全文获取类型
收费全文 | 1575篇 |
免费 | 190篇 |
出版年
2023年 | 7篇 |
2022年 | 9篇 |
2021年 | 65篇 |
2020年 | 29篇 |
2019年 | 40篇 |
2018年 | 39篇 |
2017年 | 37篇 |
2016年 | 58篇 |
2015年 | 91篇 |
2014年 | 79篇 |
2013年 | 93篇 |
2012年 | 113篇 |
2011年 | 116篇 |
2010年 | 57篇 |
2009年 | 73篇 |
2008年 | 97篇 |
2007年 | 71篇 |
2006年 | 78篇 |
2005年 | 65篇 |
2004年 | 74篇 |
2003年 | 57篇 |
2002年 | 50篇 |
2001年 | 29篇 |
2000年 | 18篇 |
1999年 | 21篇 |
1998年 | 17篇 |
1997年 | 16篇 |
1996年 | 10篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 26篇 |
1991年 | 18篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 19篇 |
1987年 | 11篇 |
1986年 | 10篇 |
1985年 | 11篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1981年 | 4篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 4篇 |
1975年 | 4篇 |
1969年 | 4篇 |
1958年 | 5篇 |
1939年 | 3篇 |
1938年 | 4篇 |
排序方式: 共有1765条查询结果,搜索用时 62 毫秒
1.
Jeremy D. Hogan Katharine M. Jack Fernando A. Campos Urs Kalbitzer Linda M. Fedigan 《American journal of primatology》2019,81(7)
Primates have long been used as indicator species for assessing overall ecosystem health. However, area‐wide census methods are time consuming, costly, and not always feasible under many field conditions. Therefore, it is important to establish whether monitoring a subset of a population accurately reflects demographic changes occurring in the population at large. Over the past 35 years, we have conducted 15 area‐wide censuses in Sector Santa Rosa, Costa Rica. These efforts have revealed important trends in population growth patterns of capuchin monkeys following the protection and subsequent regeneration of native forests. During this same period, we have also intensively studied a subset of the capuchin groups. Comparing these two datasets, we investigate whether the population structures of the closely monitored groups are reliable indicators of area‐wide demographic patterns. We compare the overall group size and the individual age/sex class compositions of study groups and nonstudy groups (i.e., those contacted during area‐wide censuses only). Our study groups contained more individuals overall with a larger proportion of infants, and there were indications that the proportion of adult and subadult males was lower. These differences can be ascribed either to sampling errors or real differences attributable to human presence and/or better habitat quality for the study groups. No other sex/age classes differed, and major demographic changes were simultaneously evident in both study and nonstudy groups. This study suggests that the Santa Rosa capuchin population is similarly impacted by large‐scale ecological patterns observable within our study groups. 相似文献
2.
Cephapirin, a cephalosporin antibiotic, is used by the majority of dairy farms in the US. Fecal and urinary excretion of cephapirin could introduce this compound into the environment when manure is land applied as fertilizer, and may cause development of bacterial resistance to antibiotics critical for human health. The environmental loading of cephapirin by the livestock industry remains un-assessed, largely due to a lack of appropriate analytical methods. Therefore, this study aimed to develop and validate a cephapirin quantification method to capture the temporal pattern of cephapirin excretion in dairy cows following intramammary infusion. The method includes an extraction with phosphate buffer and methanol, solid-phase extraction (SPE) clean-up, and quantification using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The LOQ values of the developed method were 4.02 µg kg−1 and 0.96 µg L−1 for feces and urine, respectively. This robust method recovered >60% and >80% cephapirin from spiked blank fecal and urine samples, respectively, with acceptable intra- and inter-day variation (<10%). Using this method, we detected trace amounts (µg kg−1) of cephapirin in dairy cow feces, and cephapirin in urine was detected at very high concentrations (133 to 480 µg L−1). Cephapirin was primarily excreted via urine and its urinary excretion was influenced by day (P = 0.03). Peak excretion (2.69 mg) was on day 1 following intramammary infusion and decreased sharply thereafter (0.19, 0.19, 0.08, and 0.17 mg on day 2, 3, 4, and 5, respectively) reflecting a quadratic pattern of excretion (Quadratic: P = 0.03). The described method for quantification of cephapirin in bovine feces and urine is sensitive, accurate, and robust and allowed to monitor the pattern of cephapirin excretion in dairy cows. This data will help develop manure segregation and treatment methods to minimize the risk of antibiotic loading to the environment from dairy farms. 相似文献
3.
