首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   60366篇
  免费   4943篇
  国内免费   31篇
  65340篇
  2022年   414篇
  2021年   854篇
  2020年   528篇
  2019年   638篇
  2018年   832篇
  2017年   772篇
  2016年   1358篇
  2015年   2392篇
  2014年   2515篇
  2013年   3335篇
  2012年   4239篇
  2011年   4247篇
  2010年   2756篇
  2009年   2440篇
  2008年   3533篇
  2007年   3591篇
  2006年   3391篇
  2005年   3421篇
  2004年   3357篇
  2003年   3155篇
  2002年   3118篇
  2001年   712篇
  2000年   533篇
  1999年   729篇
  1998年   866篇
  1997年   598篇
  1996年   604篇
  1995年   592篇
  1994年   572篇
  1993年   597篇
  1992年   536篇
  1991年   463篇
  1990年   389篇
  1989年   406篇
  1988年   403篇
  1987年   339篇
  1986年   340篇
  1985年   395篇
  1984年   444篇
  1983年   374篇
  1982年   459篇
  1981年   409篇
  1980年   364篇
  1979年   242篇
  1978年   294篇
  1977年   284篇
  1976年   231篇
  1975年   216篇
  1974年   236篇
  1973年   219篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Kubo producer-units (families and independent bachelors) could have been self-sufficient in the production of bananas but chose not to be. Nor did they seek self-sufficiency in the production of any combination of staple carbohydrate foods (bananas, tubers, sago flour) or, in the long term, strive for balance in the exchange of food with other producer-units. Despite the fact that bananas, which provided 50% of people's energy needs, were a delayed-return crop Kubo communities were very unstable. This instability and the failure to choose the option of self-sufficiency were connected and were mediated through intense intracommunity sharing that, ultimately, served to negotiate a concern with sorcery. The people grew bananas in the way they did, not out of environmental necessity, but to accommodate the crop to the needs of sharing and, thereby, facilitate community living.  相似文献   
102.
Peter I. Collins 《CMAJ》1992,146(12):2200-2201
  相似文献   
103.
Calicotyle urolophi n. sp. is proposed for calicotylines found in the cloaca of three stingaree species, Urolophus cruciatus, U. bucculentus and U. paucimaculatus, collected off the southeastern coast of Tasmania. Variations in the soft body parts were observed between specimens taken from U. cruciatus and U. bucculentus but were not considered sufficient for separation into two species. C. urolophi is differentiated from other Calicotyle spp. found in the South Pacific by the configuration of the tubular male copulatory organ, the structure of the intestinal caeca and the arrangement of the vaginae. Amended diagnoses for the subfamily Calicotylinae and the genus Calicotyle are provided.  相似文献   
104.
When grown with nitrate as terminal electron acceptor both the soluble (periplasm, cytoplasm) and the membrane fraction of Spirillum strain 5175 exhibited high nitrite reductase activity. The nitrite reductase obtained from the soluble fraction was purified 76-fold to electrophoretical homogeneity. The enzyme reduced nitrite to ammonia with a specific activity of 723 mol NO inf2 sup- × (mg protein × min)-1. The molecular mass was 58±1 kDa by SDS-PAGE compared to 59±2 kDa determined by size exclusion chromatography under nondenaturing conditions. The enzyme (as isolated) contained 5.97±0.15 heme c molecules/Mr 58 kDa. The absorption spectrum was typical for c-type cytochrome with maxima at 280, 408, 532 and 610 nm (oxidized) and at 420, 523 and 553 nm (dithionite-reduced). The enzyme (as isolated) exhibited a complex set of high-spin and lowspin ferric heme resonances with g-values at 9.82, 3,85, 3.31, 2.95, 2.30 and 1.49 in agreement with data reported for electron paramagnetic resonance spectra of nitrite reductases from Desulfovibrio desulfuricans, Wolinella succinogenes and Escherichia coli.Abbreviations DNRA dissimilatory nitrate reduction to ammonia - EPR electron paramagnetic resonance - PAGE polyacrylamide gel electrophoresis - NaPi sodium phosphate - SDS sodium dodecylsulfate  相似文献   
105.
