全文获取类型
收费全文 | 3466篇 |
免费 | 155篇 |
国内免费 | 396篇 |
出版年
2023年 | 37篇 |
2022年 | 63篇 |
2021年 | 91篇 |
2020年 | 94篇 |
2019年 | 114篇 |
2018年 | 70篇 |
2017年 | 98篇 |
2016年 | 89篇 |
2015年 | 101篇 |
2014年 | 148篇 |
2013年 | 261篇 |
2012年 | 135篇 |
2011年 | 189篇 |
2010年 | 141篇 |
2009年 | 202篇 |
2008年 | 205篇 |
2007年 | 221篇 |
2006年 | 160篇 |
2005年 | 138篇 |
2004年 | 138篇 |
2003年 | 95篇 |
2002年 | 99篇 |
2001年 | 78篇 |
2000年 | 70篇 |
1999年 | 78篇 |
1998年 | 52篇 |
1997年 | 56篇 |
1996年 | 64篇 |
1995年 | 65篇 |
1994年 | 60篇 |
1993年 | 53篇 |
1992年 | 48篇 |
1991年 | 37篇 |
1990年 | 41篇 |
1989年 | 31篇 |
1988年 | 26篇 |
1987年 | 30篇 |
1986年 | 32篇 |
1985年 | 31篇 |
1984年 | 39篇 |
1983年 | 27篇 |
1982年 | 50篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 24篇 |
1978年 | 15篇 |
1977年 | 17篇 |
1976年 | 11篇 |
1973年 | 11篇 |
1971年 | 6篇 |
排序方式: 共有4017条查询结果,搜索用时 140 毫秒
111.
Asymptotic solutions for the effectiveness factor and the concentration profile are obtained for mth-order chemical reactions inside a slab catalyst pellet with Robin boundary condition at the pellet's outer surface. Using perturbation analysis in the limit of large reaction order m, the effectiveness factor and the concentration profile are explicitly determined up to O(1/m). Higher-order solutions can be obtained in a systematic way if desired. 相似文献
112.
R. G. DAVIES K. L. BORATYSKI 《Biological journal of the Linnean Society. Linnean Society of London》1979,12(2):95-165
A quantitative comparison of the classifications of 24 species of male Diaspididae by principal component and principal coordinate analysis was extended to allow the comparison of classifications based on selected subsets of the 101 available characters. It was shown that subsets of 25–28 characters could be chosen numerically and used to construct classifications that resembled very closely the classification based on the reference set of characters. The conclusions are discussed in relation to other views on character-weighting and in relation to classifications of the Diaspididae based on traditional taxonomic characters provided by the females. 相似文献
113.
M. R. SETHURAJ 《Plant, cell & environment》1981,4(1):81-83
Abstract A theoretical analysis of yield components of Hevea brasiliensis is attempted in this paper. The effect of the major yield components, i.e. initial flow rate per unit length of tapping cut, length of the cut, percentage rubber content and plugging index on rubber yield is represented by the formula Variation in yield within and between clones can be ascribed to variation to any one of the above components. The importance of high growth rate for maintaining high yield throughout the life cycle of the tree is theoretically elucidated. While the present contention of a theoretical maximum yield of 9.5 t ha?1 with a stand of 350 trees is questioned, the theoretical possibility of attaining that yield by increasing the stand per ha to 600 is analysed. 相似文献
114.
The occurrence of mesosomes was investigated during septum formation of vegetative and sporulating cells of Bacillus cereus. It has been demonstrated that bacterial mesosomes which are considered by numerous microbiologists as an integrated constituent of Gram positive bacteria, are in reality artifacts arising during the preparation for electron microscopy. The conventional fixation methods allowed enough time for the cytoplasmic membrane to react to the changed conditions and to form the typical pocket-like membrane invaginations. With cryofixation followed by freeze-substitution it was shown in ultrathin sections that mesosomes do not occur. The extremely rapid freezing and the substitution of the ice by an organic solvent containing the fixative prevented the formation of membraneous artifacts.Non-standard abbreviations OsO4
osmium tetroxide
- UO2Ac
uranylacetate
- PHB
poly--hydroxy-butyric acid
- M
mesosome
- CW
cell wall
- CM
cytoplasmic membrane
- PF
plasmatic fracture of the cytoplasmic membrane 相似文献
115.
