |
1938年 | 4篇 |
1915年 | 3篇 |
1896年 | 2篇 |
1895年 | 3篇 |
1893年 | 3篇 |
1892年 | 2篇 |
排序方式: 共有671条查询结果,搜索用时 15 毫秒
41.
Microparasite infections often consist of genetically distinct clonal lineages. Ecological interactions between these lineages within hosts can influence disease severity, epidemiology, and evolution. Many medical and veterinary interventions have an impact on genetic diversity within infections, but there is little understanding of the long-term consequences of such interventions for public and animal health. Indeed, much of the theory in this area is based on assumptions contradicted by the available data. 相似文献
42.
Nagy AF Cravens TE Chen RH Taylor HA Brace LH Brinton HC 《Science (New York, N.Y.)》1979,205(4401):107-109
Datafrom the Pioneer Venus ion mass spectrometers are compared with model calculations of the ion density distributions appropriate for daytime conditions. The model assumes diffusive equilibrium upper boundary conditions for the major ions (O(2)(+), O(+), CO(2)(+), He(+), and H(+)); the agreement between the calculated and measured gross behavior of these ions is reasonably good except for H(+), which may be influenced strongly by convective transport processes. The distributions of five minor ions (C(+), N(+), NO(+), CO(+), and N(2)(+)) are also calculated for the chemically controlled region ( less, similar 200 kilometers); the agreements are, in general, poor, an indication that our present understanding of the Venus minor ion chemistry is still incomplete. 相似文献
43.
Hydrated and anhydrous crystals of 2-p-toluidinyl-6-naphthalene sulfonate exhibit fluorescence spectra similar to those obtained for 2-p-toluidinyl-6-naphthalene sulfonate in water and in organic solvents, respectively. The molecular structure of anhydrous 2-p-toluidinyl-6-naphthalene sulfonate suggests extended electron resonance over the whole molecule and this is one of the causes of the spectral differences. 相似文献
44.
More than 50 cores ranging in age from Pliocene to Lower Miocene have been recovered from the East Pacific Rise. Near the crestal regions the sediment cover is thin or lacking, and only Pleistocene sediments were recovered. On the flanks, the sediment thickness increases and pre-Pleistocene sediments are encountered. This pattern of increasing age and increasing sediment thickness away from the axis of the rise is in agreement with that predicted for spreading of the ocean floor. 相似文献
45.
Namoratunga, a megalithic site in northwestern Kenya, has an alignment of 19 basalt pillars that are nonrandomly oriented toward certain stars and constellations. The same stars and constellations are used by modern eastern Cushitic peoples to calculate an accurate calendar. The fact that Namoratunga dates to about 300 B.C. suggests that a prehistoric calendar based on detailed astronomical knowledge was in use in eastern Africa. 相似文献
46.
Hyman LH 《Science (New York, N.Y.)》1940,92(2379):105-106
47.
48.
Bailey LH 《Science (New York, N.Y.)》1903,17(429):441-454
49.
50.
Liu C Fu X Liu L Ren X Chau CK Li S Xiang L Zeng H Chen G Tang LH Lenz P Cui X Huang W Hwa T Huang JD 《Science (New York, N.Y.)》2011,334(6053):238-241
Periodic stripe patterns are ubiquitous in living organisms, yet the underlying developmental processes are complex and difficult to disentangle. We describe a synthetic genetic circuit that couples cell density and motility. This system enabled programmed Escherichia coli cells to form periodic stripes of high and low cell densities sequentially and autonomously. Theoretical and experimental analyses reveal that the spatial structure arises from a recurrent aggregation process at the front of the continuously expanding cell population. The number of stripes formed could be tuned by modulating the basal expression of a single gene. The results establish motility control as a simple route to establishing recurrent structures without requiring an extrinsic pacemaker. 相似文献