全文获取类型
收费全文 | 4503篇 |
免费 | 342篇 |
国内免费 | 244篇 |
专业分类
5089篇 |
出版年
2024年 | 8篇 |
2023年 | 84篇 |
2022年 | 111篇 |
2021年 | 144篇 |
2020年 | 137篇 |
2019年 | 140篇 |
2018年 | 138篇 |
2017年 | 137篇 |
2016年 | 141篇 |
2015年 | 183篇 |
2014年 | 193篇 |
2013年 | 313篇 |
2012年 | 141篇 |
2011年 | 174篇 |
2010年 | 165篇 |
2009年 | 167篇 |
2008年 | 212篇 |
2007年 | 198篇 |
2006年 | 168篇 |
2005年 | 157篇 |
2004年 | 163篇 |
2003年 | 142篇 |
2002年 | 142篇 |
2001年 | 117篇 |
2000年 | 110篇 |
1999年 | 107篇 |
1998年 | 89篇 |
1997年 | 115篇 |
1996年 | 101篇 |
1995年 | 104篇 |
1994年 | 78篇 |
1993年 | 104篇 |
1992年 | 78篇 |
1991年 | 74篇 |
1990年 | 59篇 |
1989年 | 50篇 |
1988年 | 74篇 |
1987年 | 37篇 |
1986年 | 37篇 |
1985年 | 37篇 |
1984年 | 45篇 |
1983年 | 16篇 |
1982年 | 21篇 |
1981年 | 8篇 |
1980年 | 18篇 |
1979年 | 12篇 |
1976年 | 6篇 |
1975年 | 8篇 |
1974年 | 4篇 |
1973年 | 11篇 |
排序方式: 共有5089条查询结果,搜索用时 8 毫秒
81.
Synaptic transmission is the key system for the information transfer and elaboration among neurons. Nevertheless, a synapse is not a standing alone structure but it is a part of a population of synapses inputting the information from several neurons on a specific area of the dendritic tree of a single neuron. This population consists of excitatory and inhibitory synapses the inputs of which drive the postsynaptic membrane potential in the depolarizing (excitatory synapses) or depolarizing (inhibitory synapses) direction modulating in such a way the postsynaptic membrane potential. The postsynaptic response of a single synapse depends on several biophysical factors the most important of which is the value of the membrane potential at which the response occurs. The concurrence in a specific time window of inputs by several synapses located in a specific area of the dendritic tree can, consequently, modulate the membrane potential such to severely influence the single postsynaptic response. The degree of modulation operated by the synaptic population depends on the number of synapses active, on the relative proportion between excitatory and inbibitory synapses belonging to the population and on their specific mean firing frequencies. In the present paper we show results obtained by the simulation of the activity of a single Glutamatergic excitatory synapse under the influence of two different populations composed of the same proportion of excitatory and inhibitory synapses but having two different sizes (total number of synapses). The most relevant conclusion of the present simulations is that the information transferred by the single synapse is not and independent simple transition between a pre- and a postsynaptic neuron but is the result of the cooperation of all the synapses which concurrently try to transfer the information to the postsynaptic neuron in a given time window. This cooperativeness is mainly operated by a simple mechanism of modulation of the postsynaptic membrane potential which influences the amplitude of the different components forming the postsynaptic excitatory response. 相似文献
82.
H. L. Xu J. Lopez F. Rachii N. Tremblay L. Gauthier Y. Desjardins A. Gosselin 《Physiologia plantarum》1996,96(4):722-726
Sulphate accumulates in the rhizosphere of plants grown in hydroponic systems. To avoid such sulphate accumulation and promote the use of environmentally sound hydroponic systems, we examined the effects of four sulphate concentrations (0.1, 5,2, 10.4 and 20.8 m M ) on photosynthesis, ribulose-l,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) activities and related physiological processes in greenhouse–grown tomato plants ( Lycopersicon esculentum Mill. cv. Trust). The lowest sulphate concentration (0.1 m M ) significantly decreased photosynthetic capacity (Pc ) and Rubisco activities on a leaf area basis. This result was supported by our data for dry matter per plant, which was low for plants in the 0.1 m M treatment. The photosynthesis-related variables such as leaf conductance, chlorophyll and soluble protein were lowest for the 0.1 m M treatment. Both total Rubisco activity and the activated ratio were reduced with this treatment. However, Rubisco activities expressed per g of protein or per g of chlorophyll were not significantly affected. These results suggest that sulphur deficiency depressed Pc – by reducing the amount of both Rubisco and chlorophyll and by causing an inactivation of Rubisco. The ratio of organic sulphur vs organic nitrogen (S/N) in plants of the 0.1 m M treatment was far below the normal values. This low S/N ratio might be accountable for the negative effect of low sulphate on Pc and plant growth. Pc and dry matter were not affected until sulphate concentration in the nutrient solution reached a high level of 20.8 m M . 相似文献
83.
