全文获取类型
收费全文 | 3628篇 |
免费 | 225篇 |
国内免费 | 163篇 |
出版年
2024年 | 10篇 |
2023年 | 36篇 |
2022年 | 53篇 |
2021年 | 80篇 |
2020年 | 88篇 |
2019年 | 121篇 |
2018年 | 112篇 |
2017年 | 93篇 |
2016年 | 95篇 |
2015年 | 75篇 |
2014年 | 177篇 |
2013年 | 197篇 |
2012年 | 133篇 |
2011年 | 216篇 |
2010年 | 166篇 |
2009年 | 214篇 |
2008年 | 251篇 |
2007年 | 246篇 |
2006年 | 262篇 |
2005年 | 204篇 |
2004年 | 172篇 |
2003年 | 155篇 |
2002年 | 142篇 |
2001年 | 80篇 |
2000年 | 80篇 |
1999年 | 75篇 |
1998年 | 87篇 |
1997年 | 50篇 |
1996年 | 49篇 |
1995年 | 38篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 40篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 14篇 |
1988年 | 6篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 12篇 |
1984年 | 14篇 |
1983年 | 5篇 |
1982年 | 8篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 9篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 3篇 |
排序方式: 共有4016条查询结果,搜索用时 15 毫秒
51.
本文研究结果表明低剂量的氦氖激光可以提高绵羊精清中GOT和LDH酶的活性,并对其机制作了初步的探讨。 相似文献
52.
Relative levels of hexokinase (ATP:D-hexose 6-phosphotransferase, EC 2.7.1.1) have been determined in 16 discrete regions of adult rat brain by a quantitative immunofluorescence method. The distribution of immunofluorescence in brain sections was determined by interactive laser cytometry and related to hexokinase content by comparison with standard sections containing known amounts of the enzyme. In many of these regions, referred to here as group I regions, hexokinase content was correlated with previously reported basal rates of glucose utilization. However, several regions (group II regions) in which hexokinase content exceeded that expected from basal rates of glucose utilization were also detected. Compared with the corresponding regions from albino rat brain, higher hexokinase levels were found in the dorsal and ventral lateral geniculate (group I regions) of pigmented Norway rats, a result reflecting previously reported increased glucose utilization by these regions in pigmented rats. There was no difference in hexokinase levels in the superior colliculus, a group II structure, from albino and pigmented rats, a finding implying that a reported increase in rate of glucose utilization in the superior colliculus of pigmented rats is effected without an increase in hexokinase content. It is suggested that group II regions may be adapted to sustain increases in rates of glucose utilization that are, relative to basal rates, considerably greater than those experienced by group I regions. 相似文献
53.
Summary Studies were conducted in 22 non-calcareous soils (India) to evaluate various extractants,viz. (6N HCl, 0.1N HCl, EDTA (NH4)2CO3, EDTA NH4OAc, DTPA+CaCl2 and 1M MgCl2) to find critical levels of soil and plant Zn for green gram (Phaseolus aureus Roxb.). The order of extractability by the different extractants was 6N HCl>0.1N HCl>EDTA (NH4)2CO3<EDTA NH4OAc DTPA+CaCl2>1M MgCl2.
Critical levels of 0.48 ppm DTPA × CaCl2 extractable Zn, 0.80 ppm EDTA NH4OAc extractable Zn, 0.70 ppm EDTA (NH4)2CO3 extractable Zn, and 2.2 ppm 0.1N HCl extractable Zn were estimated for the soils tested. The critical Zn concentration in 6 weeks old plants was found to
be 19 ppm. The 0.1N HCl method gave the best correlation (r=0.588**) between extractable Zn and Bray's per cent yield, while with DTPA+CaCl2, it was slightly low (r=0.542**). The DTPA + CaCl2 method gave significant (r=0.73**) correlation with plant Zn concentration. The 0.1N HCl gave the higher correlation with Zn uptake (r=0.661**) than DTPA (r=0.634**) 6N HCl and 1M MgCl2 method gave nonsignificant positive relationship with Bray's per cent yield. For noncalcareous soils apart from the common
use of DTPA+CaCl2, 0.1N HCl can also be used for predicting soil available Zn. The use of 0.1N HCl would be much cheaper than DTPA and other extractants used in the study. 相似文献
54.
Roger C. Anderson Anthony E. Liberta Joseph Packheiser Melvin E. Neville 《Plant and Soil》1980,54(1):149-152
Summary In an incubation study addition of green manure caused a reduction in the ammonia volatilized initially from both sodic and reclaimed soils but extensive volatilization occurred from the sodic soil, amended with green manure, after the tenth day till the conclusion of the experiment after 9 weeks. Volatilization from the reclaimed soil was much less. There was a slight build up of organic carbon and ammoniacal nitrogen in both the soils though greater in the reclaimed soil. More of nitrate and nitrite accumulated in the sodic soil. 相似文献
55.
Energy requirements for growth and maintenance of Scenedesmus protuberans Fritsch in light-limited continuous cultures 总被引:1,自引:0,他引:1
Scenedesmus protuberans Fritsch was grown in light-limited continuous cultures with a light-dark cycle, at temperatures of 20° and 28° C. At 20° irradiances of 12 and 38 W m–2 were used, at 28° 38 W m–2.The relationships between growth rate and light uptake rate were of diphasic linear character. With the lower growth rates the relationships were defined with the parameters
e
, i.e. the specific maintenance rate constant, and c, the true efficiency of light energy conversion into biomass. The
e
-value was dependent on temperature, the c on irradiance.In cultures, incubated in prolonged darkness, decrease rates of biomass were comparable to the derived
e
-values.Both diphasic linear relationships between growth rate and light uptake rate and the same order of magnitude of
e
-values could be derived from literature data on other green algae. 相似文献
56.
