全文获取类型
收费全文 | 10096篇 |
免费 | 811篇 |
国内免费 | 511篇 |
出版年
2024年 | 21篇 |
2023年 | 193篇 |
2022年 | 303篇 |
2021年 | 326篇 |
2020年 | 345篇 |
2019年 | 425篇 |
2018年 | 429篇 |
2017年 | 315篇 |
2016年 | 405篇 |
2015年 | 483篇 |
2014年 | 518篇 |
2013年 | 776篇 |
2012年 | 362篇 |
2011年 | 425篇 |
2010年 | 375篇 |
2009年 | 489篇 |
2008年 | 533篇 |
2007年 | 472篇 |
2006年 | 484篇 |
2005年 | 422篇 |
2004年 | 359篇 |
2003年 | 327篇 |
2002年 | 303篇 |
2001年 | 194篇 |
2000年 | 173篇 |
1999年 | 166篇 |
1998年 | 164篇 |
1997年 | 162篇 |
1996年 | 141篇 |
1995年 | 135篇 |
1994年 | 98篇 |
1993年 | 109篇 |
1992年 | 84篇 |
1991年 | 79篇 |
1990年 | 65篇 |
1989年 | 71篇 |
1988年 | 62篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 78篇 |
1984年 | 103篇 |
1983年 | 81篇 |
1982年 | 72篇 |
1981年 | 49篇 |
1980年 | 55篇 |
1979年 | 45篇 |
1978年 | 16篇 |
1977年 | 5篇 |
1974年 | 5篇 |
1973年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The sex ratio of the Arkansas 1 isolate of Heterodera glycines was determined in experiments in which ''Lee'' soybean was inoculated with either one or two larvae. A 3:1 male to female sex ratio was established for this isolate under the test conditions used. No influence of one nematode on the penetration and development to adult of another nematode in the same root was detected in double larval inoculations. 相似文献
32.
ABSTRACT The study of plant-herbivore dynamics was carried out using Aphis rumicis L. The insect is oligophagous to the genus Rumex. It utilizes its food plant by means of phloem sucking. Here, in this study, organic nitrogen as a form of a pellet fertilizer was used as a limiting factor. The major aim of the study is to find out the tripartite relationship among soil nitrogen concentration, the plant, and the insect. Observations were made from both the natural habitat site and the manipulated field site. The result shows that Aphis rumicis responded positively to increased N concentration of leaves (p< 0.005). The application of organic nitrogen did not alter the pattern of the population dynamics of Aphis rumicis over the study period. However, it may determine the time for the optimal exploitation of resources by the insect. It is also suggested that the growth efficiency of the insect relates to the C: N ratio in the phloem. 相似文献
33.
Jonathan A. Newman 《Global Change Biology》2004,10(1):5-15
Experimental evidence regarding the responses of cereal aphids to rising atmospheric CO2 has been ambiguous. Some studies suggest increased population sizes under future CO2 levels, others suggest decreased population sizes, and still others suggest little or no difference. Recently, Newman et al. (2003) constructed a general mathematical model of the aphid–grass interaction to investigate whether or not we should, in fact, expect a general aphid response to rising CO2. They concluded that aphid populations are likely to be larger under future CO2 concentrations if soil N levels are high, the aphid species' nitrogen requirement is low and the aphid species' density‐dependent response in winged morph production is weak. In that model, and in field experiments, CO2 concentration influences aphid population dynamics through the effect it has on plant quality. However, future CO2 concentrations are also likely to be accompanied by higher ambient temperatures, a combination that has received little focus to date. In the present paper, the Newman et al. model is used to consider the combined effects of increased CO2 concentrations and temperature on aphid population sizes. It is concluded that, when both factors are elevated, aphid population dynamics will be more similar to current ambient conditions than expected from the results of experiments studying either factor alone. This result has important implications for future experimentation. 相似文献
34.
Parameter estimation techniques for interaction and redistribution models: a predator-prey example 总被引:2,自引:0,他引:2
Summary The use of parameter estimation techniques for partial differential equations is illustrated using a predatorprey model. Whereas ecologists have often estimated parameters in models, they have not previously been able to do so for models that describe interactions in heterogeneous environments. The techniques we describe for partial differential equations will be generally useful for models of interacting species in spatially complex environments and for models that include the movement of organisms. We demonstrate our methods using field data from a ladybird beetle (Coccinella septempunctata) and aphid (Uroleucon nigrotuberculatum) interaction. Our parameter estimation algorithms can be employed to identify models that explain better than 80% of the observed variance in aphid and ladybird densities. Such parameter estimation techniques can bridge the gap between detail-rich experimental studies and abstract mathematical models. By relating the particular bestfit models identified from our experimental data to other information on Coccinella behavior, we conclude that a term describing local taxis of ladybirds towards prey (aphids in this case) is needed in the model. 相似文献
35.
A. Lebeda T. Jendrulek 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,75(1):194-199
Summary In gene-for-gene systems, specificity of hostparasite interactions is most often estimated qualitatively using the symbols +, –, (i.e. susceptibility and/or resistance). In large sets (interaction patterns) it becomes impossible to analyze numerous data by mere comparison. This is overcome by application of cluster analysis. In our experiments the methods in question were used to estimate the data obtained in a study on interactions between more than 220 Lactuca sativa cultivars and 12 Bremia lactucae physiological races (isolates) of Czechoslovak origin. The matrix of similarity coefficients was analyzed by hierarchical clustering. Similarity and/or dissimilarity of host R-genotypes was graphically expressed using the method of two principal components. The results obtained are related to genetic constitution of race specific resistance of the host and the possibility of predicting effective resistance sources. 相似文献
36.
