全文获取类型
收费全文 | 456050篇 |
免费 | 168312篇 |
国内免费 | 21020篇 |
专业分类
645382篇 |
出版年
2016年 | 5775篇 |
2015年 | 7643篇 |
2014年 | 8596篇 |
2013年 | 11203篇 |
2012年 | 12487篇 |
2011年 | 13006篇 |
2010年 | 11001篇 |
2009年 | 15398篇 |
2008年 | 12547篇 |
2007年 | 12907篇 |
2006年 | 11364篇 |
2005年 | 11178篇 |
2004年 | 11063篇 |
2003年 | 10254篇 |
2002年 | 10928篇 |
2001年 | 22070篇 |
2000年 | 20483篇 |
1999年 | 20799篇 |
1998年 | 12646篇 |
1997年 | 12609篇 |
1996年 | 11749篇 |
1995年 | 11870篇 |
1994年 | 11226篇 |
1993年 | 10904篇 |
1992年 | 18416篇 |
1991年 | 17964篇 |
1990年 | 18268篇 |
1989年 | 17359篇 |
1988年 | 16178篇 |
1987年 | 15005篇 |
1986年 | 14119篇 |
1985年 | 13706篇 |
1984年 | 11130篇 |
1983年 | 9359篇 |
1982年 | 8265篇 |
1981年 | 7634篇 |
1980年 | 7155篇 |
1979年 | 10300篇 |
1978年 | 8586篇 |
1977年 | 8012篇 |
1976年 | 7545篇 |
1975年 | 7858篇 |
1974年 | 8751篇 |
1973年 | 8671篇 |
1972年 | 8472篇 |
1971年 | 7725篇 |
1970年 | 6753篇 |
1969年 | 6748篇 |
1968年 | 5992篇 |
1967年 | 5056篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
1. The anoas are two species of dwarf buffalo, the lowland anoa Bubalus depressicornis and mountain anoa Bubalus quarlesi that are endemic to the island of Sulawesi, Indonesia. The classification of the subgenus Anoa within Bubalus is upheld by assessment of recent genetic and morphological research. The classification of anoas into two species is still debated, but with the absence of significant opposing evidence, this position is adopted here. 2. Information about the distribution of the two species is presented that adds to but largely supports existing reports. However, it is still uncertain whether the two putative species are sympatric or parapatric in their distribution. A review of anoa distribution from historical reports and recent field data (1990s to 2002) highlights their decline throughout Sulawesi, especially in the southern and north-eastern peninsulas. The decline has been attributed to local hunting for meat and habitat loss. Most populations are rapidly becoming fragmented, suggesting that the conservation of viable populations may eventually require management of metapopulations. 3. There is an urgent requirement for conservation efforts to: (i) protect anoas from hunting; (ii) prevent habitat loss in key sites; (iii) complete genetic studies to better determine the number of anoa taxa and Management Units and assess their distribution; and (iv) determine the status of the remaining anoa populations. 相似文献
42.
Beta-adrenergic agonists and cyclic AMP decrease intracellular resting free-calcium concentration in ileum smooth muscle 总被引:2,自引:0,他引:2
I Parker Y Ito H Kuriyama R Miledi 《Proceedings of the Royal Society of London. Series B, Containing papers of a Biological character. Royal Society (Great Britain)》1987,230(1259):207-214
Intracellular free-calcium levels were measured in strips of longitudinal smooth muscle from guinea-pig ileum; fura-2 was used as a calcium monitor. At rest the calcium concentration was about 180 nM, and this rose to 300-400 nM following electrical stimulation and during spontaneous calcium transients (all measurements at 23-25 degrees C). Isoprenaline suppressed the spontaneous calcium transients, and reduced the resting calcium level to about 130 nM. This fall in resting calcium concentration was seen even in muscle strips which did not have spontaneous activity. Elevation of intracellular cyclic AMP levels, produced by forskolin or dibutyryl cyclic AMP, mimicked the actions of isoprenaline. We conclude that the relaxant effects of beta-adrenergic agonists of visceral smooth muscle may be explained partly by a fall in intracellular resting free-calcium level, mediated via an increase in cyclic AMP. 相似文献
43.