The sequence homologies of the glucose dehydrogenase subunits of B. megaterium and B. subtilis are compared. From the known B. megaterium aminoacid sequence and the base sequence of the cloned B. subtilis structural gene we predict the B. megaterium structural glucose dehydrogenase gene. Assuming the minimal mutational changes to convert one gene into the other 23 transitions, 30 transversions, 1 inversion, 3 insertion-deletions, but no frameshifts are postulated necessary to interconvert the structural genes. The homology of both enzyme subunits of 85% reflects the close evolutionary distance between B. subtilis and B. megaterium.  相似文献   
106.
Summary Corn Bunting populations in Central England have 2 song types, during spontaneous singing they are sung approximately equally frequently and in bouts. Playback of song does not elicit song matching. The tendency to approach the source of playback wanes with repeated presentation of song — the birds habituate. The approach response is restored by playback of the other song type (dishabituation) — evidence that Corn Buntings can discriminate between song types. The functional significance of song types and the reason for differences in song type use between British and European populations remain unclear.
Strophentypen der GrauammerEmberiza calandra: Experimente zur Angleichung und zum Unterscheidungsvermögen
Zusammenfassung Die Populationen der Grauammer in Mittelengland besitzen 2 Strophentypen, die sie beim experimentell unbeeinflußten Singen jeweils in Serien und etwa gleich häufig vortragen. Tonbandvorspiel von Strophen löst keine Angleichung im Strophentyp aus. Die Tendenz, sich der Schallquelle anzunähern, läßt mit wiederholtem Vorspiel desselben Strophentyps nach, d. h. die Vögel gewöhnen sich an ihn (Habituation). Vorspiel des anderen Strophentyps führt dagegen wieder zur Annäherung (Antihabituation). Dies ist ein Beleg, daß Grauammern zwischen Strophentypen unterscheiden können. Die funktionelle Bedeutung der Strophentypen und die Ursache, warum sich britische und zentraleuropäische Populationen in der Verwendung von Strophentypen unterscheiden, bleibt weiterhin unklar.
  相似文献   
107.
Cycling adult female hamsters can be induced to mate and ovulate 24 h early by the injection of 20 IU human chorionic gonadotropin (hCG) at 1500 h on Day 3 (day before proestrus), but pregnancy is not established. Although there is evidence of decreased sperm transport in precociously ovulated females, this does not appear to be the primary cause of infertility. Reduced size and vascularity of corpora lutea (CL) in treated females suggests incomplete or failed CL activation. Control and hCG-treated females were killed by exsanguination under ether anesthesia at intervals for the first 5 days after mating. Serum luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, estradiol, and progesterone were measured by radioimmunoassay. Luteinizing hormone in treated animals was very high at 2200 h on Day 1 after mating (31 h after the hCG injection), due to endogenous release, and dropped below control levels thereafter. Follicle-stimulating hormone, by contrast, was significantly lower than controls at 2200 h on Day 1 and remained low until 2200 h on Day 3 after mating. Prolactin in treated animals was not different from that in controls, except for 1000 h on Day 4, when it showed a significant dip. Estradiol in treated animals was significantly higher than in controls at 2200 h on Day 1 (when LH was also high and FSH was low), and remained high at 1000 h and 2200 h on Day 2, dropping thereafter to control levels. Progesterone was initially at control levels but had dropped significantly by 1000 h on Day 2 and remained low for the next 24 h. These results suggest that pregnancy failure is due to inadequate activation of corpora lutea. This may be due to: 1) immaturity of follicles at the time of ovulation; 2) inappropriate timing of preovulatory events; 3) the luteolytic effects of high levels of LH or estradiol or both; 4) the low level of FSH in the early stages of corpus luteum development; or 5) a combination of the above. Abnormalities of prolactin secretion were not investigated in detail but cannot be ruled out at this time.  相似文献   
108.