Irradiation of the kinetochore region of PtK2 chromosomes by laser light of 532 nm was used to study the function of the kinetochore region in chromosome movement and
to create artificial micronuclei in cells. When the sister kinetochores of a chromosome were irradiated at prometaphase, the
affected chromosome detached from the spindle and exhibited no further directed movements for the duration of mitosis. The
chromatids of the chromosome remained attached to one another until anaphase, at which point they separated. No poleward movement
of the chromatids was observed, and at telophase they passively moved to one of the daughter cells and were enclosed in a
micronucleus. The daughter cell containing the micronucleus was then isolated by micromanipulation and followed through subsequent
mitoses. At the next mitosis, two chromosomes, each with two chromatids, condensed in the micronucleus. These chromosomes
did not attach to the spindle and showed chromatid separation, but no poleward movements at anaphase. They were again enclosed
in micronuclei at telophase. The third generation mitosis was similar to the second.
Occasionally, both the irradiation-produced and naturally occurring micronuclei exhibited no chromosome condensation at mitosis.
Feulgenstained monolayers of PtK2 cells with naturally occurring micronuclei showed that some micronuclei stain positive for DNA and others do not. This finding
raises questions about the fate of chromosomes in a micronucleus. 相似文献
57.
A system is described consisting of a mode-locked Ar ion laser and time-resolved photon-counting electronics. The system is capable of measuring fluorescence lifetimes in the subnanosecond time domain. The Ar ion laser is suitable for the excitation of flavins, since the available laser wavelengths encompass the first absorption band of the yellow chromophore. Due to the high radiation density and the short pulse, both the time and wavelength resolution of the fluorescence of very weakly emitting compounds can be measured. Experiments have been described for flavin models exhibiting single and multiple modes of decay. In these examples lifetimes were determined both from deconvolved decay curves and from direct analysis of the tail of the curve, where no interference of the exciting pulse is encountered. Both determinations showed very good agreement. Due to the highly polarized laser light the decay of the emission anisotropy could be measured directly after the exciting pulse. In principle, fast rotational motions might be detected. An anisotropy measurement conducted with a flavoprotein with a noncovalently attached FAD is presented. 相似文献
58.
Kinetic studies on cis-[Pt(NH3)2(OH2)2]2+ and various nucleobases show that this ion reacts more quickly with guanosine than with adenosine, cytidine, and thymidine, and that a monophosphoric acid unit considerably enhances the rate of reaction of guanosine; the kinetic preference of 5'-GMP over 5'-AMP may point to a greater thermodynamic selectivity. 相似文献
59.
Urban and periurban forests, which are integrated within the concept of Green Infrastructure, provide important Ecosystem Services, including air purification. In this study, we quantified the Ecosystem Service of particulate matter (PM10) and Ozone (O3) removal from urban and periurban forests in ten metropolitan cities in Italy, and its total monetary value. In order to gain a better understanding of how Ecosystem Services can be regulated on a wider scale, the vegetation ecosystem types were grouped into Physiognomic-Structural Categories of Vegetation according to morphofunctional criteria. The pollution removal was mapped using a remote sensing and GIS approach, by applying a deposition model and a stomatal flux model. We estimated, for the ten metropolitan cities, an overall pollution abatement of 7150 Mg of PM10 and 30,014 Mg of O3 in the year 2003, which was an extremely hot year. Our findings indicate that structural characteristics (i.e. Leaf Area Index) and functional diversity, linked to stomatal conductance, exert a marked influence on the provision of the regulating Ecosystem Services, whose total monetary value was estimated to be equal to 47 and 297 million USD for PM10 and O3 removal, respectively. This study represent the first national-scale assessment of the Ecosystem Services of air pollution removal in Europe, thus providing information that may be useful to stakeholders to manage Green Infrastructure more efficiently. 相似文献
60.
The degree of technological change biased to the environmental factor is crucial to industrial sustainable development. Using the stochastic frontier analysis method based on the translog production function and the panel data of 32 industrial sub-sectors in Shanghai over 1994–2011, this paper combines the evolution dynamic of the frontier technological structure with the evolution dynamic of technological change direction to estimate the output elasticities of production factors and the growth rate of green total factor productivity. Also, we investigate and compare the degrees of technological change biased to four production factors, i.e., capital, labor, energy, and carbon emissions. The results show that the industrial green total factor productivity in Shanghai presents an overall upward trend and mainly depends on the technical efficiency change. The improvements of labor productivity, R&D intensity, and energy efficiency can effectively enhance the green technical efficiency, while capital deepening has a mitigation effect on the green technical efficiency. The technological change of Shanghai's industrial production biases to energy use and capital saving, causing a high energy demand of industrial development. Under the dual impacts of economic development and energy-saving and emission-reduction policies, the degree of technological change biased to the environmental factor (carbon emissions) displays strong and weak alternations, indicating that the green bias of industrial technological change in Shanghai is not stable and that the green transformation of industrial development model needs to be further advanced. 相似文献