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   3435篇
  免费   236篇
  国内免费   154篇
  2023年   53篇
  2022年   62篇
  2021年   111篇
  2020年   119篇
  2019年   141篇
  2018年   148篇
  2017年   108篇
  2016年   105篇
  2015年   143篇
  2014年   172篇
  2013年   257篇
  2012年   129篇
  2011年   177篇
  2010年   161篇
  2009年   178篇
  2008年   189篇
  2007年   165篇
  2006年   143篇
  2005年   119篇
  2004年   122篇
  2003年   94篇
  2002年   97篇
  2001年   69篇
  2000年   68篇
  1999年   65篇
  1998年   43篇
  1997年   58篇
  1996年   60篇
  1995年   38篇
  1994年   42篇
  1993年   33篇
  1992年   32篇
  1991年   43篇
  1990年   36篇
  1989年   21篇
  1988年   22篇
  1987年   17篇
  1986年   20篇
  1985年   20篇
  1984年   13篇
  1983年   10篇
  1982年   15篇
  1981年   15篇
  1980年   9篇
  1979年   21篇
  1978年   13篇
  1977年   7篇
  1976年   14篇
  1975年   5篇
  1973年   13篇
排序方式: 共有3825条查询结果,搜索用时 31 毫秒
991.
The food versus fuel issue has attracted considerable attention with the rapid growth of global biofuel production. The previous literature examining the interconnectedness between biofuel, fossil fuel and agricultural markets employed futures and spot prices. However, food security needs to be discussed with non-financial market prices, such as wholesale prices, since retail companies and households usually do not purchase products directly from financial markets, which encompass speculative activities, making them more volatile than non-financial ones. This paper focuses on non-financial markets in the United States to investigate the price-interconnection of biodiesel, highway diesel, crude oil, and soybean, initially applying partial wavelet coherence and the Diebold-Yilmaz connectedness index to price transmission research on biodiesel, highway diesel, crude oil, and soybean. Our main results are as follows: First, significant coherence between biodiesel and soybean, biodiesel and highway diesel, and highway diesel and crude oil is identified in the short and long term. Second, crude oil and biodiesel prices are found to be net transmitters, while soybean and highway diesel prices are net receivers. Finally, the crude oil market is identified as the source of spillovers among the four markets, and strongly influences the highway diesel market.  相似文献   
992.
The introduction of steel tools and new crops radically transformed the Yanoama Indians' pre-Columbian economy. A consideration of the impact of these innovations allows for the reconstruction of that economy which suggests a higher dependence upon foraging, a lower population density, more mobile settlement pattern, and heavier workload, than today. This reconstruction challenges both current speculations about pre-Columbian Amazonian economic systems which apply Sahlins' concept of aboriginal affluence and those concerning the relationship between Amazonian environment and cultural development. This reconstruction, confirmed in part by fragmentary historical sources, explores some of the dramatic changes which must have taken place since contact. The Yanoama's agricultural subsistence base is a recent innovation, made possible since contact by new tools and new crops.This article has benefitted considerably from conversations with Emilio Fuentes to whom I offer my gratitude. I would also like to thank the anonymous reviewer who made detailed comments on a first draft.  相似文献   
993.
Using transmission and scanning electron microscopy, the early migration of juvenile Fasciola hepatica was examined in naive and resistant rats. In naive rats, the migration of flukes to the peritoneal cavity was uneventful. In resistant rats, flukes were rapidly coated with antibody whilst still in the gut lumen and a proportion of the flukes were unable to penetrate the intestinal wall. Those that did penetrate were unharmed as they crossed the gut wall, but on entering the peritoneal cavity they were coated with antibody and host cells including eosinophils, neutrophils, macrophages, and mast cells. Eosinophils were seen degranulating onto the fluke surface, and this appeared to result in the erosion of the tegumental syncytium.  相似文献   
994.
A common and widespread disease of honey bees, Apis mellifera, is caused by an unoccluded, Feulgen-positive, filamentous nuclear virus. Ovoid viral particles seen in diseased bee hemolymph consisted of a folded nucleocapsid within a viral envelope and were 0.40 by 0.10 μm. Virions with unfolded nucleocapsids were about 3060 by 60 nm. The disease was transmissible to bees both per os and by injection, but efforts to infect oriental cockroaches, Blatta orientalis, and the greater wax moth, Galleria mellonella, failed. The disease is apparently the same as that described as a rickettsial disease of European bees.  相似文献   
995.
SYNOPSIS. Epizootic outbreaks of red-sore disease in several reservoirs in the southeastern United States have been reported to cause heavy mortality among several species of fish having sport and commercial value. The etiologic agent is said to be the peritrich ciliate Epistylis sp.; secondary infection by the gram-negative bacterium Aeromonas hydrophila produces hemorrhagic septicemia which results in death. However, in recent studies on the largemouth bass Micropterus salmoides, Epistylis sp. could be isolated from only 35% of 114 lesions from 114 fish, while A. hydrophila was found in 96% of the same lesions. Transmission and scanning electron microscopy of lesions associated with red-sore disease indicate that neither the stalk nor the attachment structure of Epistylis sp. have organelles capable of producing lytic enzymes. Since other investigators have shown that A. hydrophila produces strong lytic toxins, and in absence of evidence to the contrary, it is concluded that Epistylis sp. is a benign ectocommensal and that A. hydrophila is the primary etiologic agent of red-sore disease.  相似文献   
996.
