全文获取类型
收费全文 | 3279篇 |
免费 | 154篇 |
国内免费 | 1篇 |
出版年
2021年 | 19篇 |
2018年 | 26篇 |
2017年 | 34篇 |
2016年 | 50篇 |
2015年 | 76篇 |
2014年 | 72篇 |
2013年 | 124篇 |
2012年 | 140篇 |
2011年 | 141篇 |
2010年 | 94篇 |
2009年 | 57篇 |
2008年 | 133篇 |
2007年 | 120篇 |
2006年 | 111篇 |
2005年 | 123篇 |
2004年 | 128篇 |
2003年 | 114篇 |
2002年 | 120篇 |
2001年 | 67篇 |
2000年 | 61篇 |
1999年 | 65篇 |
1998年 | 26篇 |
1997年 | 27篇 |
1996年 | 34篇 |
1995年 | 45篇 |
1994年 | 38篇 |
1993年 | 33篇 |
1992年 | 66篇 |
1991年 | 68篇 |
1990年 | 63篇 |
1989年 | 62篇 |
1988年 | 50篇 |
1987年 | 50篇 |
1986年 | 50篇 |
1985年 | 63篇 |
1984年 | 56篇 |
1983年 | 41篇 |
1982年 | 43篇 |
1981年 | 42篇 |
1980年 | 31篇 |
1979年 | 37篇 |
1978年 | 33篇 |
1977年 | 40篇 |
1976年 | 20篇 |
1975年 | 29篇 |
1974年 | 32篇 |
1973年 | 26篇 |
1971年 | 17篇 |
1970年 | 13篇 |
1969年 | 13篇 |
排序方式: 共有3434条查询结果,搜索用时 328 毫秒
51.
In 1980, the seasonal changes in nitrogen and phosphorus concentration of various plant parts of Nymphoides peltata (Gmel.) O. Kuntze, together with aspects of nitrogen and phosphorus cycling by this species were studied in an oxbow lake of the river Waal (The Netherlands). The nitrogen and phosphorus stores of the water, seston, sediment and macrophyte compartments were assessed each month.The underground Nymphoides structures had high nitrogen and phosphorus concentrations before and after the main growing season, while during summer the aboveground plant parts had high nutrient contents. Nymphoides peltata accumulated maximum amounts of nitrogen (334 mmol m−2) and phosphorus (56.6 mmol m−2) in July. The upper layers of the bottom appeared to be an enormous nutrient reservoir (94–99% of total) of which the largest part was not directly available to Nymphoides. Nutrient uptake from the sediments by N. peltata is suggested by the fact that the bottom and/or interstitial water of the sample station devoid of rooted macrophytes, contained higher concentrations of nitrogen and phosphorus than that of the Nymphoides stands. The annual flux of nutrients from Nymphoides to the detritus compartment was estimated to be ca. 1200 mmol nitrogen and 164 mmol phosphorus per m2 of littoral. During breakdown of the detritus there was a relatively fast net conversion of organically bound nitrogen and phosphorus to inorganic forms, especially at higher temperatures.Nymphoides has the potential to function as an important nitrogen and phosphorus pump, which regenerates sediment nutrients. 相似文献
52.
Henriette M. C. Put H. van Doren W. R. Warner J. Th. Kruiswijk 《Journal of applied microbiology》1972,35(1):7-27
S ummary : This review summarizes the work on the subject carried out mainly in the authors'laboratories but also in the associated laboratories of Metal Box Co. Ltd., London, and Plat Manufaktor, Malmö, Sweden. Special attention is paid to the mechanism of bacterial reinfection and how it is influenced by deviations in can construction or can handling procedures. Methods of preventing bacterial reinfection at the most critical points in the canning operation are considered and certain guiding principles are derived. 相似文献
53.
Homoserine kinase from Escherichia coli K-12: properties, inhibition by L-threonine, and regulation of biosynthesis 总被引:4,自引:1,他引:3 下载免费PDF全文
We have partially purified homoserine kinase from a genetically derepressed strain of Escherichia coli K-12. The optimum pH of the enzyme-substrate reaction was 7.8 and the K(m) values for l-homoserine and adenosine 5'-triphosphate were both 3 x 10(-4) M. K(+) (or NH(4) (+)) as well as Mg(2+) were required for its activity. The sedimentation coefficient determined by ultracentrifugation in a sucrose density gradient was 5.0 +/- 0.25S. l-Homoserine was an excellent protector against heat inactivation of homoserine kinase. l-Threonine was a competitive inhibitor of homoserine kinase, suggesting that end-product inhibition of this enzyme plays a role in vivo in the overall regulation of threonine biosynthesis. The specific activity of aspartokinase I-homoserine dehydrogenase I and of homoserine kinase showed a strong positive correlation in extracts from strains under widely varying conditions of genetic or physiological derepression; it was concluded that these two enzymes are coordinately regulated in E. coli K-12. 相似文献
54.