44.
D S Herson B McGonigle M A Payer K H Baker 《Applied and environmental microbiology》1987,53(5):1178-1180
Enterobacter cloacae attached to drinking water distribution particles was subjected to chlorination. Attachment resulted in the protection of these organisms from disinfection. This effect was found to be dependent upon both the level of chlorine in the system and attachment time. The results obtained in this study indicate that attached organisms may play an important role in coliform outbreaks. 相似文献
45.
A new species of copepod belonging to the family Aetideidae,Bradyidius slyliformis sp. nov. is described and figured. Therelationship of this species to the other 12 members of thegenus is discussed.
1Present address: 121 Jalan Athinahapan Dua, Taman Tun Dr Ismail,60000 Kuala Lumpur, Malaysia 相似文献
46.
Summary The endoglucanase component (CMCase I) ofPenicillium
funiculosum cellulase was purified to apparent homogeneity by ultrafiltration and gel chromatography. It consists of a single polypeptide chain with a molecular weight of 56000 and is a glycoprotein. Viscometric and end-product analysis revealed the randomness of enzyme action. Multifunctional characteristic of CMCase I was studied with various carbohydrate substrates.NCL Communication No.: 4307 相似文献
47.
Transforming potential of a myc-containing variant of feline leukemia virus in vitro in early-passage feline cells. 总被引:5,自引:4,他引:1 下载免费PDF全文
We studied a naturally occurring variant of feline leukemia virus (FeLV) in which the oncogene myc has substituted for a portion of the viral structural genes (myc-FeLV). myc-FeLV was rescued by replication in the presence of FeLV as helper, and its biological activity was examined in early-passage feline cells in vitro. Infection of leukocytes from peripheral blood, spleen, or thymus, or of kitten fibroblasts did not immortalize these cells or alter them morphologically. Northern blot (RNA blot) analysis of virion RNA prepared from the supernatant of infected cells demonstrated the 8.2-kilobase genome of FeLV, but did not demonstrate the 5.0-kilobase genome of myc-FeLV. Apparently, the myc-FeLV genome was lost in the absence of the selective pressure of transformation. In contrast, infection of embryonic fibroblasts with myc-FeLV(FeLV) rendered these cells capable of greatly increased, if not infinite, proliferative potential. The cells were morphologically altered compared with controls and were only loosely adherent to the substrate. The cells failed to proliferate in semisolid medium and did not form tumors when inoculated subcutaneously into athymic mice. Blot analyses demonstrated the presence and expression of integrated proviral DNAs of both FeLV and myc-FeLV in these cells. They appear, then, to represent cells partially transformed by infection with myc-FeLV(FeLV). The action of feline v-myc in early-passage cells in vitro was compared to that of avian v-myc. 相似文献
48.
Intracellular crystals of aragonite have been identified by selected area electron diffraction (SAED) in a species of the freshwater filamentous alga Spirogyra from the Thames River, Ontario, Canada. The crystals are 2 to 24 μm in diameter, and characterized by a unique cross-shaped morphology, in which needle-like, or prismatic outgrowths develop from a common axis. Crystals may be dispersed throughout filaments, but tend to cluster as aggregates towards the centre . 相似文献
49.
A method for the multicomponent analysis of estrogens in urine after initial hydrolysis of the conjugates is described. Following protection of the carbonyl functions by ethoximation, estrogen conjugates were extracted on Sep-Pak C18 cartridges and purified on the acetate form of DEAE-Sephadex. The samples were subsequently hydrolysed by Helix pomatia juice and the hydrolysate was purified on the acetate form of QAE-Sephadex. Estrogens with vicinal cis-hydroxyls and diphenolic compounds were fractionated on the borate and bicarbonate form of QAE-Sephadex, respectively. Neutral steroids were removed by the free base form of DEAE-Sephadex after which estrogens were separated into two groups using Lipidex 5000 in a straight phase system. Following trimethylsilyl ether derivatization estrogens were analysed by selected ion monitoring (SIM). The method allows the quantitation of all the important estrogen metabolites including catechol estrogens. It is precise, accurate and sensitive permitting the quantitation of estrogens in urine of males and non-pregnant females. 相似文献
50.