V. Russo D. Harman K. Hayden A. Iwen T. Portle J. Spreenberg J. Stone A. Pappelis 《Mycopathologia》1980,72(2):107-110
Spore suspensions from young (10–14 da; young spores) and old (4 mo; old spores) colonies of PColletotrichum dematium var circinans were placed on slides. Coverslips were left off, placed on in the normal manner, or supported on shims. Slides were placed in moist chambers and incubated in light or dark for up to 48 hrs. Germination and post-germination development were studied. Shimming had some beneficial effect on germination, especially for old spores in dark. In general, more germ-tubes and appressoria were produced on spores under shims than spores with other coverslip treatments. By 48 hrs more old spores under shims germinated, and greater numbers of germ-tubes and appressoria were produced than on other old spores under different coverslip treatments. However, numbers produced were lower than those predicted for comparably treated young spores. Spore age, incubation regime, and placement of coverslips did not affect germ-tube initiation. For all treatments more germ-tubes were initiated from spore tops than bottoms or tips. Fewer germ-tubes were initiated from spore centers than other locations on tops and bottoms, and from both tips than one tip. Approximately 26 % of all appressoria were produced sessile. A higher percentage of sessile appressoria were produced on old spores (80 %) than on young spores (20%). 相似文献
4.
Katharine Lloyd-Williams 《BMJ (Clinical research ed.)》1950,2(4689):1170-1171
5.
Katharine Y. Ku Fred R. Butcher 《Biochimica et Biophysica Acta (BBA)/General Subjects》1980,631(1):70-78
Calmodulin coupled to Sepharose has provided a rapid and sensitive means of isolating a cyclic nucleotide phosphodiesterase activity which is stimulated by the calmodulin-Ca2+ complex, from rat parotid gland. Initial experiments established that phosphodiesterase activity sensitive to calmodulin and Ca2+ could not be demonstrated in crude extracts of rat parotid gland or after partial purification of rat parotid phosphodiesterase over DEAE-cellulose. However, it was possible to readily demonstrate the presence of a cyclic nucleotide phosphodiesterase activity regulated by calmodulin if the extracts were first purified by batch ion-exchange chromatography over DEAE-cellulose followed by affinity chromatography with calmodulin coupled to Sepharose. The batch ion-exchange chromatography step removed the major portion of free parotid calmodulin which could compete with calmodulin-coupled Sepharose for the proteins regulated by calmodulin. Thus, by employing an initial chromatography step over DEAE-cellulose to separate phosphodiesterase activity from calmodulin, it was possible to increase the recovery of calmodulin-sensitive phosphodiesterase after affinity chromatography with calmodulin coupled to Sepharose. This approach should be useful for demonstrating the presence of and for purifying other parotid proteins regulated by calmodulin. 相似文献
6.
J M Vanderkooi R Landesberg G W Hayden C S Owen 《European journal of biochemistry》1977,81(2):339-347
The luminescent properties of metal-free, tin(IV) and zinc(II) cytochromes c have been used to characterize the interaction of cytochrome c with mitochondria and cytochrome oxidase. Diminution in the fluorescence yields of tin and zinc cytochrome c occur when these derivates bind to cytochrome oxidase or mitochondria. Based upon spectral overlap and quantum yield, the distance between the porphyrin rings of cytochrome a and cytochrome c is estimated according to Forster theory to be in the neighborhood of 3.5 nm. Measurements of the polarized emission of metal-free 'porphyrin' cytochrome c when bound to oriented layers of cytochrome c oxidase indicate that the porphyrin is bound obliquely to the plane of the oxidase layers with an angle of about 70 degrees C from heme plane to membrane plane. It is proposed that these data have significance for elucidation of electron transfer mechanisms. 相似文献
7.
8.
9.
Initiation of bacteriophage P22 DNA packaging series. Analysis of a mutant that alters the DNA target specificity of the packaging apparatus 总被引:10,自引:0,他引:10
Bacteriophage P22 is thought to package its double-stranded DNA chromosome from concatemeric replicating DNA in a "processive" sequential fashion. According to this model, during the initial packaging event in such a series the packaging apparatus recognizes a nucleotide sequence, called pac, on the DNA, and then condenses DNA within the coat protein shell unidirectionally from that point. DNA ends are generated near the pac site before or during the condensation reaction. The opposite end of the mature chromosome is created by a cut made in the DNA after a complete chromosome is condensed within the phage head. Subsequent packaging events on that concatemeric DNA begin at the end generated by the headful cut of the previous event and proceed in the same direction as the previous event. We report here the identification of a consensus nucleotide sequence for the pac site, and present evidence that supports the idea that the gene 3 protein is a central participant in this recognition event. In addition, we tentatively locate the portion of the gene 3 protein that contacts the pac site during the initiation of packaging. 相似文献
10.
Complete diphallia was detected in a six-month-old Poodle cross dog. Duplication of the urinary bladder, right renal hypoplasia, bifurcation of the descending colon and bilateral cryptorchidism were also present. The left urethra was patent, and emptied the left urinary bladder; the right urethra ended blindly at a point 2.5 cm from the tip of the right penis. The left urinary bladder was joined medially to a right urinary bladder with no interbladder communication; however, the right bladder opened caudally into a blind sac which drained into the left prostatic urethra. Pyelonephritis of the left kidney was present, and was suspected to be due to ascending bacterial infection of the right urinary bladder and the right segment of the bifurcated descending colon, which terminated at the apex of the right urinary bladder. Diphallia and associated urogenital anomalies are reviewed. 相似文献