全文获取类型
收费全文 | 35721篇 |
免费 | 3070篇 |
国内免费 | 19篇 |
专业分类
38810篇 |
出版年
2023年 | 112篇 |
2022年 | 317篇 |
2021年 | 594篇 |
2020年 | 360篇 |
2019年 | 476篇 |
2018年 | 595篇 |
2017年 | 524篇 |
2016年 | 924篇 |
2015年 | 1565篇 |
2014年 | 1695篇 |
2013年 | 2073篇 |
2012年 | 2808篇 |
2011年 | 2871篇 |
2010年 | 1786篇 |
2009年 | 1676篇 |
2008年 | 2368篇 |
2007年 | 2421篇 |
2006年 | 2260篇 |
2005年 | 2106篇 |
2004年 | 2084篇 |
2003年 | 1926篇 |
2002年 | 1869篇 |
2001年 | 409篇 |
2000年 | 275篇 |
1999年 | 405篇 |
1998年 | 475篇 |
1997年 | 340篇 |
1996年 | 305篇 |
1995年 | 274篇 |
1994年 | 239篇 |
1993年 | 259篇 |
1992年 | 230篇 |
1991年 | 174篇 |
1990年 | 156篇 |
1989年 | 172篇 |
1988年 | 139篇 |
1987年 | 131篇 |
1986年 | 101篇 |
1985年 | 137篇 |
1984年 | 160篇 |
1983年 | 111篇 |
1982年 | 128篇 |
1981年 | 116篇 |
1980年 | 96篇 |
1979年 | 58篇 |
1978年 | 72篇 |
1977年 | 66篇 |
1976年 | 42篇 |
1974年 | 35篇 |
1973年 | 39篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
A recently introduced approach far estimating the photosynthetic quantum efficiency (φ) of a freshwater or marine phytoplankton community has been applied for the first time to high latitude polar ecosystems, namely four lakes of southern Victoria Land, Antarctica. Values for φ at various depths ranged from 0.0022–0.1560 when calculated using a recommended mean extinction coefficient for phytoplankton (i.e. k?c= 0.016). By contrast, φ ranged from 0.0037–0.0760 when calculated using an empirically estimated value for k?c of 0.0328. If the recommended k?c= 0.016 more closely approaches an accurate estimate, then the φ valves indicate that the phytoplankton convert light to organic carbon more efficiently than elsewhere. However, if the empirically derived k?c= 0.0328 more closely approaches an accurate estimate, then the φ values indicate the phytoplankton trap light more efficiently than elsewhere. Although we have not resolved whether light conversion (φ) or light trapping are more efficient, the results show that the phytoplankton of these Antarctic lakes are well adapted to performing photosynthesis under extremely low light conditions. 相似文献
92.
The natural 13C abundance (13C value) of the field-grown leguminousplants (soybean, kidney bean, pea, azuki bean, mung bean, peanutand cowpea) was investigated by mass spectrometry with a precisionbetter than %0.2 for 13C. Among organs of premature plants,the leaves had the most negative values, and the nodules generallyhad the least negative values, and other organs, fruits, stemsand roots, showed intermediate values. In the soybeans so farinvestigated, the grains of nodulating plants exhibited higher13C values than nonnodulating lines. The 13C values of the grainsvaried depending on the species: peanuts showed the most negativevalues. Possible causes underlying these variations are discussed. (Received March 2, 1983; Accepted May 27, 1983) 相似文献
93.
Does New Guinea cannibalism have nutritional value? 总被引:1,自引:0,他引:1
This paper examines the question of the nutritional value of cannibalism. Although other authors have concluded that the practice does not have such value, we argue that this cannot properly be determined except in the context of the total subsistence economy and local human ecology. The paper also presents a format for the empirical investigation of foodgetting and new ethnographic information about New Guinea cannibalism. Our major conclusion is that this practice does have nutritional value for certain human groups, specifically tropical peoples living at lowmedium population densities and exploiting a diverse range of animal foods. 相似文献
94.
