全文获取类型
收费全文 | 6209篇 |
免费 | 712篇 |
国内免费 | 146篇 |
出版年
2023年 | 155篇 |
2022年 | 89篇 |
2021年 | 188篇 |
2020年 | 253篇 |
2019年 | 305篇 |
2018年 | 277篇 |
2017年 | 272篇 |
2016年 | 273篇 |
2015年 | 268篇 |
2014年 | 336篇 |
2013年 | 448篇 |
2012年 | 261篇 |
2011年 | 296篇 |
2010年 | 245篇 |
2009年 | 311篇 |
2008年 | 348篇 |
2007年 | 361篇 |
2006年 | 258篇 |
2005年 | 245篇 |
2004年 | 221篇 |
2003年 | 166篇 |
2002年 | 192篇 |
2001年 | 159篇 |
2000年 | 124篇 |
1999年 | 122篇 |
1998年 | 94篇 |
1997年 | 70篇 |
1996年 | 73篇 |
1995年 | 60篇 |
1994年 | 58篇 |
1993年 | 47篇 |
1992年 | 46篇 |
1991年 | 65篇 |
1990年 | 32篇 |
1989年 | 34篇 |
1988年 | 32篇 |
1987年 | 36篇 |
1986年 | 26篇 |
1985年 | 29篇 |
1984年 | 28篇 |
1983年 | 13篇 |
1982年 | 47篇 |
1981年 | 22篇 |
1980年 | 16篇 |
1979年 | 12篇 |
1978年 | 8篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 11篇 |
1972年 | 12篇 |
排序方式: 共有7067条查询结果,搜索用时 156 毫秒
121.
122.
Wang Yusheng 《Plant Ecology》1992,98(1):83-95
(1) | The influence of sheep grazing of moderate grazing (MG), heavy grazing (HG), inordinate grazing (IG), over grazing (OG), and extinct grazing (EG) on vegetation succession was studied in the Stipa baicalensis steppe during 1984–1986. |
(2) | The relationships between the relative sum of dominance ratio (RSDR) and the degree of succession (DS) at all stages of grazing succession in the Stipa baicalensis steppe were revealed in terms of the method of mathematical analysis under five grazing intensities mentioned above. |
(3) | The dynamic models of grazing succession that are based on accurate grazing intensities in relation to succession courses were constructed using RSDR5, and DS. The models may explain the rules of grazing succession in the Stipa baicalensis steppe. |
123.
Machiguenga Indians of the Peruvian Amazon, like other low populationdensity, technologically “simple” peoples, produce ample food for a nutritious diet. Assuming that this is an intended outcome of their foodproduction strategy, to what extent is it a labor-efficient solution to the problem of producing a “balanced diet”? A linear-programming model of the “diet problem” is constructed with parameters reflecting the Machiguenga economy, and solutions are computed. These are then compared to observed Machiguenga food production; the degree of fit between model and behavior is examined and reasons for discrepancies are discussed. 相似文献
124.
A study of cultural transmission in Taiwan 总被引:1,自引:0,他引:1
Kuang -Ho Chen L. L. Cavalli-Sforza M. W. Feldman 《Human ecology: an interdisciplinary journal》1982,10(3):365-382
Our study of cultural transmission in Taiwan is based on a survey of 1000 students, their families, and friends, for characters ranging from religion to various customs and beliefs, as well as entertainment and hygienic habits. The effects of father, mother, and an older sib on a propositus are tested by an additive model of transmission, using a novel statistical procedure, and compared with correlations with friends. For many traits there exist significant influences; older sibs are almost as important as father and mother, with effects differing somewhat with their sex. Formulas for recurrences and equilibrium frequency of a cultural character for which father, mother, and sib are active in transmission are given in the appendices along with formulas for estimation of their effects from real data.This research was supported in part by grant NIH GM 20467 and NIH GM 20816. In early part of this investigation K.H.C. was supported by a grant to SIMS from the Alfred P. Sloan Foundation. 相似文献
125.
Erkki Oja 《Journal of mathematical biology》1982,15(3):267-273
A simple linear neuron model with constrained Hebbian-type synaptic modification is analyzed and a new class of unconstrained learning rules is derived. It is shown that the model neuron tends to extract the principal component from a stationary input vector sequence. 相似文献
126.