The rate of inhibition of cyclic photophosphorylation in chloroplast thylakoids by the arginine reagent phenylglyoxal was enhanced in the light, i.e., under conditions where membrane energization occurred. Uncouplers, but not energy-transfer inhibitors, prevented the effect of light. Chemical modification of chloroplast thylakoids by phenylglyoxal under dark or in light conditions affected differently the light-induced exchange of tightly bound ADP. In both cases the exchange was less inhibited than photophosphorylation. Complete inhibition of ATPase activity of soluble CF1 was correlated with the incorporation of 8 mol [14C]phenylglyoxal per mol enzyme. About 50% of the incorporated radioactivity was lost at different rates depending on the buffer present and suggesting a change in the stoichiometry of the adduct from 2:1 to 1:1. Inhibition of ATPase and photophosphorylating activities of chloroplasts by modification with [14C]phenylglyoxal in the dark was associated with the incorporation of 1 and 2 mol reagent per mol membrane-bound CF1, respectively. In the light the rate of incorporation was enhanced and both reactions were inactivated when 2 mol were bound. In all the labelling experiments the radioactivity was mainly recovered from the α- and β-subunits. 相似文献
116.
R. L. Jantz C. H. Hawkinson H. Brehme H. W. Hitzeroth 《American journal of physical anthropology》1982,57(3):311-321
This paper addresses the question of the extent to which finger ridge-count data are useful features with which to study population variation in Subsaharan Africa. Each subject was represented by a vector of 20 ridge-counts, a radial and an ulnar count for each digit. Such data were available from 11 African groups, nine of which were indigenous Africans, and two, the South African Colored and South African Indians, contained a portion of non-African ancestory. The ridge-counts were first transformed to principal component scores and these were subjected to multivariate analysis of variance and distance analysis to elucidate intergroup variation. The primary findings were that ridgecounts provide a good reflection of variation on at least two levels, that of African versus non-African, and variation among Africans. Also, the principal components that reveal variation at these two levels are very different. We conclude that ridge-counts can only be useful in population studies if full account is taken of their multicomponent nature. 相似文献
117.
118.
为进一步阐明大肠杆菌AE 109青霉素G酰化酶(PA,E.C.3.5.1.11)的结构与功能关系,研究了数种修饰剂对酶活性的影响;同时测定了四种作用物存在下对各修饰剂修饰酶的影响。结果表明Ser残基处于酶的活性部位,Met残基可能处于与底物结合的部位,His和Cys残基与酶的活性无关。 相似文献
119.
B. Markert 《Plant Ecology》1991,95(2):127-135
To get a first impression of the degree of pollution in the Forest Biosphere Reserve 350 km northwest of Moscow, samples of Polytrichum commune and peat profiles were measured by ICP/MS after decomposition by high-pressure ashing. Comparing the results with data found in similar systems in Central Europe the Forest Biosphere Reserve can be described than medium-polluted by heavy metals. Pb shows lower values as in comparable systems in Germany, due to the fact of lower traffic density in Sowjetunion. The status of the nutritional elements Ca, Mg and K is in the same order of magnitude as found in German forest ecosystems. 相似文献
120.
Gerard J. Niemann Jos B. M. Pureveen Gert B. Eijkel Hendrik Poorter Jaap J. Boon 《Oecologia》1992,89(4):567-573
Summary Eleven grass species varying in potential relative growth rate (RGR) were investigated for differences in chemical composition by pyrolysis mass spectrometry. The spectral data revealed correlations between RGR and the relative composition of several biopolymers. Species with a low potential RGR contained relatively more cell wall material such as lignin, hemicellulose, cellulose, polysaccharide-bound ferulic acid and hydroxyproline-rich protein, whereas species with a high potential RGR showed relatively more cytoplasmic elements such as protein (other than those incorporated in cell walls) and sterols. 相似文献