Iodinated vinculin, metavinculin and α-actinin were used to probe the interaction of these proteins with electrophoretically separated cytoskeletal proteins. Using the gel overlay technique, we detected strong binding of 125I-vinculin and 125I-metavinculin to α-actinin, 175 kDa polypeptide, talin, vinculin and metavinculin themselves, and moderate binding to actin.125I-α-actinin was capable of interacting with vinculin and metavinculin. The specific binding of 125-I-α-actinin to vinculin and metavinculin immobilized on a polysterene surface was also demonstrated. We suggest that the ability of vinculin and α-actinin to form a complex may be realized in microfilament-membrane linkages. 相似文献
37.
Summary Physical parameters of membrane bilayers were studied for their effect on the binding of hematoporphyrin derivative (Hpd), which is used as a sensitizer in photodynamic therapy of cancerous tissues. The purpose of this study was to clarify which parameters were relevant, under physiological conditions, to the selectivity of Hpd binding to cancer cells. Fluorescence spectroscopy was used to measure the relative partitioning of the dye between the lipid and aqueous media. Increasing the microviscosity of the liposomes' membranes by various bilayer additives results in a strong reduction of Hpd binding, to an extent independent of the specific additive. The effect of temperature near the physiological value as well as the effect of cross membrane potential are small. Surface potential does not affect the binding constant, indicating that the binding species does not carry a net electric charge. 相似文献
38.
Michael A. Keller 《Entomologia Experimentalis et Applicata》1987,43(1):55-59
Walking speeds of female Trichogramma exiguum Pinto & Platner were fastest on maize and soybean (12 cm/min), intermediate on tomato (8 cm/min), and slowest on woolly mullein, Verbascum thapsus (3 cm/min). Similarly, rates of turning along the paths of walking T. exiguum were smallest on maize (median angle=0°±15°), intermediate on soybean and tomato, and greatest on V. thapsus (median angle=30°±15°). Leaf trichome density and morphology influenced walking behavior. Walking was slowed and flight initiation delayed for T. exiguum walking on Amaranthus hybridus leaves compared to either maize or filter paper. When inundative releases are conducted, the effects of plant surfaces on searching rates and arrestment should be considered in determining release rates of Trichogramma spp.
Résumé La vitesse de marche a été déterminée en lâchant des individus sur chaque substrat végétal et en traçant leur parcours sur une plaque de verre placée à 9 mm au dessus du sujet. Les déviations angulaires de portions successives des tracés, longues de 1 mm, ont été utilisées pour mesurer les taux de changement de direction. Dans une deuxième expérience, des individus ont été lâchés au milieu d'une cercle de 40 mm de diamètre sur chaque substrat et les temps écoulés avant l'envol ou pour atteindre le bord du cercle à la marche ont servi à évaluer respectivement la propension au vol et la vitesse de déambulation.Les vitesses de marche ont été plus rapides sur maïs et soja (12 cm/min), moyennes sur tomate (8 cm/min) et les plus lentes sur Verbascum thapsus (Scrophulariaceae) (3 cm/min). De le même façon, les angles de changement de direction au cours des marches effectuées par T. exiguum ont été plus petits sur maïs, moyens sur soja et tomate, et plus grands sur V. thapsus. Chez T. exiguum marchant sur des feuilles d'Amaranthus hybridus L. (Amaranthaceae), la marche a été plus lente et l'envol plus tardif que sur maîs ou papier filtre.Lors de lâchers inondatifs, les effets des surfaces végétales sur les vitesses d'exploration et d'arrêt devraient être pris en compte pour déterminer les vitesses de lâcher des Trichogramma spp.相似文献
39.
Parathyroid hormone (PTH), the plasma concentration of which is raised in uremia, has been suggested as one of the agents responsible for the myocardial changes commonly seen in uremia. The effect of intact [1–84] PTH on rat heart cells grown in tissue culture has been studied. Addition of the hormone to the media significantly stimulated beating rate. The stimulation was directly proportional to the amount of PTH in the medium. Excessively high concentration of PTH caused immediate cessation of the beating, which was reversed by the addition of calcium to the medium. The extent of stimulation by PTH was inversely proportional to the calcium concentrations in the medium. Isoproterenol and phenylephrine at excessively high concentrations in the medium did not mimic the PTH effect either alone or together with PTH. When beating ceased due to verapamil the effect was not reversed by the addition of calcium to the medium.Calcium added to the myocytes seen after beating ceased reversed the effect and the cells started to beat again. Cells kept for a longer period in the arrested state were not revived by the addition of calcium. 相似文献
40.
Leaf-specific thionins of barley (Hordeum vulgare L.) have been identified as a novel class of cell-wall proteins toxic to plant-pathogenic fungi and possibly involved in the defence mechanism of plants. The distribution of these polypeptides has been studied in the host-pathogen system of barley and Erisyphe graminis DC.f.sp. hordei Marchal (powdery mildew). Immunogold-labelling of thionins in several barley cultivars indicates that resistance or susceptibility may be attributed to the presence or absence of thionins at the penetration site in walls and papillae of epidermal leaf cells.All of the leaf-specific thionin genes are confined to the distal end of the short arm of chromosome 6 of barley. None of the genes for cultivarspecific resistance to powdery mildew which have previously been mapped on barley chromosomes are found close to this locus. 相似文献