An investigation of the phosphorus loading characteristics of 31 southern African man-made was lakes made. The lakes were characterized by low water retention times, with most of the lakes having retention times of less than one year. Catchment phosphorus export rates showed wide variation (1–162 mg P m-2 y-1) with those lakes experiencing excessive municipal wastewater inputs having export rates in excess of 53 mg m-2 y-1. The phosphorus data were tested against the Vollenweider (1976) and Dillon & Rigler (1974) phosphorus budget models which predict in-lake steady state concentrations of phosphorus. It was found that both models displayed good potential for the prediction of steady state concentrations of phosphorus, with better results being obtained from the Dillon & Rigler (1974) model. However, because phosphorus concentrations within these lakes may not necessarily be related to trophic status the use of these models as a predictive tool for eutrophication control still requires further development. 相似文献
127.
Measurement of membrane potential inBacillus subtilis: A comparison of lipophilic cations,rubidium ion,and a cyanine dye as probes 总被引:6,自引:0,他引:6
Summary Two of the commonly used probes for measuring membrane potential—lipophilic cations and the cyanine dye diS-C3(5)—indicated nominally opposite results when tetraphenylarsonium ion was added as a drug to suspensions of metabolizingBacillus subtilis cells. [3H]-Triphenylmethylphosphonium uptake was enhanced by the addition, indicating hyperpolarization, yet fluorescence of diS-C3(5) was also enhanced, indicating depolarization. Evidence is presented that both effects are artifactual, and can occur without any change in membrane potential, as estimated by86Rb+ uptake in the presence of valinomycin. The fluorescence studies suggest that tetraphenylarsonium ion displaces the cyanine dye from the cell envelope, or other binding site, into the aqueous phase.The uptake characteristics of the radiolabeled lipophilic cations were quite unusual: At low concentrations (e.g., less than 10 m for triphenylmethylphosphonium) there was potential-dependent uptake of the label to a stable level, but subsequent addition of nonradioactive lipophilic cation caused further uptake of label to a new stable level. Labeled triphenylmethylphosphonium ion taken up to the first stable level could be displaced by 10mm magnesium ion, whereas86Rb+ uptake was unperturbed. Association of the lipophilic cations with the surface of de-energized cells was concentration-dependent, but there was no evidence for cooperative binding. This phenomenon of stimulated uptake inB. subtilis (which was not seen inEscherichia coli cells or vesicles) is consistent with a two-compartment model with access to the second compartment only being possible above a critical cation concentration. We tentatively propose such a model, in which these compartments are the cell surface and the cytoplasm, respectively.Triphenylmethylphosphonium up to 0.5mm exhibited linear binding to de-energized cells; binding of tetraphenylphosphonium and tetraphenylarsonium was nonlinear but was not saturated at the highest concentration tested (1mm). The usual assumption, that association of the cation with cell surfaces is saturated and so can be estimated on de-energized cells, therefore leads to undercorrected estimates of cytoplasmic uptake inB. subtilis, and hence to overestimates of membrane potential. We describe a more realistic procedure, in which the estimate of extent of binding is based on a mean aqueous concentration related both to the external concentration and to the much higher internal concentration that exists in energized cells. Using this procedure we estimate the membrane potential inB. subtilis to be 120 mV, inside-negative. The procedure is of general applicability, and should yield more accurate estimates of membrane potential in any system where there is significant potential-dependent binding.Work performed while on sabbatical leave from Department of Biology, Ben-Gurion University of the Negev, Beer-Sheva, Israel. 相似文献
128.
129.
Integral equation models for endemic infectious diseases 总被引:6,自引:0,他引:6
Summary Endemic infectious diseases for which infection confers permanent immunity are described by a system of nonlinear Volterra integral equations of convolution type. These constant-parameter models include vital dynamics (births and deaths), immunization and distributed infectious period. The models are shown to be well posed, the threshold criteria are determined and the asymptotic behavior is analysed. It is concluded that distributed delays do not change the thresholds and the asymptotic behaviors of the models.This work was partially supported by NIH Grant AI 13233. 相似文献
130.
Ebbe Thue Poulsen 《Journal of mathematical biology》1979,8(4):325-343
The life-cycle of a species with separate generations is divided into a reproduction phase and a growing-up phase. In the reproduction phase we assume random mating and selection due to genotype differences in fecundity of the parents and viability of the offspring. During the growing-up phase we assume a (deterministic) death process in continuous time with death rates for the genotypes which increase linearly with the genotype population sizes.In the absence of genotype differences the model gives logistic population regulation. With genotype differences the model generalizes the usual separate generations selection patterns. In addition to these we exhibit cases with three polymorphic equilibria or with a stable cycle. 相似文献