全文获取类型
收费全文 | 90170篇 |
免费 | 6437篇 |
国内免费 | 10445篇 |
专业分类
107052篇 |
出版年
2023年 | 1637篇 |
2022年 | 2244篇 |
2021年 | 2613篇 |
2020年 | 2629篇 |
2019年 | 3588篇 |
2018年 | 3159篇 |
2017年 | 2729篇 |
2016年 | 2610篇 |
2015年 | 2611篇 |
2014年 | 4532篇 |
2013年 | 6042篇 |
2012年 | 3585篇 |
2011年 | 4291篇 |
2010年 | 3376篇 |
2009年 | 4228篇 |
2008年 | 4332篇 |
2007年 | 4671篇 |
2006年 | 4199篇 |
2005年 | 3611篇 |
2004年 | 3167篇 |
2003年 | 2975篇 |
2002年 | 2680篇 |
2001年 | 2177篇 |
2000年 | 1849篇 |
1999年 | 1688篇 |
1998年 | 1598篇 |
1997年 | 1432篇 |
1996年 | 1322篇 |
1995年 | 1334篇 |
1994年 | 1299篇 |
1993年 | 1164篇 |
1992年 | 1097篇 |
1991年 | 1099篇 |
1990年 | 827篇 |
1989年 | 843篇 |
1988年 | 788篇 |
1987年 | 689篇 |
1986年 | 701篇 |
1985年 | 1122篇 |
1984年 | 1485篇 |
1983年 | 1035篇 |
1982年 | 1192篇 |
1981年 | 1024篇 |
1980年 | 970篇 |
1979年 | 860篇 |
1978年 | 680篇 |
1977年 | 645篇 |
1976年 | 588篇 |
1975年 | 421篇 |
1973年 | 458篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
71.
E. Cortés A.A. Kirk J.A. Goolsby P.J. Moran A.E. Racelis M.A. Marcos-García 《Biocontrol Science and Technology》2011,21(11):1369-1373
Arundo donax L. (Poaceae) is native to Mediterranean Europe and invasive in the Rio Grande Basin of North America. Rhizomes from nine sites in France and Spain infested with a candidate control agent, the armoured scale Rhizaspidiotus donacis (Hemiptera: Diaspididae) weighed 50% less than those from nine sites without scale. 相似文献
72.
Focal contacts of spreading platelets with the substratum 总被引:1,自引:0,他引:1
Contacts with glass substratum formed by the spreading rabbit platelets were examined by an antibody-exclusion method; monoclonal antibodies against 80 kD bovine serum protein were used. It was found that platelets form focal contacts in the course of spreading. The size of the largest focal contacts formed by platelets is smaller than that of the contacts formed by fibroblasts. The antibody-exclusion method revealed focal contacts of platelets much more clearly than interference reflection microscopy (IRM). The similarity of reactions involved in spreading platelets and of large nucleus-containing tissue cells is discussed. 相似文献
73.
The copy frequency distribution of a transposable element family in a Drosophila melanogaster natural population is generally characterised by the values of the Charlesworths' model parameters α and β (Charlesworth & Charlesworth, 1983). The estimation of these parameters is made using the observed distribution of the occupied sites in a population sample. Several results have been interpreted as due either to the influence of stochastic factors or to deterministic factors (transposition, excision, selection…). The accuracy of this method was tested by estimations performed on samples from simulated populations. The results show that with the sample size usually used for natural population studies, the confidence intervals are too large to reasonably deduce either the element copy number distribution or the values of transposition and excision rate and selective coefficients. 相似文献
74.
There is a major risk that many of the remaining semi-natural pastures in Swedish forest dominated regions will lose their grazing in the near future with lost biodiversity as a result. The reason is the high costs of grazing small pastures with cattle from generally small herds. The approaching decoupling of the present EU income support per head of cattle will increase the risk. Calculations based on economies of scale in beef production and opportunity cost of forest and arable land suggest that re-creating extensive pasture-forest mosaics consisting of existing semi-natural pastures and adjacent arable fields and forests can secure economically sustainable grazing. The risk of local extinction of grassland species due to habitat isolation is also lower in large mosaics than in small, scattered pastures. 相似文献
75.
Since the first North Sea Conference (1984, Bremen), all countries bordering the North Sea made commitments to reduce discharge of hazardous substances into the North Sea. From Belgium and The Netherlands, large reductions (upto 90) in heavy metal emissions from land-based sources have been reported between 1985 and 2000. Recently, some studies in the Western Scheldt estuary (WS) have shown that total metal concentration in the water, sediments and suspended particles have decreased compared to levels in the 70s. However, data on aquatic organisms is still very limited and it is therefore difficult to confirm whether the reductions in pollution input and generally improving water quality in the WS have a corresponding impact on the levels of heavy metals in aquatic organisms. The current study measured metal concentrations in the soft tissues of mussels, Mytilus edulis (known to be good indicators of environmental metal pollution) during the period 1996–2002. Spatial (salinity and pollution gradients), temporal and seasonal variations were also studied. Results showed a down-stream decreasing trend for the metals studied (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb and Zn) during all sampling campaigns. There was also a significant seasonal effect on tissue metal concentrations, with a peak observed around spring in both WS and the nearby less polluted Eastern Scheldt (ES). On temporal trends, a clear drop of metals in mussels was observed in the early 80s coinciding with the start of the efforts to reduce chemical pollution input into the North Sea. Since those early reductions, metal concentrations in mussels generally remained unchanged upto mid 90s. However, in recent times metal concentration in mussels have increased significantly, for example Cd in 2002 was almost 10 times the values in 1983 and similar to levels seen during the peak in the 70s. Other metals also increased in the 90s also reaching levels seen in the 70s. As there is no indication of recent increase in metal input into the estuary, we suggest that increased metal concentrations observed in mussels in recent years especially in the upper estuary are most likely a result of changes in physical and chemical speciation and metal bioavailability. Such changes may be caused by changes in some water quality parameters in the estuary (i.e. increased dissolved oxygen, concentration of organic matter), resulting in conditions that favour releases of sediment-bound metals into the water column. The relationship between metal content and season showed very similar annual profiles in the polluted WS and less polluted ES. Thus, seasonal variations in metal concentrations appear to be largely controlled by biological processes, while total body burdens are dependent on environmental levels and bioavailability. 相似文献
76.
