全文获取类型
收费全文 | 4490篇 |
免费 | 26篇 |
国内免费 | 62篇 |
专业分类
系统科学 | 32篇 |
丛书文集 | 83篇 |
教育与普及 | 64篇 |
理论与方法论 | 7篇 |
现状及发展 | 2000篇 |
研究方法 | 217篇 |
综合类 | 2153篇 |
自然研究 | 22篇 |
出版年
2012年 | 62篇 |
2011年 | 113篇 |
2010年 | 42篇 |
2009年 | 106篇 |
2008年 | 98篇 |
2007年 | 127篇 |
2006年 | 78篇 |
2005年 | 133篇 |
2004年 | 138篇 |
2003年 | 89篇 |
2002年 | 91篇 |
2001年 | 218篇 |
2000年 | 193篇 |
1999年 | 142篇 |
1998年 | 32篇 |
1997年 | 29篇 |
1996年 | 21篇 |
1995年 | 31篇 |
1992年 | 101篇 |
1991年 | 90篇 |
1990年 | 92篇 |
1989年 | 70篇 |
1988年 | 78篇 |
1987年 | 80篇 |
1986年 | 59篇 |
1985年 | 98篇 |
1984年 | 82篇 |
1983年 | 62篇 |
1982年 | 66篇 |
1981年 | 60篇 |
1980年 | 76篇 |
1979年 | 146篇 |
1978年 | 135篇 |
1977年 | 107篇 |
1976年 | 80篇 |
1975年 | 73篇 |
1974年 | 98篇 |
1973年 | 104篇 |
1972年 | 90篇 |
1971年 | 111篇 |
1970年 | 100篇 |
1969年 | 96篇 |
1968年 | 96篇 |
1967年 | 87篇 |
1966年 | 87篇 |
1965年 | 58篇 |
1959年 | 24篇 |
1958年 | 41篇 |
1957年 | 32篇 |
1956年 | 29篇 |
排序方式: 共有4578条查询结果,搜索用时 31 毫秒
991.
H. S. Lee Y. S. Kim S. B. Kim B. E. Choi B. H. Woo K. C. Lee 《Cellular and molecular life sciences : CMLS》1999,55(4):679-682
A mistletoe lectin was isolated from water extracts of Korean mistletoe, a subspecies of Viscum album, grown on Quercus mongolica using CM-Sepharose chromatography followed by an affinity chromatography on a concanavalin A-Sepharose column. The compound
proved to be a mistletoe lectin II with D-galactose and N-acetyl-D-galactosamine specificity. Matrix-assisted laser desorption time-of-flight mass spectroscopy showed it to have an
average molecular mass of 62.7 kDa and to consist of two subunits of 30.6 kDa and 32.5 kDa. It was a basic protein with isoelectric
points of 9.4 and 9.6 by capillary isoelectric focusing and was cytotoxic to Molt4 cell.
Received 17 November 1998; received after revision 3 March 1999; accepted 3 March 1999 相似文献
992.
993.
994.
金属丰度和对流超射对恒星演化影响的初步探讨 总被引:1,自引:0,他引:1
文章采用了pd90程序模拟了5 M⊙的恒星,在主序和主序后的演化情况。并且根据其程序给出的计算结果,绘出了5 M⊙恒星的赫罗图。选择金属丰度和对流超射的不问参数,形成六条赫罗图线。在赫罗图中,对5 M⊙的恒星其金属丰度和对流超射对恒星演化的影响进行讨论。从赫罗图中的lgL/L⊙和lgTeff,与演化时间序列图,可以直观的得出金属丰度和对流超射变化对恒星演化的影响。 相似文献
995.
Indian ricegrass ( Achnatherum hymenoides [Roem. & Schult.] Barkworth) is a rangeland species native to western North America with populations that may exhibit distinct seed-size morphs borne on separate plants. These morphs are often associated with very different levels of seed dormancy. We compared RAPD profiles of the relatively infrequent and highly dormant jumbo seed morph to its more germinable small or large (non-jumbo) companion seed morphs collected from 9 sites where they were sympatric. Jumbo seed accessions from 9 additional sites that did not exhibit seed polymorphism were included to better sample this seed morph across the species' range. Sympatric seed morphs were consistently distinguishable by their RAPD profiles, and sympatric seed morphs were no more likely to cluster together than nonsympatric seed morphs. The ability of sympatric seed morphs to maintain their genetic identity is compatible with the hypothesis of a predominately autogamous breeding system. Furthermore, these data support the hypothesis that local polymorphic populations result from overlapping waves of migration with distinct geographical origins. The jumbo seed morphs exhibited moderate correlation between genetic distance and geographic distance, but the non-jumbo seed morphs exhibited no such correlation. Jumbo seed morphs that were collected at or east of the Continental Divide in New Mexico and Colorado are more genetically homogeneous than those west of the Continental Divide, and non-jumbo seed morphs from the east are more genetically variable than jumbo seed morphs from the same locations. This suggests that gene flow eastward across the Rocky Mountains has been more frequent for the non-jumbo seed morphs than for the jumbo seed morphs. 相似文献
996.
Succession, nutrient cycling, production, and competition studies in plant communities require estimation of plant biomass. This is often accomplished by relating weight to easily measured plant dimensions via allometric equations. Dimensions of basal area and crown volume were used to predict phytomass of singleleaf pinyon ( Pinus monophylla Torr. & Frem.). Two regression methods for fitting the allometric equation to data for phytomass prediction were tested. These methods were the more commonly used logarithmic transformation of both data variables followed by linear regression analysis and an iterative nonlinear analysis without data transformation. The first was consistently less effective for predicting both individual tree and total plot phytomass for pinyon. Basal area was a better predictor of phytomass on a site than was crown volume. Prediction equations were highly site specific. Age of the trees in the stand used to derive the equations significantly affected the phytomass estimates. Other site and stand factors also appear to affect the phytomass-allometric parameter relationship. 相似文献
997.
Preparatory changes in neural activity before the execution of a movement have been documented in tasks that involve an instructed delay period (an interval between a transient instruction cue and a subsequently triggered movement). Such preparatory activity occurs in many motor centres in the brain, including the primary motor cortex, premotor cortex, supplementary motor area and basal ganglia. Activity during the instructed delay period reflects movement planning, as it correlates with parameters of the cue and the subsequent movement (such as direction and extent), although it occurs well before muscle activity. How such delay-period activity shapes the ensuing motor action remains unknown. Here we show that spinal interneurons also exhibit early pre-movement delay activity that often differs from their responses during the subsequent muscle activity. This delay activity resembles the set-related activity found in various supraspinal areas, indicating that movement preparation may occur simultaneously over widely distributed regions, including spinal levels. Our results also suggest that two processes occur in the spinal circuitry during this delay period: the motor network is primed with rate changes in the same direction as subsequent movement-related activity; and a superimposed global inhibition suppresses the expression of this activity in muscles. 相似文献
998.
我们利用月球侦察轨道器(LRO)的窄角相机(NAC)数据,使用一种新的光学遥感方法—相比率影像法,来评估月表亚分辨率尺度的粗糙度.我们用该方法对月面谷Vallis Schrteri地区的三维相比率影像进行分析后发现,在月面谷侧面上风化物的复杂运动模式与撞击坑并无关联,而是由风化物和岩屑形成的,而非冲击熔融. 相似文献
999.
The substrate- and inhibitor-related characteristics of monoamine oxidase (MAO) were studied on chick brain mitochondria. It was found that neither 5-hydroxytryptamine nor beta-phenylethylamine is the specific substrate for type A and type B MAO in chick brain. 相似文献
1000.