全文获取类型
收费全文 | 5551篇 |
免费 | 541篇 |
国内免费 | 2篇 |
出版年
2022年 | 35篇 |
2021年 | 71篇 |
2020年 | 51篇 |
2019年 | 58篇 |
2018年 | 74篇 |
2017年 | 65篇 |
2016年 | 112篇 |
2015年 | 213篇 |
2014年 | 184篇 |
2013年 | 295篇 |
2012年 | 344篇 |
2011年 | 328篇 |
2010年 | 196篇 |
2009年 | 181篇 |
2008年 | 283篇 |
2007年 | 321篇 |
2006年 | 282篇 |
2005年 | 290篇 |
2004年 | 264篇 |
2003年 | 261篇 |
2002年 | 232篇 |
2001年 | 54篇 |
2000年 | 44篇 |
1999年 | 67篇 |
1998年 | 75篇 |
1997年 | 56篇 |
1996年 | 60篇 |
1995年 | 75篇 |
1994年 | 37篇 |
1993年 | 55篇 |
1992年 | 56篇 |
1991年 | 53篇 |
1990年 | 36篇 |
1989年 | 39篇 |
1988年 | 30篇 |
1987年 | 46篇 |
1986年 | 37篇 |
1985年 | 51篇 |
1984年 | 46篇 |
1983年 | 46篇 |
1982年 | 58篇 |
1981年 | 57篇 |
1980年 | 58篇 |
1978年 | 46篇 |
1977年 | 32篇 |
1976年 | 35篇 |
1975年 | 27篇 |
1974年 | 31篇 |
1972年 | 27篇 |
1969年 | 27篇 |
排序方式: 共有6094条查询结果,搜索用时 0 毫秒
31.
Response of grasshoppers (Orthoptera: Acrididae) to livestock grazing in southeastern Arizona: differences between seasons and subfamilies 总被引:3,自引:0,他引:3
Summary Grasshopper densities were compared between grazed and ungrazed semidesert grassland sites in southeastern Arizona. Bouteloua-dominated perennial grass cover was about 1.5 times greater on the livestock exclosure. Grasshoppers were 3.7 times more abundant on the protected area in the summers of 1983 and 1984, when dominant species were grass-feeding members of the subfamily Gomphocerinae. In fall 1984, grasshoppers were 3.8 times more common on the grazed site, when dominants were mainly herb-feeders in the subfamily Melanoplinae. These results indicate important seasonal and taxonomic differences in the responses of grasshoppers to the activities of vertebrate grazers. 相似文献
32.
33.
Bettina M. Diekamp H. Carl Gerhardt 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1992,171(2):245-250
1. We derived audiograms from recordings of multiunit activity in the torus semicircularis of 10 males and 6 females of the spring peeper from central Missouri, USA. We used free-field stimulation with tone bursts that had temporal properties similar to typical advertisement calls and that ranged in frequency from 500-6000 Hz. 2. Audiograms from different electrode positions in the same animal had the same general shape. There was no evidence of tonotopy. 3. Audiograms showed two regions of maximal sensitivity: a low-frequency region (500-700 Hz); and a high-frequency region (2000-4000 Hz). Absolute thresholds and frequencies of maximum sensitivity varied considerably from individual to individual. 4. Audiograms derived from all individuals of each sex indicated that in the high-frequency region, corresponding to the frequency range of advertisement calls, males were more broadly tuned than females. However, tuning in both sexes was relatively weak, and the data predict relatively little selectivity in behavioral responses over the entire range of variation in frequency of the advertisement call in local populations. 5. The results are discussed in terms of behavioral experiments with both males and females from the same populations in central Missouri. We show that merely summarizing the audiograms based on estimates of minimum thresholds of a population or species may mask significant individual differences in tuning. Moreover, most behavioral studies are conducted at playback levels considerably above threshold. For these reasons, behavioral selectivity is not always accurately predicted by inspection of "average" audiograms. 相似文献
34.
Summary Semi-thin sections of three-dimensional reaggregates from adult female rat pituitary, cultured in serum-free defined medium, were stained for prolactin, gonadotropin, thyrotropin, growth hormone and S-100, using the double immunolabelling technique. The frequency of juxtaposition between lactotrophs and gonadotrophs was enumerated and compared with the expected frequency at random distribution of polygonal cell profiles in a hexagonal configuration. The proportions of lactotrophs and gonadotrophs in the aggregate sections were determined using stereometrical analysis. The observed frequency of juxtaposition did not differ significantly from the expected frequency. Hence, no reason was found to assume a selective adhesion between lactotrophs and gonadotrophs in adult female rat pituitary reaggregates. A constant proportion of lactotrophs was found to meet the criteria of a cup-shaped morphology, and 70%±9% (mean ±S.D.) of these so-called cupshaped lactotrophs were found to be juxtaposed at their concave side to gonadotrophs. Administration of 0.01 nM 17-oestradiol to the culture medium resulted in a significant reduction of the proportion of cup-shaped lactotrophs but did not affect the selectivity of juxtaposition to gonadotrophs. The selectivity of juxtaposition between cup-shaped lactotrophs and gonadotrophs may be the morphological correlate of the functional relationship between these cells, which are known to be involved in an intra-pituitary paracrine communication system. 相似文献
35.
36.
37.
38.
Rates of oxidation of Mn(II) were measured by an in situ incubation technique in the water column of Toolik Lake, Alaska. Measured rates were lower than those observed in other aquatic systems but were sufficient to oxidize all Mn(II) in the lake within a 3-month period. Measured rates compared favorably with rates estimated from a previous study of the geochemical cycling of Mn in Toolik Lake. The Mn(II) oxidation was largely microbially mediated, as indicated by inhibition of oxidation rates by sodium azide. Azide had been previously demonstrated to be a suitable microbial poison for studying Mn(II) oxidation in seawater. This study demonstrates that azide is also a suitable poison for freshwaters and that it inhibits microbial but not abiotic oxidation of Mn(II). Manganese(II) oxidation rates were similar during cold, under-ice conditions in early spring and during warmer summer conditions. This observation suggests that Mn(II) concentration, rather than temperature or oxygen concentration, is the most important factor regulating Mn(II) oxidation rates in Toolik Lake. 相似文献
39.
40.
David A. Priestley Brenda G. Werner A. Carl Leopold Murray B. McBride 《Physiologia plantarum》1985,64(1):88-94
In view of their possible role in oxidative deterioration of seeds and pollen, organic free radicals were measured by electron spin resonance in embryonic axes and cotyledons of soybean [ Glycine max (L.) Merr], embryo and endosperm fractions of corn [ Zea mays L.] and pollen of cattail [ Typha latifolia L.]. A pronounced decline in the radical signal ensued when hydration increased above about 7% (wet weight basis) in both the seed materials and in pollen. Moderate hydration of the soybean axis followed by drying led to a small decrease in organic free radicals compared to untreated material, especially if the desiccation step was performed under nitrogen. In a comparison of soybeans of various ages under normal storage, organic free radical levels in the axis showed little or no increase with age. In marked contrast, over 5 days of accelerated aging at 40°C and near-saturating humidity, organic radical levels approximately doubled in the axis. This pronounced increase in free radical content was not associated with a decrease in the proportion of polyunsaturated fatty acids. The data suggest that hydration of seed and pollen causes a release of free radicals from the trapped state. 相似文献