77.
E. É. Kolesnikova 《Neurophysiology》2008,40(1):48-52
Results of a comparative study of the sensitivity of the system of respiratory control to increases in the CO2 concentration and the intensity of free-radical processes in young and elderly subjects are described. It is shown that normal
(natural) aging is accompanied by a decrease in the sensitivity of the respiratory system to hypercapnic stimulation and a
parallel significant decrease in the activity of catalase in the blood of examined subjects. Mechanisms responsible for the
modifications of the sensitivity of the system of respiratory control to hypercapnia are discussed; these shifts can be at
least partly related to changes in the intensity of production of free radicals observed in elderly subjects.
Neirofiziologiya/Neurophysiology, Vol. 40, No. 1, pp. 53–57, January–February, 2008. 相似文献
78.
Hiroyuki Akita 《Biocatalysis and Biotransformation》1996,13(3):141-156
Water-insoluble compounds can be substrates for enzymatic reactions when lipases are immobilized properly and suitable organic solvents are used. In this review, three type of lipase immobilization method and their application to the asymmetric syntheses of complex molecules are described. Lipases immobilized with Celite or synthetic prepolymers such as urethane prepolymer and photo-crosslinkable resin prepolymer have been applied for the kinetic resolution of many kinds of water-insoluble substrate.
Phospholipid-lipase aggregates with ether linkages are novel and have been found to function effectively as immobilized lipases in asymmetric hydrolysis or esterification reactions in water-saturated organic solvent. The phospholipid-lipase aggregates are considered to have a stacked bilayer based on X-ray diffraction analysis structure of the lipid in the crystalline phase. 相似文献
Phospholipid-lipase aggregates with ether linkages are novel and have been found to function effectively as immobilized lipases in asymmetric hydrolysis or esterification reactions in water-saturated organic solvent. The phospholipid-lipase aggregates are considered to have a stacked bilayer based on X-ray diffraction analysis structure of the lipid in the crystalline phase. 相似文献
79.
I. N. Konareva 《Neurophysiology》2006,38(3):201-208
Modifications of different EEG rhythms induced by a single neurofeedback session (by the EEG characteristics) directed toward
an increase in the ratio of the spectral powers (SPs) of the α vs θ oscillations were compared with the psychological characteristics of the tested subjects (the group included 30 persons).
A generally accepted neurofeedback technique was used; the intensity of acoustic white noise served as the feedback signal.
EEG potentials were recorded from the C3 and C4 leads. Psychological testing was carried out using Eysenck’s (EPQ), Rusalov’s
(OST), and (16 PF) questionnaires. The directions of changes in the SPs of EEG frequency components were found to significantly
correlate with some individuality-related peculiarities of the tested subjects. The SP of the δ rhythm correlated with the
EPQ scale “neuroticism,” OST scale “social plasticity,” and 16 PF factors H (“parmia”), I (“premsia”), and Q3 (“self-control of behavior”). The SP of the θ component demonstrated correlations with the OST scales “ergisity,” “plasticity,”
and “social temp” and with 16 PF factors M (“autia”), Q4 (“frustration”), and Q1 (“exvia”). The SP of the α rhythm correlated with 16 PF factors Q3 (“self-control of behavior”), G (“strength of superEgo”), O (“hypothymia”), L (“protension”), and N (“shrewdness”). The SP
of the β rhythm correlated with the OST scale “emotionality,” while that of the γ rhythm showed correlations with the 16 PF
indices L (“protension”) and M (“autia”). Changes in the ratio of the α vs θ SPs correlated with the EPQ scale “neuroticism.” Thus, our data confirm the statement that a high individual variability
of the results of a single (first in the series) neurofeedback session is to a great extent related to peculiarities of the
individual psychological pattern of the subject.
Neirofiziologiya/Neurophysiology, Vol. 38, No. 3, pp. 239–247, May–June, 2006. 相似文献
80.
D. F. Roberts 《International Journal of Anthropology》1996,11(2-4):185-191
As a conclusion, this paper reviews briefly the content of the volume. The wealth of demographic data has not been adequately
exploited in anthropology; this is why this publication is valuable in showing attempts to apply demographic data in a variety
of anthropological problems. This symposium has explored many interesting points which we recall here. Yet it has also opened
up a whole range of further questions on the material presented as well as in this broad field. Several directions of research
could be developed, for instance, testing among human populations, over long periods, the ecological thoughts of ecosystems
evolving as a cascade of instabilities, rather than a succession of equilibrium states. Let us also recall the pervasive nature
of demographic facts in topics such as the energy cycle or the genetic structure and evolution of human populations. 相似文献