Isobel Braidman Mark Carroll Norman Dance Donald Robinson 《The Biochemical journal》1974,143(2):295-301
Hexosaminidase C was separated from human brain supernatant by immunoadsorption of the A and B forms on to a column of immobilized antibody followed by preparative starch-block electrophoresis. There were some differences in the properties of hexosaminidase C preparations after each of these stages, shown by comparison of their heat-inactivation characteristics and filtration through Bio-Gel P-200. The C form prepared by both separation steps had properties which differed markedly from those of the A and B isoenzymes; its molecular weight was much larger, greater than 200000, it had optimum activity between pH6 and 7 and could not be successfully eluted from DEAE-cellulose, even with high salt concentrations, or from Sephadex G-200. These results seem to support the proposal that the C form is under a separate genetic control from the others. 相似文献
95.
96.
Evidence for a Reductive Pathway for the Anaerobic Metabolism of Benzoate 总被引:18,自引:6,他引:12 下载免费PDF全文
Evidence is presented for a reductive pathway for the anaerobic metabolism of benzoate by Rhodopseudomonas palustris. 相似文献
97.
Mark L. Fleischman 《Primates; journal of primatology》1968,9(3):273-282
- The chimpanzee vocal system is an emotionally linked vocal response developed on a standard pre-symbolic primate system.
- Differences in number of sounds recorded by different observers may be due to personal choice, or due actual presence or absence of certain sound types in different living situations. Further research is needed in this case before comparisons can be made.
- Idiosyncratic use of certain signals may serve as a basis for growth of local dialects in different areas. Spread of such dialects may be related to the amount of respect accorded to dominant animals. More study may shed light on the growth and development of pre-symbolic systems.
98.
Benedict Mark hall 《Economic botany》1965,19(1):46-52
Mixed cultures of bacteria anaerobically attacking cellulosic materials produce sufficient acetic, butyric, lactic and formic acids and ethanol and glucose to account for most of the original carbon. These products contain 60 to 90% of the free energy of the cellulose and are all usable as carbon sources for the aerobic growth of yeast. 相似文献
99.
Further Studies on the Lyo and Desmo Components of Several Hydrolytic Enzymes and Their Histochemical Significance 总被引:1,自引:1,他引:0 下载免费PDF全文
This report describes additional studies of the lyo and desmo components of esterase, alkaline phosphatase, acid phosphatase, leucine aminopeptidase, and β-glucuronidase. The techniques used have already been reported (7). Enzyme diffusion occurs to different degrees in different fixatives, and varies somewhat with each enzyme. Loss of enzymatic activity during fixation occurs as a result of both inactivation due to the chemical reaction of the fixative with the enzymic protein, and diffusion of the lyo component into the fixative. The amount of diffusion into formalin can be reduced by the addition of salts, sucrose, or methocel. The pH of the aqueous medium significantly influences the removal of the lyo fraction from the tissue section. A striking similarity can be noted in the proportions of each fraction of enzyme present in the kidney of the rat, dog, and man. The procedure of fixation and paraffin embedding of tissue blocks does not wholly prevent the diffusion of the lyo component from the tissue sections when they are subsequently immersed in the aqueous incubation medium. 相似文献
100.
A dialysis unit was used to test whether direct physical contact between serum albumin and hamster spermatozoa is required for capacitation and/or the acrosome reaction. Sperm and bovine serum albumin (BSA) were incubated cither together (direct incubation) or separated by a dialysis membrane (indirect incubation). Sperm viability was supported with “sperm motility factors” (hypotaurine and epinephrine) and polyvinylalcohol (PVA). Spermatozoa became capacitated and underwent acrosome reactions when directly incubated in medium containing BSA (TALP-PVA), but did not undergo acrosome reactions when indirectly incubated with BSA (medium TLP-PVA). When sperm were first incubated for 4 hr indirectly with BSA, followed by 4 hr direct incubation with BSA, capacitation did not occur during indirect incubation. These findings indicate that an “intimate association” is necessary between serum albumin and spermatozoa to support capacitation under in vitro incubation conditions. The data are consistent with the concept of direct transfer of compounds from sperm to albumin and/or vice versa during sperm capacitation. 相似文献