Fine structure of the dorsal ocellus of the worker honeybee 总被引:1,自引:0,他引:1
The three dorsal ocelli of worker honeybees have been studied by light and electron microscopy. Each ocellus has a single flattened spheroidal lens and about 800 elongated retinular cells. Retinular cells are paired and form a two-part plate-like rhabdom between their distal processes. Each rhabdomere comprises parallel microvilli projecting laterally from the apposed retinular cells. Primary receptor cell axons synapse within the ocellus with ocellar nerve fibers of two different calibers. Each ocellus has eight thick fibers ca 10 m?m in diameter and several thinner ones less than 3 m?m in diameter. Fine structural evidence suggests that retinular axons end presynaptically on both types of ocellar nerve fibers. Since all retinular cells apparently synapse repeatedly with the thick fibers this involves a convergence of about 100:1. Thick fibers always terminate postsynaptically within the ocellus while thin fibers terminate presynaptically on other thin fibers, thick fibers or retinular axons. Structural evidence for synaptic polarization indicates that retinular cells and thick fibers are afferent, thin fibers efferent. Thus complex processing of the ocellar visual input can occur before the secondary neurons of the three ocelli converge to form the single short ocellar nerve which runs to the posterior forebrain. 相似文献
55.
The existence of a soluble complex formed by polyuridylic acid (poly (U)) and 3′,5′-cyclic AMP (cAMP) is demonstrated by u.v. extinction vs. temperature curves, optical rotation, equilibrium dialysis, and reaction calorimetry. The complex hasthe stoichiometry of 2 poly (U)-cAMP and its formation is accompanied by an enthalpy change of ?13.0 kcal/mole of base triplet. The introuction of an empirical factor α in the equations given by Damle2 and Crothers2 leads to the evolution of a ΔH value of ?13.4 keal/mole. The parameter α is considered as a correction factor for the concentration dependence of the binding process. There is no relation between α and the reduction of monomer activity due to self-association of monomers. The study of the binding process at several temperatures showed that the cooperativity parameter, σ, is independent of temperature and its value of 6.5 × 10?3 is in good agreement with σ = 5 × 10?3 for the poly (U)·poly(A) system.3 相似文献
56.
H. J. Th. Goos 《Cell and tissue research》1969,97(1):118-124
Summary In Xenopus laevis tadpoles the relation between a paired nucleus of bio-amine producing neurons in the caudal hypothalamus and the pars intermedia of the hypophysis was studied.Treatment of the animals (stage 49 to 50 of Nieuwkoop and Faber's normal table) with reserpine caused aggregation of the skin melanophores within one hour, followed by redispersion five to six hours after the beginning of the experiment. This was at exactly the same time as the bio-amines in the caudal hypothalamus disappeared. However, the drug was ineffective if the nuclei had been removed. This indicates that reserpine acts via these nuclei and does not influence the skin melanophores directly.It was concluded that the initial aggregation of the melanophores may be the result of a reduced extrusion of MSH from the pars intermedia, caused by an increased output of a MIF by the bio-amine producing nuclei. The redispersion was explained by assuming that the bio-amines were depleted and the nuclei stopped with the extrusion of the MIF. This does not mean that the production of a MIF is the only function of the paired bio-amine producing nucleus in the caudal hypothalamus.The author thanks Prof. Dr. P. G. W. J. van Oordt for his helpful comments and criticism. Mr. J. H. I. J. M. ten Berge and Mr. E. W. A. Kamperdijk provided great assistance during the course of the experiments. Mr. H. van Kooten made the diagram and the photograph. 相似文献
57.
58.
Th. Butterfass 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1968,38(7):294-297
Zusammenfassung Bei diploiden Zuckerrüben entsprechen die Zellgrößen des Mesophylls, die durch Endotetraploidisierung entstehen, im Mittel dem Erfordernis für einen optimalen Ertrag. Bei tetraploiden Zuckerrüben jedoch werden die Zellen der meisten Pflanzen endooktoploid und damit zu groß. Es ist nicht ratsam, die Endopolyploidie der Tetraploiden ganz wegzüchten zu wollen; nur eine Verminderung ihres Ausmaßes erscheint vorteilhaft.
Endopolyploidy and yield in diploid and tetraploid sugar beets II
Summary In diploid sugar beet the mesophyll cell sizes, arising from endotetraploidization, are well adapted to optimal yield requirements. In tetraploid sugar beet, however, the mesophyll cells of most plants become endooctoploid and hence too large. Trying to eliminate all endopolyploidy from tetraploids is not advisable, but some reduction of its extent seems advantageous.相似文献
59.
60.