全文获取类型
收费全文 | 5949篇 |
免费 | 737篇 |
国内免费 | 366篇 |
出版年
2024年 | 16篇 |
2023年 | 136篇 |
2022年 | 92篇 |
2021年 | 182篇 |
2020年 | 286篇 |
2019年 | 286篇 |
2018年 | 256篇 |
2017年 | 238篇 |
2016年 | 253篇 |
2015年 | 234篇 |
2014年 | 265篇 |
2013年 | 494篇 |
2012年 | 251篇 |
2011年 | 240篇 |
2010年 | 213篇 |
2009年 | 225篇 |
2008年 | 270篇 |
2007年 | 294篇 |
2006年 | 261篇 |
2005年 | 237篇 |
2004年 | 240篇 |
2003年 | 180篇 |
2002年 | 211篇 |
2001年 | 240篇 |
2000年 | 182篇 |
1999年 | 149篇 |
1998年 | 117篇 |
1997年 | 123篇 |
1996年 | 78篇 |
1995年 | 77篇 |
1994年 | 88篇 |
1993年 | 71篇 |
1992年 | 55篇 |
1991年 | 48篇 |
1990年 | 48篇 |
1989年 | 38篇 |
1988年 | 36篇 |
1987年 | 34篇 |
1986年 | 35篇 |
1985年 | 35篇 |
1984年 | 84篇 |
1983年 | 18篇 |
1982年 | 24篇 |
1981年 | 18篇 |
1980年 | 24篇 |
1979年 | 13篇 |
1978年 | 21篇 |
1977年 | 10篇 |
1976年 | 10篇 |
1973年 | 6篇 |
排序方式: 共有7052条查询结果,搜索用时 15 毫秒
41.
Shigeru Kumano Masashi Ihira Yasuo Maeda Misako Yamauchi Eiji Matsumoto Isao Matsuda 《Ecological Research》1990,5(2):221-235
Diatom assemblages of sediments obtained from three sites on Kushiro Moor were analyzed to investigate the Holocene sedimentary
history. The results showed that: 1) The Takkobu site was originally at the bottom of the paleo-Kushiro Bay, and after-wards
the paleo-Takkobu Lagoon developed, became sealed off, and changed to a freshwater lake. The succession to peat moor probably
began about 2000 yr B.P. at the Takkobu site. 2) The Tsurui site was originally at the bottom of the paleo-Kushiro Bay, then
changed to the paleo-Kushiro Lagoon and became peat moor as a result of the first Holocene regression, which finished about
3600 yr B.P. The site then returned to a brackish lake again, probably due to the second Holocene transgression between 3600
and 3000 yr B.P., thereafter passing through brackish lake and freshwater lake stages, and eventually becaming peat moor at
about 2000 yr B.P., 3) At the Chuo site, the second paleo-Kushiro Bay developed again as a result of the second Holocene transgression,
which finished about 3000 yr B.P. Thereafter, brackish or freshwater lakes, rivers, and then peat moor developed in the central
area of Kushiro Moor. 4) The second marine diatom zone (MD2 Zone), which indicates the second Holocene transgression, complete by about 3000 yr B.P., is detected only at the Chuo site
in the central area of Kushiro Moor. 相似文献
42.
R. Chima Wokocha 《Plant and Soil》1988,106(1):146-148
Populations of viable sclerotia ofSclerotium rolfsii were highest in soil in a field in which tomato was planted for three successive years before sampling and in one in which
tomato followed groundnut in the 2 years prior to this study. The lowest sclerotial numbers were recorded in fields in which
groundnut followed maize or in which maize or sorghum was the last crop before sampling. 相似文献
43.
The influences of diets having different fatty acid compositions on the fatty-acid content, desaturase activities, and membrane fluidity of rat liver microsomes have been analyzed. Weanling male rats (35–45 g) were fed a fat-free semisynthetic diet supplemented with 10% (by weight) marine fish oil (FO, 12.7% docosahexaenoic acid and 13.8% eicosapentaenoic acid), evening primrose oil (EPO, 7.8% γ-linolenic acid and 70.8% linoleic acid) or a mixture of 5% FO-5% EPO. After 12 weeks on the respective diets, animals fed higher proportions of (n-3) polyunsaturated fatty acids (FO group) consistently contained higher levels of 20:3(n-6), 20:5(n-3), 22:5(n-3), and 22:6(n-3), and lower levels of 18:2(n-6) and 20:4(n-6), than those of the EPO (a rich source of (n-6) polyunsaturated fatty acids) or the FO + EPO groups. Membrane fluidity, as estimated by the reciprocal of the order parameter SDPH, was higher in the FO than in the EPO or the FO + EPO groups, and the n-6 fatty-acid desaturation system was markedly affected. 相似文献
44.
45.
