全文获取类型
收费全文 | 5832篇 |
免费 | 455篇 |
国内免费 | 1075篇 |
专业分类
7362篇 |
出版年
2024年 | 30篇 |
2023年 | 118篇 |
2022年 | 133篇 |
2021年 | 170篇 |
2020年 | 235篇 |
2019年 | 219篇 |
2018年 | 227篇 |
2017年 | 217篇 |
2016年 | 233篇 |
2015年 | 233篇 |
2014年 | 329篇 |
2013年 | 512篇 |
2012年 | 233篇 |
2011年 | 305篇 |
2010年 | 257篇 |
2009年 | 336篇 |
2008年 | 319篇 |
2007年 | 335篇 |
2006年 | 343篇 |
2005年 | 264篇 |
2004年 | 242篇 |
2003年 | 239篇 |
2002年 | 190篇 |
2001年 | 182篇 |
2000年 | 140篇 |
1999年 | 132篇 |
1998年 | 100篇 |
1997年 | 84篇 |
1996年 | 96篇 |
1995年 | 92篇 |
1994年 | 75篇 |
1993年 | 91篇 |
1992年 | 56篇 |
1991年 | 91篇 |
1990年 | 44篇 |
1989年 | 46篇 |
1988年 | 38篇 |
1987年 | 36篇 |
1986年 | 42篇 |
1985年 | 35篇 |
1984年 | 47篇 |
1983年 | 37篇 |
1982年 | 34篇 |
1981年 | 31篇 |
1980年 | 20篇 |
1979年 | 19篇 |
1978年 | 19篇 |
1977年 | 13篇 |
1976年 | 11篇 |
1972年 | 7篇 |
排序方式: 共有7362条查询结果,搜索用时 0 毫秒
101.
102.
对分子置换法中积分半径选取方法的探讨 总被引:1,自引:0,他引:1
通过对晶胞中帕特逊向量的统计分布的研究,从理论上阐述了根据模型结构单元的线度确定旋转函数的积分半径这种作法的合理性,并且指出了估算积分半径取值范围的具体方法,以及自身旋转函数与交叉旋转函数的积分半径取值范围的区别。经过我们将此方法应用于酚胰岛素B链羰端六肽胰岛素的旋转函数求解,计算结果证实了这种积分半径的估算方法的可靠性。 相似文献
103.
The cross-immunoreactivity, topography, and fiber projections of the alpha MSH-immunoreactive specific neurons in the forebrain of the rat appear to be distinctly different from that of the neurons in the hypothalamic arcuate opiocortin system. The cell bodies, immunoreactive only to -MSH, have a specific pattern of distribution in the dorsal and lateral hypothalamic regions from the level of the retrochiasmatic region to the premammillary area of the posterior hypothalamus. Immunoreactive fibers of these cells appear to extend into regions of the cerebral cortex and hippocampus. An antomical relationship between the immunostained fibers and/or terminals of the arcuate opiocortin pool of neurons and the -MSH-immunoreactive perikarya is described utilizing the ABC (Avidin-Biotin-Peroxidase Complex) and ABC-GO (Glucose Oxidase) or glucose oxidase-antiglucose oxidase complex methods of immunocytochemistry in which two tissue antigens with contrasting colors are demonstrated in the same tissue section. 相似文献
104.
Georg R. Beilharz Carl R. Middlehurst Philip W. Kuchel Glenn E. Hunt Gordon F.S. Johnson 《Analytical biochemistry》1984,137(2):324-329
Detailed operating conditions are reported for the determination of choline in human erythrocytes using proton nuclear magnetic resonance spectroscopy in conjunction with the inversion-recovery spin-echo pulse sequence. The results of the NMR method were in excellent agreement with those obtained using an enzymatic (choline oxidase) assay; however, they were approximately three times higher than those reported using gas chromatography/mass spectrometry techniques. The differences may be partly due to the method of preparing or sampling cells since there is a distribution of choline in cells of different ages. However, choline levels were not affected by the methods used in the present study for storing or preparing cells. 相似文献
105.
