全文获取类型
收费全文 | 42498篇 |
免费 | 3026篇 |
国内免费 | 5711篇 |
出版年
2024年 | 189篇 |
2023年 | 913篇 |
2022年 | 1146篇 |
2021年 | 1410篇 |
2020年 | 1423篇 |
2019年 | 2004篇 |
2018年 | 1612篇 |
2017年 | 1450篇 |
2016年 | 1368篇 |
2015年 | 1323篇 |
2014年 | 2138篇 |
2013年 | 3086篇 |
2012年 | 1715篇 |
2011年 | 2015篇 |
2010年 | 1549篇 |
2009年 | 2022篇 |
2008年 | 1991篇 |
2007年 | 2186篇 |
2006年 | 1930篇 |
2005年 | 1761篇 |
2004年 | 1563篇 |
2003年 | 1417篇 |
2002年 | 1286篇 |
2001年 | 1015篇 |
2000年 | 898篇 |
1999年 | 821篇 |
1998年 | 802篇 |
1997年 | 703篇 |
1996年 | 598篇 |
1995年 | 570篇 |
1994年 | 546篇 |
1993年 | 498篇 |
1992年 | 487篇 |
1991年 | 466篇 |
1990年 | 355篇 |
1989年 | 347篇 |
1988年 | 354篇 |
1987年 | 282篇 |
1986年 | 316篇 |
1985年 | 541篇 |
1984年 | 637篇 |
1983年 | 351篇 |
1982年 | 510篇 |
1981年 | 496篇 |
1980年 | 428篇 |
1979年 | 332篇 |
1978年 | 241篇 |
1977年 | 253篇 |
1976年 | 217篇 |
1973年 | 188篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
971.
972.
Size, age and number of offspring at the first and subsequent tworeproductions were compared in Daphnia treated with cues frominvertebrate predators (adult Cyclopoida fed Daphnia)and from crushed conspecifics. Daphnia in the presence ofinvertebrate predators had earlier maturation, greater size atfirst and subsequent reproductions and a larger number of offspringthan control animals, while the opposite (later maturation andsmaller size at subsequent reproductions) happened when Daphnia were treated with homogenate from crushedconspecifics. 相似文献
973.
Several aspects of community organization wereanalyzed comparatively in a small side-arm of theParaná River (Correntoso) and a shallowfloodplain lake (El Tigre) (31° 41 S and60° 42 W), in relation to the hydrology of thesystem. Taxonomic and morphological composition inthe river differed from that in the lake: the riverhad lower species richness (151 vs 218),different contributions of some Classes to totalspecies number (higher Cyano-, Zygo- andDiatomophyceae vs higher Chlorophyceae), anddiffent proportions of nannoplanktonic algae (67.5%vs 80.7%) and netplanktonic filamentousspecies (18.2% vs 4.2%). Phytoplanktonbiomass, higher in the lake than in the river due tothe retention time, was mostly dominated bynannoplankton and netplankton. Loticphytoplankton was dominated by typical fluvialspecies of Diatomophyceae (R-strategists). Riverconditions seem to maintain a subclimacticcommunity, which was little impacted by the flushingof populations from floodplain lakes. Water levelwas the main factor controlling phytoplanktonbiomass, species diversity (H), evenness (E) andcommunity change rate () in the river. Inthe lake, phytoplankton had an autogenicsuccessional sequence during the isolation phase (C-to S-strategists) and other responses todisturbance, mainly during the flood(R-strategists). Frequent changes in phytoplanktoncomposition, biomass, H, E and , revealed aenvironmental instability in the lake, which may beexplained by interactions of external factors(hydrology and climatology) and those of internalorigin, such as nutrients and grazing. 相似文献
974.
I determined whether selection of breeding sites by thered-neckedgrebe Podiceps grisegena (Boddaert, 1783) was affectedbycompetition for food with fish. During 1987–89, ten ponds inScania, southern Sweden were investigated, five with(breedingsites) and five without breeding red-necked grebe(non-breedingsites). I quantified the biomass of fish and macrozoans(here:larger invertebrates and small vertebrates) and measured thephysical and chemical conditions of the ponds.Breeding sites had significantly lower relative fish abundancethannon-breeding sites. They were either devoid of fish or hadonly nine-spined sticklebackPungitius pungitius. The biomass (dry weight) ofmacrozoans,many of which represent a food resource for grebes, was 16timesgreater at breeding than at non-breeding sites. The physicalandchemical conditions of the ponds did not differsignificantly.These results are consistent with the hypothesis thatcompetitionfor macrozoans occurs between red-necked grebe and fish andthat adense population of many fish species caused red-necked grebestoavoid otherwise acceptable breeding sites. 相似文献
975.
