全文获取类型
收费全文 | 43100篇 |
免费 | 3870篇 |
国内免费 | 33篇 |
专业分类
47003篇 |
出版年
2023年 | 166篇 |
2022年 | 309篇 |
2021年 | 599篇 |
2020年 | 395篇 |
2019年 | 489篇 |
2018年 | 641篇 |
2017年 | 555篇 |
2016年 | 1044篇 |
2015年 | 1758篇 |
2014年 | 1880篇 |
2013年 | 2456篇 |
2012年 | 2979篇 |
2011年 | 3057篇 |
2010年 | 1945篇 |
2009年 | 1657篇 |
2008年 | 2517篇 |
2007年 | 2507篇 |
2006年 | 2271篇 |
2005年 | 2338篇 |
2004年 | 2290篇 |
2003年 | 2181篇 |
2002年 | 2130篇 |
2001年 | 505篇 |
2000年 | 406篇 |
1999年 | 510篇 |
1998年 | 606篇 |
1997年 | 462篇 |
1996年 | 411篇 |
1995年 | 393篇 |
1994年 | 355篇 |
1993年 | 345篇 |
1992年 | 385篇 |
1991年 | 327篇 |
1990年 | 270篇 |
1989年 | 272篇 |
1988年 | 274篇 |
1987年 | 251篇 |
1986年 | 234篇 |
1985年 | 302篇 |
1984年 | 321篇 |
1983年 | 257篇 |
1982年 | 351篇 |
1981年 | 275篇 |
1980年 | 255篇 |
1979年 | 190篇 |
1978年 | 222篇 |
1977年 | 208篇 |
1976年 | 178篇 |
1974年 | 188篇 |
1973年 | 194篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
Culture of Clostridium pasteurianum in Defined Medium and Growth as a Function of Sulfate Concentration 下载免费PDF全文
Clostridium pasteurianum strain W-5 was selected as an anaerobe which may be grown from large inocula in defined media with sulfate as its primary sulfur source. Since it is important to keep inocula small in minimizing transfer of sulfur sources, culture conditions were optimized. The medium devised decreased lag period and generation time when compared with other media, but growth could not be induced consistently with 6 x 10(6) cells per ml or less. Addition of trace elements, chelating agents, reducing agents, metabolites, and spent medium from various stages of growth did not stimulate growth from small inocula. Generation time was 85 min on inoculation with 10(7) or more cells per ml taken from young stocks, but the lag period decreased somewhat with larger inocula. On the other hand, generation time and lag period increased with age of the inoculum. The total yield of cells increased when buffer capacity was increased. Growth of C. pasteurianum W-5 was dependent upon sulfate at relatively low sulfate concentrations, and the organism is thus suitable for study of sulfur metabolism. No evidence of a maintenance requirement for sulfate was detected. 相似文献
63.
64.
65.
Abstract. 1. Clip cages have been used widely by experimental ecologists to contain insects on plants.
2. Under controlled conditions, the effect of applying clip cages alone and clip cages and the chrysomelid beetle Gastrophysa viridula on systemic leaf expansion of Rumex obtusifolius was investigated. Treatments were applied to the fully expanded fourth leaf and expansion of leaf 8 was measured over a period of 22 days.
3. The application of clip cages reduced the rate at which leaf area increased and led to reductions in final leaf areas.
4. Clip cages have systemic effects on plant development and these effects are sustained even after the clip cage is removed. Investigators should take this into account in designing experiments. 相似文献
2. Under controlled conditions, the effect of applying clip cages alone and clip cages and the chrysomelid beetle Gastrophysa viridula on systemic leaf expansion of Rumex obtusifolius was investigated. Treatments were applied to the fully expanded fourth leaf and expansion of leaf 8 was measured over a period of 22 days.
3. The application of clip cages reduced the rate at which leaf area increased and led to reductions in final leaf areas.
4. Clip cages have systemic effects on plant development and these effects are sustained even after the clip cage is removed. Investigators should take this into account in designing experiments. 相似文献
66.
67.
The defence reaction of operculum closing in response to the presence of the molluscivorous leech Glossiphonia complanata (L.) and the non-molluscivorous Erpobdella octoculata (L.) was studied in four species of freshwater prosobranch gastropod. Bithynia tentaculata (L.) and Valvata piscinalis (Müller) can distinguish between the leeches, reacting only to G. complanata. V. piscinalis is capable of a greater degree of distance chemoreception of the leech ‘scent’. Valvata cristata Müller and Potamopyrgus jenkinsi (Smith) did not react to either leech. V. cristata may not be a potential prey item for G. complanata, while P. jenkinsi is fed on by the leech, but is a relative newcomer to the freshwater fauna.
Animal Ecology Research Group, Department of Zoology, University of Oxford
Commonwealth Forestry Institute, Department of Plant Sciences, University of Oxford 相似文献
68.
The embryonic development of total specific activities as well as of molecular forms of acetylcholinesterase (AChE, EC 3.1.1.7) and of butyrylcholinesterase (BChE, EC 3.1.1.8) have been studied in the chick brain. A comparison of the development in different brain parts shows that cholinesterases first develop in diencephalon, then in tectum and telencephalon; cholinesterase development in retina is delayed by about 2-3 days; and the development in rhombencephalon [not studied until embryonic day 6 (E6)] and cerebellum is last. Both enzymes show complex and independent developmental patterns. During the early period (E3-E7) first BChE expresses high specific activities that decline rapidly, but in contrast AChE increases more or less constantly with a short temporal delay. Thereafter the developmental courses approach a late phase (E14-E20), during which AChE reaches very high specific activities and BChE follows at much lower but about parallel levels. By extraction of tissues from brain and retina in high salt plus 1% Triton X-100, we find that both cholinesterases are present in two major molecular forms, AChE sedimenting at 5.9S and 11.6S (corresponding to G2 and G4 globular forms) and BChE at 2.9S and 10.3S (G1 and G4, globular). During development there is a continuous increase of G4 over G2 AChE, the G4 form reaching 80% in brain but only 30% in retina. The proportion of G1 BChE in brain remains almost constant at 55%, but in retina there is a drastic shift from 65% G1 before E5 to 70% G4 form at E7.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
69.
70.
Bacterivorous nanoflagellates (microflagellates) have been routinely enumerated in marine and freshwater samples using either a Most Probable Number (MPN) culture method or by a direct microscopical counting method (DC). These two techniques typically yield highly disparate estimates of the density of nanoflagellates in natural samples. We compared these methods with seawater and marine snow (macroscopic detrital aggregate) samples collected from surface waters throughout the North Atlantic and in freshwater samples collected at three stations in Lake Ontario. Densities of nanoflagellates determined by the two methods differed by as much as four orders of magnitude; the MPN estimate rarely exceeded 10% of the microscopical count, and averaged 1% of this count. The MPN estimate constituted a higher percentage of the DC value in environments with high concentrations of nanoflagellates relative to environments with low concentrations of nanoflagellates. The ratio of the culture count to the microscopical count (MPNDC) increased along an environmental gradient from oligotrophy to eutrophy, and was positively correlated with the density of bacteria in the samples. In laboratory experiments with two species of bacterivorous nanoflagellates, the MPN count constituted a much greater percentage of the DC count during the exponential growth phase of the nanoflagellate than during the stationary growth phase. Differences in the estimates of nanoflagellate density obtained with these two techniques probably can be explained by the trophic mode of these protozoa, their growth stage, and the amenability of these species to laboratory culture. 相似文献