首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2419篇
  免费   95篇
  国内免费   124篇
  2638篇
  2023年   24篇
  2022年   34篇
  2021年   69篇
  2020年   58篇
  2019年   63篇
  2018年   67篇
  2017年   91篇
  2016年   87篇
  2015年   60篇
  2014年   102篇
  2013年   146篇
  2012年   67篇
  2011年   91篇
  2010年   75篇
  2009年   157篇
  2008年   168篇
  2007年   165篇
  2006年   127篇
  2005年   103篇
  2004年   95篇
  2003年   80篇
  2002年   64篇
  2001年   34篇
  2000年   35篇
  1999年   59篇
  1998年   37篇
  1997年   24篇
  1996年   33篇
  1995年   37篇
  1994年   30篇
  1993年   26篇
  1992年   35篇
  1991年   24篇
  1990年   17篇
  1989年   19篇
  1988年   17篇
  1987年   14篇
  1986年   30篇
  1985年   23篇
  1984年   27篇
  1983年   8篇
  1982年   19篇
  1981年   20篇
  1980年   12篇
  1979年   18篇
  1978年   8篇
  1977年   19篇
  1976年   9篇
  1975年   3篇
  1974年   3篇
排序方式: 共有2638条查询结果,搜索用时 0 毫秒
91.
The types of the parasitic wasps described in the families Proctotrupidae, Diapriidae, and Scelionidae (Hymenoptera) from the Upper Eocene Florissant locality (Colorado, United States) are revised. All the specimens are shown to belong to the family Proctotrupidae, representing the genera Oxyserphus, Mischoserphus, Nothoserphus, and Palaeoteleia. The known species are redescribed and two new species, Mischoserphus bruesi sp. nov. and Nothoserphus rasnitsyni sp. nov., are described. A new combination, Oxyserphus exhumatus (Brues, 1910), comb. nov., and a new synonymy, Paramesius defectus Brues, 1910, syn. nov. = Oxyserphus exhumatus (Brues, 1910), are established.  相似文献   
92.
Pinus mutoi is described as a new species on the basis of a permineralized seed cone from the Upper Cretaceous of Hokkaido, Japan. The cone is at least 20 cm long and up to 6 cm in diameter, consisting of a cone axis and numerous cone-scale complexes that are arranged helically around the axis. Two winged seeds are borne on the adaxial surface of each ovuliferous scale. Each complex receives a single trace from the vascular cylinder of the cone axis. In the scale base, all the resin canals occur abaxially to the vascular strand. The spatulate bract of the fossil is unique to the specimen among the cones of both living and fossil Pinus. The central umbo, broad sclerotic cortex of cone axis, and absence of serotinous features of the fossil cone suggest affinity with the subsection Sylvestres of the section Pinus, subgenus Pinus. This is the first record of permineralized preserved Pinus cone from the Cretaceous of Eastern Eurasia.  相似文献   
93.
Macroalgae harbour specific microbial communities on their surface that have functions related to host health and defence. In this study, the bacterial biofilm of the marine brown alga Fucus spiralis was investigated using 16S rRNA gene amplicon-based analysis and isolation of bacteria. Rhodobacteraceae (Alphaproteobacteria) were the predominant family constituting 23% of the epibacterial community. At the genus level, Sulfitobacter, Loktanella, Octadecabacter and a previously undescribed cluster were most abundant, and together they comprised 89% of the Rhodobacteraceae. Supported by a specific PCR approach, 23 different Rhodobacteraceae-affiliated strains were isolated from the surface of F. spiralis, which belonged to 12 established and three new genera. For seven strains, closely related sequences were detected in the 16S rRNA gene dataset. Growth experiments with substrates known to be produced by Fucus spp. showed that all of them were consumed by at least three strains, and vitamin B12 was produced by 70% of the isolates. Since growth of F. spiralis depends on B12 supplementation, bacteria may provide the alga with this vitamin. Most strains produced siderophores, which can enhance algal growth under iron-deficient conditions. Inhibiting properties against other bacteria were only observed when F. spiralis material was present in the medium. Thus, the physiological properties of the isolates indicated adaption to an epiphytic lifestyle.  相似文献   
94.
The results of 6-year-long observations on the size of some fish species of the Far East State Marine Reserve (Peter the Great Bay, Sea of Japan) suggest that resident fishes attain maximum size in the waters of the reserve. Compared to previously known sizes of fish, record values were found for the white-spotted greenling Hexagrammos stelleri (540 mm), the frog sculpin Myoxocephalus stelleri (490 mm), the snowy sculpin M. brandti (418 mm); the fringed blenny Chirolophis japonicus (520 mm), and some other fishes. These data confirm the opinion that the Marine Reserve waters are the reservation of the genofond of the inhabiting fish.  相似文献   
95.