Protein Phosphorylation and Calcium Uptake into Rat Forebrain Synaptosomes: Modulation by the σ Ligand, 1,3-Ditolylguanidine 总被引:1,自引:0,他引:1
Paul J. Brent Lynn Herd Heather Saunders Alistair T. R. Sim Peter R. Dunkley 《Journal of neurochemistry》1997,68(5):2201-2211
Abstract: The σ ligand 1,3-di- O -tolylguanidine (DTG) increased basal dynamin and decreased depolarization-stimulated phosphorylation of the synaptosomal protein synapsin Ib without having direct effects on protein kinases or protein phosphatases. DTG dose-dependently decreased the basal cytosolic free Ca2+ concentration ([Ca2+ ]i ) and blocked the depolarization-dependent increases in [Ca2+ ]i . These effects were inhibited by the σ antagonists rimcazole and BMY14802. The nitric oxide donors sodium nitroprusside (SNP) and 8-( p -chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate decreased basal [Ca2+ ]i and the KCl-evoked rise in [Ca2+ ]i to an extent similar to DTG. SNP, but not DTG, produced a rise in cyclic GMP levels, suggesting that the effect of DTG on [Ca2+ ]i was not mediated via downstream regulation of cyclic GMP levels. DTG increased 45 Ca2+ uptake and efflux under basal conditions and inhibited the 45 Ca2+ uptake induced by depolarization with KCl. The KCl-evoked rise in [Ca2+ ]i was inhibited by ω-conotoxin (ω-CgTx)-GVIA and -MVIIC but not nifedipine and ω-agatoxin-IVA. The effect of DTG on decreasing the KCl-evoked rise in [Ca2+ ]i was additive with ω-CgTx-MVIIC but not with ω-CgTx-GVIA. These data suggest that DTG was producing some of its effects on synapsin I and dynamin phosphorylation and intrasynaptosomal Ca2+ levels via inhibition of N-type Ca2+ channels. 相似文献
976.
Michio Masuda Tsuyoshi Abe Shinji Sato Teruaki Suzuki Minoru Suzuki 《Journal of phycology》1997,33(2):196-208
Many morphologically similar, but chemically distinct, populations have been found in the marine red alga Laurencia nipponica Yamada (Rhodomelaceae, Ceramiales) growing in Japan. Each chemical type is characterized by a specific end-product of halogenated secondaly metabolite synthesis: chamigrane-type sesquiterpenoids such as prepacifenol and halochamigrene epoxide and C15 bromoethers such as laurencin, laureatin, isoprelaurefucin, epilaurallene, and kumausallene. These seven types of secondary metabolite syntheses remained the same in the wild and under various culture conditions. Because bromoethers and terpenoids are probably synthesized by different metabolic pathways, it is virtually certain that different sets of enzymes participate in their synthesis. Prepacifenol- and laureatin-producing populations were selected as representatives of terpenoid and bromoether groups, respectively. F1 tetrasporophytes derived from crosses between reciprocal, female and male gametophytes of prepacifenol- and laureatin-producing strains bore both types of metabolites, suggesting that the genes Producing these enzyme systems are encoded by nuclear genomes. The F1 gametophytes resulting from the reciprocal crosses produced either prepacifenol or laureatin, and the four individuals derived from spore tetrads (a set of tetraspores derived from a single tetrasporangium) produced either prepacifenol or laureatin in a 1:1 ratio, indicating that genes participating in terpenoid synthsis and those participating in bromoether synthesis are on different loci of homologous chromosomes and are segregated at meiosis (tetrasporogenesis). One individual of this interpopulational F1 gamtophyte produced both parental types of metabolite, perhaps indicating the occurrence of a recombination type. Natural hybrid individuals, including such recombination-type gametophytes, were found in a sympatric locality at which these two chemical types occur. F1 tetrasporophytes derived from crosses between respective prepacifenol- and laureatin-producing strains and their F1 gametohytes produced only parental-type metabolite-producing plants. These results indicate that the diverse chemical types can be referred to as races (chemical races). 相似文献
977.
