全文获取类型
收费全文 | 1285597篇 |
免费 | 110247篇 |
国内免费 | 1472篇 |
专业分类
1397316篇 |
出版年
2021年 | 17930篇 |
2020年 | 12689篇 |
2019年 | 16277篇 |
2018年 | 17481篇 |
2017年 | 16222篇 |
2016年 | 28052篇 |
2015年 | 42534篇 |
2014年 | 50587篇 |
2013年 | 76871篇 |
2012年 | 35130篇 |
2011年 | 23095篇 |
2010年 | 42919篇 |
2009年 | 44547篇 |
2008年 | 22305篇 |
2007年 | 19703篇 |
2006年 | 25595篇 |
2005年 | 26541篇 |
2004年 | 25796篇 |
2003年 | 23383篇 |
2002年 | 21541篇 |
2001年 | 29073篇 |
2000年 | 26130篇 |
1999年 | 27415篇 |
1998年 | 24892篇 |
1997年 | 24717篇 |
1996年 | 24346篇 |
1995年 | 22363篇 |
1994年 | 22310篇 |
1993年 | 21310篇 |
1992年 | 24246篇 |
1991年 | 22828篇 |
1990年 | 21405篇 |
1989年 | 22568篇 |
1988年 | 20552篇 |
1987年 | 19555篇 |
1986年 | 18585篇 |
1985年 | 20622篇 |
1984年 | 20898篇 |
1983年 | 18511篇 |
1982年 | 19644篇 |
1981年 | 19062篇 |
1980年 | 17753篇 |
1979年 | 17039篇 |
1978年 | 16406篇 |
1977年 | 16065篇 |
1976年 | 15275篇 |
1975年 | 14587篇 |
1974年 | 15027篇 |
1973年 | 15382篇 |
1972年 | 12886篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
211.
Stability and procured instability characterize two opposing types of swimming, steady and maneuvering, respectively. Fins can be used to manipulate flow to adjust stability during swimming maneuvers either actively using muscle control or passively by structural control. The function of the dorsal fins during turning maneuvering in two shark species with different swimming modes is investigated here using musculoskeletal anatomy and muscle function. White‐spotted bamboo sharks are a benthic species that inhabits complex reef habitats and thus have high requirements for maneuverability. Spiny dogfish occupy a variety of coastal and continental shelf habitats and spend relatively more time cruising in open water. These species differ in dorsal fin morphology and fin position along the body. Bamboo sharks have a larger second dorsal fin area and proportionally more muscle insertion into both dorsal fins. The basal and radial pterygiophores are plate‐like structures in spiny dogfish and are nearly indistinguishable from one another. In contrast, bamboo sharks lack basal pterygiophores, while the radial pterygiophores form two rows of elongated rectangular elements that articulate with one another. The dorsal fin muscles are composed of a large muscle mass that extends over the ceratotrichia overlying the radials in spiny dogfish. However, in bamboo sharks, the muscle mass is divided into multiple distinct muscles that insert onto the ceratotrichia. During turning maneuvers, the dorsal fin muscles are active in both species with no differences in onset between fin sides. Spiny dogfish have longer burst durations on the outer fin side, which is consistent with opposing resistance to the medium. In bamboo sharks, bilateral activation of the dorsal in muscles could also be stiffening the fin throughout the turn. Thus, dogfish sharks passively stiffen the dorsal fin structurally and functionally, while bamboo sharks have more flexible dorsal fins, which result from a steady swimming trade off. J. Morphol. 274:1288–1298, 2013. © 2013 Wiley Periodicals, Inc. 相似文献
212.
Abstract In Arabidopsis thaliana, cell fate in developing ovules is determined by the action of the homeodomain factor BELL1 (BEL1) and of the MADS-box factors SEEDSTICK (STK), SHATTERPROOF1 (SHP1) and SHP2. The analysis of the bel1 and the stk shp1 shp2 mutants revealed that the functional megaspore is formed, however, it does not proceed into megagametogenesis. In the bel1 stk shp1 shp2, quadruple mutant megasporogenesis does not take place. In this article we describe a detailed morphological analysis of the quadruple mutant, and we discuss the possibility that BELL1, STK, SHP1 and SHP2 not only control integument identity determination and development, but that they might also play a role during megasporogenesis. 相似文献
213.