Endoparasitic copepods are very numerous in Indo-West Pacific corals. In West Indian corals they were thought to be absent, but recent studies have shown that a varied endoparasitic copepod fauna exists as well. Striking is the taxonomic composition of the coral-inhabiting copepods:In the Indo-West Pacific Lichomolgidae and Xarifiidae are the dominant families, both are absent in the West Indies. On the other hand, Corallovexiidae and Asterocheridae dominate in the West Indies; the former family is absent and the latter is apparently rare and not very diversified in the Indo-West Pacific. 相似文献
46.
The migratory movements of reproductive potamodromus fish established in man-made tropical lakes in the dry zone of Sri Lanka were determined. It was shown that the nine indigenous riverine species do not spawn in these lakes but move into the upstream channels when sexually mature. The pattern of upstream movement was found to be species-specific although most species showed a diurnal periodicity. It is suggested that the development of totally lacustrine forms of riverine species is very unlikely to occur in the man-made lakes of Sri Lanka because of their relatively short duration time. 相似文献
47.
Species of the genus Paradoxostoma Fischer are an important component of marine and estuarine ostracod faunas throughout the world. This revision of the genus in British waters deals with 17 species of which six are described as new. Each species is diagnosed and illustrated in detail, and comments are made on their ecology and distribution. The specialized mouthparts which characterize the genus are illustrated by means of the scanning electron microscope for the first time. A key to the genus is given. 相似文献
48.
ANDREW M. SUGDEN F.L.S. 《Botanical journal of the Linnean Society. Linnean Society of London》1985,90(4):231-241
A survey has been made of the leaf anatomy of 43 woody species in three montane vegetation types of Isla Margarita, Venezuela, differing in levels of cloud cover (and hence wetness) and exposure to wind. The only character that varies significantly with increasing probability of periodic drought is specific leaf area, which can be related to the higher proportion of deciduous species in the drier habitat. Leaves become significantly smaller and thicker with increased exposure to wind, and have thicker outer epidermal walls and cuticles. Most characters, including stomatal density, guard cell length, palisade: non-palisade ratios, and the incidence of such features as hypodermis, sclerenchyma, crystals and secretory structures are highly variable and show few or no trends according to habitat. Some of these characters are apparently more constrained by phylogeny than by immediate ecological circumstances. As a contribution to the continuing debate on the significance of xeromorphy in tropical montane forest leaves, it is suggested that low light levels due to cloud cover can be discounted as an important determining factor; in the case of the Isla Margarita vegetation, the need to avoid excessive leaf temperatures may be of greater significance. 相似文献
49.
Joseph M. Gennity Nestor R. Bottino Ralph A. Zingaro Andrew E. Wheeler Kurt J. Irgolic 《Phytochemistry》1985,24(12):2817-2821
Two unicellular marine algae cultured in media containing sodium selenite were examined for glutathione peroxidase activity. The 400 g supernatant from disrupted cells of both the green alga Dunaliella primolecta and the red alga Porphyridium cruentum were able to enhance both the H2O2 and the tert-butyl hydroperoxide dependent oxidation of glutathione. The glutathione peroxidation activity of D. primolecta was reduced only slightly by heating the 400 g supernatant, a 30% decrease in the rate with H2O2 and 10% decrease in the rate with t-BuOOH being observed. Heating caused the H2O2 dependent activity in P. cruentum to be reduced by only 30%, but the activity with t-BuOOH was reduced by 90%. Freezing decreased the t-BuOOH dependent activity of P. cruentum by 90%, but did not lower the t-BuOOH dependent activity of D. primolecta or the H2O2 dependent activity of either alga. It was concluded that the heat and cold stable, glutathione peroxidation was non-enzymatic in nature. A variety of small molecules (ascorbate, Cu(NO3)2, selenocystine, dimethyldiselenide and selenomethionine) were shown to be able to enhance the hydroperoxide dependent oxidation of glutathione in the assay system employed in this study. Such compounds could be responsible for the activity observed in algae. The heat and cold labile t-BuOOH reductase activity of P. cruentumwas possibly enzymatic, but was not attributable to the presence of glutathione-S-transferase. Both algae, when cultured in the presence of added selenite, displayed an approximate doubling of the non-enzymatic H2O2 and t-BuOOH dependent glutathione oxidase activities. The heat and cold labile t-BuOOH reductase activity of P. cruentum was unaltered when the alga was grown in the presence of added selenite. These observations are consistent with the hypothesis that selenium compounds present in the algae are responsible for the selenium induced glutathione peroxidation. 相似文献
50.
Some Crustacea Copepoda from Venezuela 总被引:5,自引:5,他引:0
B. H. Dussart 《Hydrobiologia》1984,113(1):25-67
The study of 38 samples of aquatic fauna from Venezuela increased the number of known species here from 28 to 66. Fifteen new species for science are described. A list of Copepoda known from the other regions of South America is presented. From this list, it can be said that only 50% of the inland water Copepoda living actually in Venezuela are known. 相似文献