SYNOPSIS. Oocysts of Eimeria nieschulzi from the laboratory rat, Rattus, norvegicus , were studied by scanning and transmission electron microscopy. Oocysts had a rough outer wall with apparent random depressions. The oocyst wall is composed of 2 layers: an osmiophilic outer layer consisting of a rough external and smooth internal surface, and a relatively thick, electron-lucent inner layer. The outer layer is composed of a dense, coarsely granular matrix. The inner layer consists of homogeneous fine granular material interspersed with coarse osmiophilic granules and contains one closely applied membrane on the outermost surface. Several raised lenticular areas are seen on the coarse outer surface of the inner layer. These layers are 102 (75–128) and 176 (135–204) nm thick, respectively.
The sporocyst wall is thin, consisting of 3 to 4 unit membranes, and measures 27 (18–34) nm thick.  相似文献   
997.
SYNOPSIS. The structure and morphogenesis of the ventral ciliature of Paraurostyla hymenophora (Stokes) are described. The oral primordium apparently originates in association with transverse cirrus #6, from which it migrates anteriorly simultaneous with kinetosomal proliferation. The primordium eventually forms an elongate ciliary field from which the future opisthe's fronto-ventro-transverse (FVT) and undulating membrane primordial fields arise. Concomitantly, the future proter's FVT primordial field is initiated by the disaggregation of frontal cirri #4, #5, and #6. Primordia then develop simultaneously within marginal and ventral cirral rows by a disaggregation of cirri within the respective rows, and do not give rise to new cirri until the FVT fields complete segregation into discrete cirri. Near the completion of cirral production from the FVT primordia, each ventral cirral primordium (VCP) forms the 2 rightmost transverse cirri. Segregation of new cirri within the marginal cirral primordia and VCP then occurs, eventually replacing all old cirri within their respective marginal and ventral cirral rows. At the end of cortical morphogenesis, all old ciliary organelles, with the exception of the adoral zone of membranelles, are either reorganized or replaced. These results suggest an evolutionary affinity between the ventral and marginal cirral rows and raise questions about the control of the developmental competence of individual primordia.  相似文献   
998.
Summary The development of the olfactory organ in the rainbow fish, Nematocentris maccullochi, was studied using scanning and transmission electron microscopy; it was compared with the developmental process in other teleosts, especially in the closely related atherinids and cyprinodonts. The formation of the nares parallels that in atherinids, salmonids, cyprinids and heterosomats, but differs from that found in cyprinodonts. Another ontogenetic feature in which the olfactory organs of the rainbow fish and also of atherinids differ from those of cyprinodonts, is the occurrence of transitory kinociliary cells which disappear during the postlarval period. The divergent evolutionary pathways are discussed with reference to experimental investigations. During development, ciliated and microvillous receptor cell types occur. At the primary larval stage ciliated receptor neurons are exclusively present. At a later stage the microvillous type develops and becomes equal in frequency. Thus, the microvillous receptor represents a separate type of olfactory neuron and is not a progenitor of the ciliated receptor cell.This work was supported by the Deutsche ForschungsgemeinschaftThe authors would like to acknowledge Prof. G. Hartmann (Hamburg) and Prof. V. Storch (Kiel) for use of the SEM, Prof. K. Seifert (Neumünster) for valuable discussion. For skillful technical assistance we are indebted to Miss C. Heim, Mrs. K. Hoffmann, and Mr. H.P. Dreyer  相似文献   
999.
Heat-inactivated serum from chickens with transmission-blocking immunity to Plasmodium gallinaceum prevented the in vitro development of oökinetes from gametocytes of P. gallinaceum only when present during the period between the initiation of gametogeriesis and the release of the microgametes. When added after this time immune serum failed to suppress oökinete development. Immune serum did not prevent the formation of gametes from gametocytes. These results are interpreted to indicate that immune serum contains factors which prevent fertilization of the malarial gametes but which do not affect the development of the zygote once fertilization has taken place. Two distinct reactions of malarial gametes with serum from chickens with transmission-suppressing immunity are described—the gamete-agglutination (AG) reaction and the microgamete surface-fixation (SF) reaction. Both reactions were associated with the immunoglobulin fraction of immune serum. The presence of SF antibodies during a blood infection correlated closely with effective transmission-blocking immunity in vivo; AG antibodies, on the other hand, were present in various circumstances in the absence of transmission-blocking immunity. AG and SF antibodies occurred not only in birds immunized with P. gallinaceum-gamete preparations but also during or following infections in unimmunized birds; SF antibodies appeared only following the peak of asexual infection in unimmunized birds and were of low titer. In immunized birds blood infections following live challenge invariably boosted low levels of SF antibodies. The results of immunization of chickens and Rhesus monkeys with gametes of their respective malaria parasites, P. gallinaceum and P. knowlesi, are compared.  相似文献   
1000.
Immunization of chickens by intravenous inoculation of preparations derived from blood infected with Plasmodium galinaceum led to reduced infectivity to mosquitoes (Aedes aegypti) during subsequently induced blood infection but had little effect on the course of asexual parasitemia. Immunization with preparations containing intracellular gametocytes was much less effective than immunization with preparations in which the extracellular gametes had been released during gametogenesis (emergence and exflagellation) prior to inoculation. By far the most effective material were preparations of partially purified gametes of both sexes. Three weekly inoculations of this material resulted in 99.99% suppression of infectivity to mosquitoes during subsequently induced blood infection. Preparations of purified gamete material from which gametes of one or another sex were absent were considerably less effective as transmission-blocking immunogens than the mixed gamete preparation. It is possible that the two sexes of gamete act synergistically to induce transmission-blocking immunity.  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