全文获取类型
收费全文 | 46609篇 |
免费 | 3152篇 |
国内免费 | 4808篇 |
专业分类
54569篇 |
出版年
2023年 | 747篇 |
2022年 | 1022篇 |
2021年 | 1162篇 |
2020年 | 1131篇 |
2019年 | 1789篇 |
2018年 | 1598篇 |
2017年 | 1287篇 |
2016年 | 1167篇 |
2015年 | 1128篇 |
2014年 | 2292篇 |
2013年 | 2872篇 |
2012年 | 1764篇 |
2011年 | 2157篇 |
2010年 | 1627篇 |
2009年 | 2118篇 |
2008年 | 2190篇 |
2007年 | 2417篇 |
2006年 | 2165篇 |
2005年 | 1758篇 |
2004年 | 1498篇 |
2003年 | 1482篇 |
2002年 | 1320篇 |
2001年 | 1139篇 |
2000年 | 947篇 |
1999年 | 821篇 |
1998年 | 788篇 |
1997年 | 746篇 |
1996年 | 734篇 |
1995年 | 729篇 |
1994年 | 719篇 |
1993年 | 630篇 |
1992年 | 634篇 |
1991年 | 626篇 |
1990年 | 477篇 |
1989年 | 493篇 |
1988年 | 440篇 |
1987年 | 436篇 |
1986年 | 414篇 |
1985年 | 661篇 |
1984年 | 890篇 |
1983年 | 720篇 |
1982年 | 751篇 |
1981年 | 599篇 |
1980年 | 610篇 |
1979年 | 544篇 |
1978年 | 432篇 |
1977年 | 403篇 |
1976年 | 371篇 |
1974年 | 243篇 |
1973年 | 255篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
1. The working hypothesis that neuropeptide gene expression in a neuron is an indicator of that neuron's physiological activity is discussed. 2. Representative examples from the literature are presented to support the hypothesis. 3. Further, we discuss the regulation of expression of two opioid peptides, preproenkephalin and preprodynorphin, in laminae I and II of the spinal cord and in nucleus caudalis of the trigeminal nuclear complex, where they may play a role in pain modulation. 4. The expression of the opioid peptide genes can be induced by both painful and nonnoxious stimuli in neurons in time-dependent and sensory-specific fashions. 相似文献
82.
P. Lieberman 《Human Evolution》1988,3(1-2):3-18
The evolution of human speech and syntax, which appear to be the defining characteristics of modern human beings, is discussed. Speech depends on the morphology of the mouth, tongue, and larynx which yield the human «vocal tract», and neural mechanisms that facilitate the perception of speech and make possible the control of the articulatory gestures that underly speech. The neural mechanisms that underly human syntax may have derived by means of the Darwinian process of preadaption from the structures of the brain that first evolved to facilitate speech motor control. Recent data consistent with this theory are presented; deficits in the comprehension of syntax of normal aged people are correlated with a slowdown in speech rate. 相似文献
83.
The intrinsic rates of increase of insects of different sizes 总被引:2,自引:0,他引:2
K. J. GASTON 《Ecological Entomology》1988,13(4):399-409
ABSTRACT.
- 1 A negative relationship between intrinsic rate of increase, r, and body size has only clearly been shown using data for species drawn from a number of phyla and covering several orders of magnitude in size. Analyses for more closely related species are equivocal.
- 2 Data for ninety-one species of insects, from nine orders, were used to examine the correlation between intrinsic rate of increase and size.
- 3 Intrinsic rate of increase was negatively correlated with both length and weight across orders, but no relationship could be shown within orders.
- 4 Generation times were positively related to body size, but there was no relationship between net reproductive rate (RQ) and size.
- 5 These results support the hypothesis that documented relationships between species size and colonization success in insects could be a consequence of the scaling of intrinsic rate of increase with size.
84.
Summary In a geographically wide distribution the life cycles of different populations of the cabbage moth Mamestra brossicae are adapted to a remarkable diversity of climatic conditions. This is undoubtedly a proof of its success in adaptation. Some populations living in regions characterized by a drought period interrupting the growth season are capable of distinguishing between one critical day length signalling the onset of the drought period and another signalling the end of the growth season. This study, therefore, is primarily concerned with the geographical patterns in the variability of the adaptional responses of populations exposed to environmental conditions requiring different strategies and tactics in, synchronizing individual, life cycles. It is also a contribution to our understanding of evolutionary mechanisms maintaining median responses to photoperiodically inductive day lengths in geographically different populations. The populations investigated originated from regions differing in predictability of the incidence, onset and duration of a drought period: Freiburg (48.0°N, Southern Germany), Avignon (44.0°N, Southern France), and Argelès (42.5°N, Southern France). Geographical variation with respect to both onset and duration of a drought period consequently results in clinal variation of the variability of innate day length thresholds triggering aestival dormancy and of innate duration of aestivation. In this paper we considered the influence of geographically changing temperatures on aestival dormancy induction. Even in southern populations of M. brassicae a temperature dependent switch off-mechanism exists which prevents aestival dormancy under certain environmental conditions. The effective temperatures vary geographically, too. What the geographical patterns in adaptive responses really are, is discussed.This research was supported by the Deutsche Forschungsgemeinschaft (Sa 259/3-1) 相似文献
85.