Michle Guillaume Leif Lapidus Per Bjrntorp Andr Lambert 《Obesity (Silver Spring, Md.)》1997,5(6):549-556
GUILLAUME, MICHÈLE, LEIF LAPIDUS, PER BJÖRNTORP, ANDRE LAMBERT. Physical activity, obesity, and cardiovascular risk factors in children. The Belgian Luxembourg Child Study II. Physical activity was measured in relation to cardiovascular (CV) risk factors in a randomly selected population of 1028 children from Province de Luxembourg in Belgium, a mainly rural area with a high prevalence of such risk factors among adults and children. Physical activity was estimated as participation in sport activities, a major indicator of leisure-time physical activity in schoolchildren, and physical inactivity was estimated as frequency and duration of television (TV) watching. Boys participated more frequently in sport activities than girls did (p=0. 001). A majority of the children watched TV daily. After age adjustment, bodyweight (girls, p<0. 012; boys, p<0. 027) and, in boys, body mass index (BMI) (p<0. 039) were related to days per week of TV watching. No significant relationships with other CV risk factors remained after adjustments for BMI. In analyses of independent contributions of age, TV watching, and sports activity on CV risk factors, age showed highly significant relationships. In boys, TV showed relationships with BMI (P<0. 04) and (borderline) with systolic blood pressure, independent of age and sports activity, whereas the latter was significantly related to subscapular skinfold (p<0. 04) and (borderline) with triceps skinfold and cholesterol. In girls, no significant independent contributions to risk factor associations were found. The father's education was directly associated with sports activities, whereas the mother being a housewife showed negative relationships to physical activity and positive to TV watching in their children, suggesting socioeconomic influence on the activity patterns of children. Furthermore, registrations suggested less physical activity in the most rural part of the area. It is concluded that children in this mainly rural area watch TV frequently. In boys, physical inactivity, measured both as TV watching and as registrations of sports activities, contributes independently to body fat mass. In girls, no contribution or weaker contributions of physical inactivity were found. This suggests that contributory factors leading to obesity might be different in girls and boys. 相似文献
978.
By kinetic methods, functional relation of TAN radical, produced in the process of TEMPONE trapping O2, vs. time during photosensitization was established. Accordingly relative rate constants of generating all kinds of active intermediates through types I and II mechanism of photosensitization can be calculated. Using the formula and experimental results, the relative rate constants of generating O2, O2 and PS2 of three kinds of perylenequinone photosensitizer: HA, HB and CP in DMF-H20 and DMSO-H2O system were calculated, and then the structure-activity relationship of perylenequinone photosensitizer and the relation between photosensitivity and solvent was studied. 相似文献
979.
周荣华 《中国科学:生命科学英文版》1997,40(6):657-664
Using genomicin-situ hybridization (GISH) technique, 7 translocation-addition lines, 6 translocation and translocation-addition lines, 2 ditelosomic
addition lines and 1 translocation line were identified fromTriticum aestivum L. -Psathyrostachys juncea (Fisch.) Nevski intergeneric hybrids, of which translocation-addition and translocation and translocation-addition lines
were not found in other reports. No substitutions and disornic additions were detected in the, hybrids and breakages occurred
in allP. juncea chromosomes studied. Results have shown that the improved GISH technique is a rapid and economical method for use in this
field. 相似文献
980.
草鱼垂体生长激素分离纯化及其抗体制备的研究 总被引:11,自引:3,他引:8
应用碱性抽提,亲和层析,凝胶过滤和离子交换层析等技术从草鱼垂体中分离提纯出纯度高的草鱼生长激素(gcGH)用酶联免疫吸附测定法(ELISA)检测表明草鱼GH与大马哈鱼GH抗血清具有明显的免疫交叉反应,而与大马哈鱼催乳素(PRL)抗血清没有交叉反应,聚丙烯酰胺凝胶电泳表明草鱼GH为单一蛋白带,其分子量约为22500道尔顿,等电聚焦揭示出草鱼GH主要由等电点为4.7和5.0的两条蛋白带组成,用纯化的草 相似文献