Haruya Takahashi Kosuke Kamakari Hideyuki Suzuki Shinsuke Mohri Tsuyoshi Goto Nobuyuki Takahashi 《Bioscience, biotechnology, and biochemistry》2013,77(10):1761-1764
We previously reported that the two peroxisome proliferator-activated receptor-α agonists, 9- and 13-oxo-octadecadienoic acids (oxo-ODAs), were found in the tomato fruit. However, their localization remains unknown. Herein, we showed that oxo-ODAs localize primarily in the fruit peel and their amount increases after the homogenization of the tomato fruit. 相似文献
84.
Jouke P. Kardolus Herman J. van Eck Ronald G. van den Berg 《Plant Systematics and Evolution》1998,210(1-2):87-103
Using the AFLP technique highly informative DNA fingerprints were generated from 19 taxa ofSolanum sect.Petota (potatoes) and three taxa ofSolanum sect.Lycopersicum (tomatoes). Both phenetic and cladistic analyses were conducted from the individual genotypic level to the species level. An AFLP fingerprint, using a combination of suitable AFLP primers, generated 12 to 71 scorable fragments per genotype which was sufficient for taxonomic interpretation. The classifications based on the molecular markers were generally in agreement with current taxonomic opinions. Unexpectedly,S. microdontum was associated with ser.Megistacroloba rather than with ser.Tuberosa, andS. demissum (ser.Demissa) and species of ser.Acaulia appeared closely affiliated. AFLP is an efficient and reliable technique to generate biosystematic data and therefore a promising tool for evolutionary studies. 相似文献
85.
G. R. Rodríguez L. Sequin G. R. Pratta R. Zorzoli L. A. Picardi 《Biologia Plantarum》2008,52(3):548-552
Pericarp polypeptide profiles were analyzed at three ripening stages in the F1 hybrid and the F2 population from the cross between the accessions: LA1385 (Lycopersicon esculentum var. cerasiforme) and 804627 (L. esculentum, a homozygous genotype for the nor mutant). Six polymorphic polypeptides were observed in LA1385, while no polymorphic polypeptides among ripening stages was
observed in 804627. On the other hand, some polypeptides in the F1 hybrid were not observed in the parents whereas others were present in both parental genotypes and were unnoticeable in the
hybrid genotype. From a cluster analysis on the protein profiles of the F2 population, the differential expression of proteins allowed to distinguish mature green (MG) stage from the others two stages,
while for breaker stage (BR) and red ripe stage, the genetic background was more important in forming groups. The differential
expression of proteins could be associated with fruit morphology traits such as a 72 kDa polypeptide present in MG stage with
fruit diameter, height and mass and a 47 kDa polypeptide found in BR with fruit shelf life. 相似文献
86.
87.
88.
89.
90.
Ian Towle Joel D. Irish Kris H. Sabbi Carolina Loch 《American journal of primatology》2022,84(1):e23349
Dental caries has been reported in a variety of primates, although it is still considered rare in wild populations. In this study, 11 catarrhine primate taxa (n = 339 individuals; 7946 teeth) were studied for the presence of caries. A differential diagnosis of lesions in interproximal regions of anterior teeth was undertaken, since they had been previously described as both carious and non-carious in origin. Each permanent tooth was examined macroscopically, with severity and position of lesions recorded. Two specimens were examined further, using micro-CT scans to assess demineralization. Differential diagnosis confirmed the cariogenic nature of interproximal cavities on anterior teeth (ICATs). Overall results show 3.3% of all teeth (i.e., anterior and posterior teeth combined) were carious (n = 262), with prevalence varying among species from 0% to >7% of teeth affected. Those with the highest prevalence of ICATs include Pan troglodytes verus (9.8% of anterior teeth), Gorilla gorilla gorilla (2.6%), Cercopithecus denti (22.4%), Presbytis femoralis (19.5%), and Cercopithecus mitis (18.3%). ICATs make up 87.9% of carious lesions on anterior teeth. These results likely reflect dietary and food processing differences among species, but also between the sexes (e.g., 9.3% of all female P. troglodytes verus teeth were carious vs. 1.8% in males). Processing cariogenic fruits and seeds with the anterior dentition (e.g., wadging) likely contributes to ICAT formation. Further research is needed in living primate populations to ascertain behavioral/dietary influences on caries occurrence. Given the presence of ICATs in frugivorous primates, their diagnosis in archaeological and paleontological specimens may shed light on diet and food processing behaviors in fossil primates. 相似文献