Manickam Sugumaran Steven J. Saul Victor Semensi 《Archives of insect biochemistry and physiology》1988,9(4):269-281
The mechanism of formation of quinone methide from the sclerotizing precursor N-acetyldopamine (NADA) was studied using three different cuticular enzyme systems viz. Sarcophaga bullata larval cuticle, Manduca sexta pharate pupae, and Periplaneta americana presclerotized adult cuticle. All three cuticular samples readily oxidized NADA. During the enzyme-catalyzed oxidation, the majority of NADA oxidized became bound covalently to the cuticle through the side chain with the retention of o-diphenolic function, while a minor amount was recovered as N-acetylnorepinephrine (NANE). Cuticle treated with NADA readily released 2-hydroxy-3′,4′-dihydroxyacetophenone on mild acid hydrolysis confirming the operation of quinone methide sclerotization. Attempts to demonstrate the direct formation of NADA-quinone methide by trapping experiments with N-acetylcysteine surprisingly yielded NADA-quinone-N-acetylcysteine adduct rather than the expected NADA-quinone methide-N-acetylcysteine adduct. These results are indicative of NADA oxidation to NADA-quinone and its subsequent isomerization to NADA-quinone methide. Accordingly, all three cuticular samples exhibited the presence of an isomerase, which catalyzed the conversion of NADA-quinone to NADA-quinone methide as evidenced by the formation of NANE—the water adduct of quinone methide. Thus, in association with phenoloxidase, newly discovered quinone methide isomerase seems to generate quinone methides and provide them for quinone methide sclerotization. 相似文献
86.
The relative appearance of the parietal muscles in the development of the zooids has been studied in several ctenostomatous and “cheilostomatous” species. A comparison of the different on-togenetical sequences demonstrated that a “cheilostomatous” type of organization of the zooids with a great probability has been achieved in minimum three times independentl and originated from different ctenostomatous sub-grous: the Membranidea from plesiomorgic victorelloids (ancestors of Bulbella with not yet developed peristomial tube), the Inoviceiata (Aetea) from advanced forms of victorelloids with reduced primary parietal muscles (perhaps stcies related to Pottsiella), and Penetrantia from arachnidioid or vesicularioid ancestors (?). Therefore, the classical orders α“Ctenostomata” and “Cheilostomata” represent only “stage groups” but no monohyletic systematical units. Because of the new concept and interpretation I propose a new name for the united group: Cteno-Cheilostomata, supra-ord. nov. 相似文献
87.
88.
Abstract Listeriolysin, an SH-activated haemolysin probably involved in Listeria pathogenicity, has been cloned into the cosmid vector pHC79 and was expressed in Escherichia coli HB101 cells. Chromosomal DNA of Listeria monocytogenes serovar 1/2 was partially digested with Mbo I and ligated to the Bam HI cleaved cosmid. From 2000 recombinant clones examined, 12 (0.6%) produced haemolysin in solid and liquid media. All of them contained chromosome fragments of Listeria of about 40 kb. The cloning of the listeriolysin determinant will lead to a better understanding of the basis of Listeria pathogenicity. 相似文献
89.
Diploschistes hensseniae
Lumbsch & Elix is described as new to science. It is characterized by its terricolous habitat, perithecioid closed ascomata, relatively small spores, cylindrical asci, and the presence of diploschistesic and orsellinic acids in addition to lecanoric acid. It occurs on soil in arid regions of Australia. 相似文献
90.
Bernhard Knig Patricia A. DiNitto Peter M. Blumberg 《Journal of cellular biochemistry》1985,29(1):37-44
The major phorbol ester receptor is the Ca++-activated, phospholipid-dependent protein kinase C. Diacylglycerol stimulates protein kinase C in a fashion similar to the phorbol esters. Likewise, it inhibits phorbol ester binding competitively. Both results suggest that diacylglycerol is the/an endogenous phorbol ester analogue. Alternatively, the diacylglycerol might simply be acting to modify the phospholipid environment of the protein. If diacylglycerol were indeed functioning as an analogue, it should interact with the receptor stoichiometrically. This interaction can be quantitated by measuring the perturbation in apparent diacylglycerol binding affinity as a function of the ratio of diacylglycerol to receptor. We report here that 1,2-dioleoylglycerol interacts with the receptor with the predicted stoichiometry. 相似文献