    
Zusammenfassung Bei einem Amsel- (Turdus merula) gelang es erstmals, während des Singens (Subsong) radiotelemetrisch die Herzfrequenz (HR) zu messen. Gegenüber der Ruhe-Herzfrequenz (R-HR) erhöhte sich die HR während 4 Gesangsphasen um 33–72 %. In den Gesangspausen und nach Beendigung des Singens fiel die HR innerhalb von 6–8 s auf Werte, die um 15–20 % über der Ruhe-HR lagen. Der aus entspannter Situation heraus erfolgende Subsong stellt eine physiologische Belastung für den Vogel dar, die seine Einstufung als unbeschwert oder gar funktionslos unwahrscheinlich macht.
Bird song and heart rate — radiotelemetry measurements to the subsong on Blackbird (Turdus merula)
Summary On a male Blackbird (Turdus merula), heart rate during the bird's singing (subsong) was measured by means of radiotelemetry. As compared to its resting values (R-HR) heart rate (HR) increased by 33–72 % during 4 phases of singing. When singing was interrupted HR decreased again within 6–8 s down to values which were still about 15–20 % above R-HR. Though subsong was uttered in an apparently relaxed situation it seemed to be to a certain degree straining for the bird. Hence subsong should not be regarded as a behaviour pattern free of stress or function.


Prof. Dr. Erwin Tretzel zum 65. Geburtstag  相似文献   
109.
The peptide subunit pentapeptide H-L-Ala-D-Glu(L-Lys-D-Ala-D-Ala-OH)-NH2 of peptidoglycan was localized in the cell walls of several Gram-positive bacteria employing the indirect immunoferritin technique. Specific antibodies to the D-alanyl-D-alanine moiety of non-crosslinked peptide subunit pentapeptide were raised in rabbits by immunization with synthetic immunogen albumin-(CH2CO-Gly-L-Ala-L-Ala-D-Ala-D-Ala-OH)39. Specificity of these antibodies for the peptide subunit pentapeptide and not for the peptide subunit tetrapeptide was corroborated in a Farr-type radio-active hapten binding assay. Specificity of labelling with ferritin was established by immunoelectron microscopic controls, and by the excellent correlation between specific labelling of cells with ferritin and the particular peptidoglycan primary structure of bacterial strains investigated. Cells of Lactobacillus gasseri, Streptococcus pyogenes and Staphylococcus aureus revealing non-crosslinked peptide subunit pentapeptides in their peptidoglycans could specifically be labelled. Lactobacillus acidophilus and Bacillus subtilis, on the contrary, missing such pentapeptides, failed in labelling.The implication of this method to possibly localize the points of attack of penicillin or cycloserine is discussed.Abbreviations used meso-A2pm meso-diaminopimelic acid - DSM Deutsche Sammlung für Mikroorganismen, Göttingen, FRG This paper is dedicated to Professor Gerhart Drews on the occasion of his 60th birthday  相似文献   
110.
This communication reports the DNA level identification of class I and class II sequences associated with 20 RT1 haplotypes which have been assigned previously to eight RT1 groups. Sixteen to 22 bands in genomic blots hybridized with the mouse pH-2III class I cDNA probe. Only the three RT1 khaplotypes associated with identical class I restriction fragment patterns. Differences in restriction bands between putatively identical RT1 haplotypes were either less than or equal to 6%, or greater than 50%, suggesting a relatively high level of recombination between serologically identified RT1.A genes and the majority of class I sequences. Restriction fragment patterns associated with three RT1 uhaplotypes differed by less than 6%. However, intra-RT1 a,intra-RT1 b,and intra-RT1 lrestriction fragment differences were between 50 and 64%. In specific cases, different RT1 haplotypes associated with identical class I restriction patterns, e.g., RT1 m(MNR) and RT1 d(MR); higher resolution confirmed the difference (two bands) between RT1 mand RT1 d.Results of hybridization with the human DC1 probe confirmed that the AVN RT1 aand NSD RT1 bhaplotypes were generated by recombinations within the vicinity of the RT1.B : RT1.D regions. These results demonstrate that a previous classification of RT1 haplotypes was incomplete and did not include the majority of class I and class II sequences which distinguish RT1 haplotypes.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号