Summary The Upper Triassic Dachsteinkalk of the Hochk?nig Massif, situated 50 km south of Salzburg in the Northern Calcareous Alps, corresponds to a platform margin reef complex of exceptional thickness. The platform interior limestones form equally thick sequences of the well known cyclic Lofer facies. Sedimentation in the reef complex was not so strongly controlled by low-amplitude sea-level oscillations as was the Lofer facies. The westernmost of the 8 facies of the reef complex is an oncolite-dominated lagoon, in which wave-resistant stromatolite mounds with a relief of a few metres were periodically developed. The transition to the central reef area is accomplished across the back-reef facies. In the back-reef facies patch reefs and calcisponges appear. The proportion of coarse bioclastic sediment increases rapidly over a few hundred metres before the central reef area is encountered. The central reef area consists of relatively widely spaced small patch reefs that did not develop wave-resistant reef framework structures. The bulk of the sediment in the central reef area is coarse bioclastic material, provided by the dense growth of reef organisms and the wave-induced disintegration of patch reefs. Collapse of the reef margin is recorded by the supply of large blocks of patch reef material to the upper reef slope. Additionally, coarse, loose bioclastic debris was supplied to the upper reef slope and this was incorporated into debris flows on the reef slope and turbidites found at the base of the slope and in the off-reef facies. Partially lithified packstones and wackestones of the lower to middle reef slope were modified by mass movement to form breccia and rudstone sheets. The latter reach out hundreds of metres into the off-reef facies environment. A reef profile is presented which was derived by the restoration of strike and dip information. In conjunction with constraints imposed by sedimentary facies related to slope processes, the angle of slope in the reef margin area ranged from 11° to 5°, forming a concave (dished downwards) slope. Water depth estimations require that the central reef area did not develop in water of less than 10 metres depth. At the reef margin water depths were about 30 metres, at the base of the reef slope 200 metres and deepening in the off-reef facies to 250 metres. While previous work on reef complexes from this type of setting suggests growth in a heavily storm-dominated environment, the present author finds little evidence for the storm generation of the fore reef breccias, although there is good evidence for storm-influenced sedimentation and reworking in the central reef area. Post-depositional processes were characterised by continued slope processes causing brecciation and hydraulic injection of red internal sediments downwards into the reef slope and off-reef limestones. Hydrothermal circulation caused a number of phases of post-depositional (diagenetic) brecciation. There appears not to have been an important period of emergence at the Triassic/Jurassic boundary.  相似文献   
96.
The Kimmeridgian Series situated at the North ofLisbon is included in the Portuguese Lusitanian basin. Its sandy clays pelagic facies with planctonic foraminifera contains a new species of bivalvia: Aulacomyella abadiensis related to A. neogeaeImlay from the Middle Kimmeridgian of Mexico and from the Kimmeridgian of Crimea. This species confirms the Middle Kimmeridgian.  相似文献   
97.
Vertebrate remains and nummulites have been investigated from the Late Eocene—Early Oligocene transgressive sequence of the «Chaînes subalpines south of the lake Annecy. The sedimentary environments range from fluvio-lacustrine shales and conglomerates at the base to hemipelagic Globigerina shales at the top. The fluvio-lacustrine white marls contain mammals of the upper, probably terminal Ludian, with affinities to the assemblage of Saint-Capraise. The brackish Cerithium beds have furnished charophytes of the Bembridge zone. The nummulite limestones are not dated precisely, but isolated, probably redeposited nummulites from the base of the Globigerina shales are of terminal Eocene or basal Oligocene age. The Globigerina shales themselves belong to the Oligocene.  相似文献   
98.
Places of refuge for ships in distress is a topic before the International Maritime Organization as a result of several recent well-publicized refusals by maritime authorities of coastal states to allow such ships to enter sheltered waters within national jurisdiction. The traditional right of refuge of the crew, ship, and cargo is pitted against threat perceptions held by coastal states resulting in a "not in my backyard" syndrome. Instances of modern state practice seem to restrict the right of refuge to a purely humanitarian dimension. There is a need to reevaluate the right of refuge and to establish a system of places of refuge on the basis of regional cooperation to counter the potential threat of stricken ships that are unable to effect necessary repairs in sheltered areas within national jurisdiction.  相似文献   
99.
The novel marine pyrrole alkaloid neolamellarin A derived from sponge has been shown to inhibit hypoxia-induced HIF-1 activity. In this work, we designed and synthesized neolamellarin A and its series of derivatives by a convergent synthetic strategy. The HIF-1 inhibitory activity and cytotoxicity of these compounds were evaluated in Hela cells by dual-luciferase reporter gene assay and MTT assay, respectively. The results showed that neolamellarin A 1 (IC50 = 10.8 ± 1.0 μM) and derivative 2b (IC50 = 11.9 ± 3.6 μM) had the best HIF-1 inhibitory activity and low cytotoxicity. Our SAR research focused on the effects of key regions aliphatic carbon chain length, aromatic ring substituents and C-7 substituent on biological activity, providing a basis for the subsequent research on the development of novel pyrrole alkaloids as HIF-1 inhibitors and design of small molecule probes for target protein identification.  相似文献   
100.
The purpose of this study was to characterize responses in oxygen uptake ( V·O2), heart rate (HR), perceived exertion (OMNI scale) and integrated electromyogram (iEMG) readings during incremental Nordic walking (NW) and level walking (LW) on a treadmill. Ten healthy adults (four men, six women), who regularly engaged in physical activity in their daily lives, were enrolled in the study. All subjects were familiar with NW. Each subject began walking at 60 m/min for 3 minutes, with incremental increases of 10 m/min every 2 minutes up to 120 m/min V·O2 , V·E and HR were measured every 30 seconds, and the OMNI scale was used during the final 15 seconds of each exercise. EMG readings were recorded from the triceps brachii, vastus lateralis, biceps femoris, gastrocnemius, and tibialis anterior muscles. V·O2 was significantly higher during NW than during LW, with the exception of the speed of 70 m/min (P < 0.01). V·E and HR were higher during NW than LW at all walking speeds (P < 0.05 to 0.001). OMNI scale of the upper extremities was significantly higher during NW than during LW at all speeds (P < 0.05). Furthermore, the iEMG reading for the VL was lower during NW than during LW at all walking speeds, while the iEMG reading for the BF and GA muscles were significantly lower during NW than LW at some speeds. These data suggest that the use of poles in NW attenuates muscle activity in the lower extremities during the stance and push-off phases, and decreases that of the lower extremities and increase energy expenditure of the upper body and respiratory system at certain walking speeds.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号