Lauren Alfred Schroeder 《Entomologia Experimentalis et Applicata》1984,35(3):255-261
Gravimetric and a combination areal-gravimetric methods for determining dry matter budgets for leaf eating Lepidoptera were compared. The gravimetric method is based on dry weight/live weight ratios of the leaves fed to the larvae. In the areal-gravimetric method, the quantity of food offered to the larvae is determined from the area of leaf tracings and the dry matter content per unit area of the leaves. The areal-gravimetric method permits the use of larger leaf sprays and an open, gauze enclosed rearing chamber. There were no consistent differences in budget factors (growth, ingestion or egestion), nor were there any differences in the observed variability of the data attributable to the method used. However, the expected variability based on instrument precision for the gravimetric method is less than for the areal-gravimetric method. Experimental factors inherent in the gravimetric method introduce variability to the measurements that are not present in the areal method. Thirty to 60% of the variability in budget factors was attributed to intrinsic properties of the larvae, even though the larvae were taken from the same egg masses.
Résumé Les budgets en matière sèche consommée par des lépidoptères ont été comparés par les méthodes gravimétrique et planimétrique. La méthode gravimétrique est basée sur le rapport poids sec/poids frais de feuilles consommées par les chenilles. Avec la méthode planimétrique, la quantité d'aliment proposée aux chenilles est déterminée par les tracés de la surface des feuilles et le contenu de matière sèche par unité de surface des feuilles. La méthode de planimétrie permet l'utilisation de plus grands rameaux de feuilles et de cages d'élevage extérieures en gaze. Il n'y avait pas de différence appréciable dans les éléments du budget (croissance, ingestion et déjection), ni aucune différence dans la variabilité observée des données attribuable à la méthode utilisée. Cependant, la variabilité attendue d'après la précision des mesures avec la méthode gravimétrique est inférieure à celle de la méthode planimétrique. est inférieure à celle de la méthode planimétrique. Des éléments expérimentaux, inhérents à la méthode gravimétrique, introduisent une variabilité dans les mesures que l'on n'a pas avec la méthode planimétrique. 30–60% de la variabilité dans la consommation ont été attribués à des paramètres internes à la chenille, même quand elles provenaient toutes de la même ooplaque.相似文献
106.
Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars
Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The15N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N2 fixed in soybean than at 20 kg N/ha in the first year of the study. However, N2 fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N2 fixed as measured by the difference method. Further proof of differences in N2 fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N2 at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N2 at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The15N methodology offers a unique tool to evaluate germplasms directly in the field for their N2 fixation abilities at different N fertilizer levels. 相似文献
107.
Abstract A micro method for the isolation and characterization of the penicillin-binding sites in penicillin-binding proteins (PBPs) was developed. Only 10 nmol of a pure PBP are required for the whole procedure which is based on high-pressure liquid chromatography (HPLC). We showed that serine 44 in PBP 5 from Escherichia coli binds penicillin covalently. 相似文献
108.
Abstract The kinetic parameters of NH+ 4 -uptake in yeast cells were determined by a method that is based on the following changes in the external NH+ 4 concentration in cell suspensions by using NADH-dependent glutamate formation from NH+ 4 and 2-oxoglutarate. The kinetics of the observed NADH oxidation were analyzed by computer and enabled an estimation of V max and K m of the NH+ 4 -uptake system of the cells. 相似文献
109.
Dr. O. E. Millhouse 《Cell and tissue research》1978,191(3):473-491
Two sorts of neurons are recognized in Golgi impregnations of the rat ventromedial hypothalamic nucleus (HVM). The two cell types, category I and II neurons, are differentiated on the basis of their somatic, dendritic, and axonal characteristics. Category I neurons form most of the neuronal population and are located throughout HVM. The small number of category II neurons that have been studied occur in lateral HVM. Two varieties of neuronal profile, "common" and "uncommon cells", are seen in thin sections of HVM. The "uncommon cells", in comparison with the "common ones", appear to have a larger soma, a more electron-dense cytoplasmic matrix, an abundance of Nissl bodies, and a population of dense-cored vesicles (100--130 nm in diameter). Some of the somata and proximal dendrites of "common", but not "uncommon" cells, are wrapped in multiple layers of astrocytic processes. Although the correlation is tentative, it is argued that category I neurons correspond to "common cells" and category II, to "uncommon cells". One possible implication of this correspondence is discussed regarding neuronal alteration in response to change in the endocrinological environment of the brain. 相似文献
110.
A probability model for the number of complete conceptions (that is, live births) taking into account foetal wastages, occurring
in a couple during a specified period of time (T0, T0+T) is developed assuming that the data was collected starting a long time after marriage. A method of estimating some of
the underlying parameters is given. The model is applied to data obtained in a Varanasi Survey in 1969–70. 相似文献