Abstract Brassica rupestris Raf. is a chasmophyte species that includes two subspecies, both endemic to Central-Western Sicily (Italy). Inter-Simple Sequence Repeat (ISSR) markers were used to detect genetic diversity within and among eight populations representative of the species' distribution range. High levels of genetic diversity were revealed both at the population (PPB = 53.88%, H S = 0.212, Sh = 0.309) and at the species level (PPB = 96.55%, H T = 0.307, Sh = 0.464). The correlation between genetic and geographical distances was negative (Mantel test, r = ?0.06, P < 0.95). The two subspecies of B. rupestris, subsp. rupestris and subsp. hispida, showed remarkable genetic similarity and molecular data did not unequivocally support their distinctness. The pattern of genetic variation revealed by our study bears important consequences for conservation management: It is desirable to preserve B. rupestris populations in situ with a “dynamic” strategy, while, ex situ conservation programmes might be improved to safeguard maximum genetic diversity. 相似文献
214.
215.
216.
Bidens cordylocarpa is a high polyploid species restricted in distribution to stream sides in the mountains of Jalisco, Mexico. The morphologically
enigmatic species was originally described as a member of the genus Coreopsis, but later transferred to Bidens, largely because the involucral bracts appear most similar to Bidens. Characters of the cypselae, often useful in generic placement, are of no value for this species because the fruits have
features not detected in either Bidens or Coreopsis. Sequences from the internal transcribed spacer region of nuclear ribosomal DNA (ITS) were used to assess the relationships
of Bidens cordylocarpa. The molecular phylogeny places B. cordylocarpa in a strongly supported clade of Mexican and South American Bidens, and provides more definitive evidence of relationships than morphology, chromosome number, or secondary chemistry. Molecular,
morphological, and chromosomal data suggest that B. cordylocarpa is an ancient polyploid, perhaps the remnant of a polyploid complex.
Received August 28, 2000 Accepted February 11, 2001 相似文献
217.
218.
Ramakrishna A. Bangaru Yatish K. Bansal A. R. M. Rao T. P. Gandhi 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2000,739(2)
A rapid, simple and sensitive reversed-phase high-performance liquid chromatographic (HPLC) method has been developed for the measurement of acyclovir concentrations in human plasma and its use in bioavailability studies is evaluated. Unchanged acyclovir has been quantified without the introduction of an internal standard using the present method. Human plasma proteins were selectively precipitated by the addition of 7% perchloric acid to spiked plasma samples or to the plasma samples obtained after acyclovir administration to human volunteers and the mixture was spun at 1000 g for 10 min. The supernatant was directly injected into a Novaflex C18 column and detected at 254 nm. The mobile phase consisted of octane sulfonic acid buffer (pH 2.5) and methanol (92:08). The limit of quantitation for acyclovir in plasma was 20 ng/ml, which enabled the determination of the area under the curve (AUC) more precisely, that is, it is much closer to its extrapolated value. The present method has been successfully applied to samples from bioavailability studies. 相似文献
219.
220.
Stephen M. Reilly 《Journal of morphology》1987,191(2):205-214
Comparison of metamorphosis of skull and hyobranchial system in two species of neotenic salamanders reveals two different types of neoteny. Ambystoma talpoideum is completely neotenic owing to delayed metamorphosis. Notophthalmus viridescens exhibits limited neoteny as a result of incomplete metamorphosis. Morphological details of neoteny are compared to life history in both species in order to discuss the ecological morphology of the two neotenic strategies. Comparisons to Taricha granulosa, Triturus vulgaris, and Ambystoma gracile indicate that these two strategies are widely employed and may represent familial patterns. 相似文献