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   342919篇
  免费   28684篇
  国内免费   108篇
  371711篇
  2018年   2852篇
  2017年   2739篇
  2016年   3942篇
  2015年   5365篇
  2014年   6281篇
  2013年   8801篇
  2012年   10021篇
  2011年   9992篇
  2010年   6826篇
  2009年   6167篇
  2008年   8878篇
  2007年   9317篇
  2006年   8612篇
  2005年   8304篇
  2004年   8026篇
  2003年   7781篇
  2002年   7792篇
  2001年   18159篇
  2000年   18394篇
  1999年   14141篇
  1998年   4245篇
  1997年   4526篇
  1996年   4338篇
  1995年   4036篇
  1994年   3973篇
  1993年   3794篇
  1992年   11477篇
  1991年   11119篇
  1990年   10674篇
  1989年   10239篇
  1988年   9478篇
  1987年   8838篇
  1986年   8077篇
  1985年   7958篇
  1984年   6388篇
  1983年   5561篇
  1982年   4102篇
  1981年   3590篇
  1980年   3352篇
  1979年   6108篇
  1978年   4568篇
  1977年   4123篇
  1976年   3759篇
  1975年   4329篇
  1974年   4497篇
  1973年   4389篇
  1972年   4090篇
  1971年   3498篇
  1970年   3168篇
  1969年   2968篇
排序方式: 共有10000条查询结果,搜索用时 12 毫秒
71.
During observations in soybean fields, 136Neoscona arabesca (Walckenaer) and 46Tetragnatha laboriosa Hentz were collected with prey. Significantly different prey compositions were recorded, withN. arabesca utilizing a large number ofColeoptera whileT. laboriosa captured primarilyHemiptera-Homoptera. Neoscona arabesca was observed to feed primarily in the evening following web construction, whileT. laboriosa fed predominantly in the morning. Although both of these species captured relatively large numbers of homopterans, most of their prey were species which are not considered to be pests in soybean.
Résumé Lors d'observations dans des champs de soja, 136Neoscona arabesca (Walckenauer) et 46Tetragnatha laboriosa Hentz furent recueillies avec leurs proies. Des différences significatives furent notées dans la composition des différents groupes de proies,N. arabesca capturant un grand nombre de Coléoptères tandis queT. laboriosa capture en premier lieu des Hémiptères (Hemiptera-Homoptera). Il fut observé queNeoscona arabesca se nourrit surtout dans la soirée qui suit la construction de la toile, alors queT. laboriosa se nourrit le plus souvent le matin. Bien que ces deux espèces aient capturé un nombre relativement important d'homoptères, la plupart de leurs proies étaient des espèces qui ne sont pas considérées comme nuisibles au soja.


This paper (82-7-20) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Director.  相似文献   
72.
Serologic and primary structural analyses of Ig chains secreted by several rabbit-mouse hybridomas have shown that these hybrid cells produce heavy (H) or light (L) chains identical to those isolated from rabbit sera. Two of the cell lines (7D2, 7D6) secreted rabbit H chains with a m.w. of 55,000 each of which expressed a full complement of variable and constant region allotypes (a3, d11, e15). These apparently normal rabbit H chains were secreted in a complex with a m.w. about 130,000, and serologic studies indicated that this complex contained a covalently linked mouse kappa L chain. Two other cell lines (4C1, 12F2) produced allotype b4 L chains with m.w. of 23,000 and 25,000, and a third (1D4P5) produced an allotype b5 L chain with a m.w. of 23,000. Serologic analyses indicated that the allotypes on these chains are equivalent to those expressed by normal rabbit Ig molecules. Partial amino acid sequence data obtained for the L chain products showed them to be typical of rabbit L chains, and to be significantly different from mouse L chains.  相似文献   
73.
74.
75.
The ultrastructure of goose intestinal epithelial cells infected with various stages of E. kotlani has been described. No changes in the ultrastructure of the nucleus and cytoplasm were detected in cells, containing asexual stages. Damage of host cell structures, except Golgi complex, was not observed until gamogenesis. It is suggested that these changes are caused by the intensive "exploitation" of differentiated epithelial cells by large coccidian stages (macro- and microgametocytes). It is concluded that the epithelial cell of the goose, as of other animals, may function beyond crypts without the regulating influence of the nucleus.  相似文献   
76.
77.
Moderate predation on meiofauna by the macrobenthos of the Wadden Sea   总被引:4,自引:0,他引:4  
The role of meiofauna in the trophic web of marine ecosystems is a controversial topic. During an experimental study on tidal flats in the eastern part of the North Sea, potential predators on meiofauna were kept in enclosures. A gobiid fish, shrimp, crabs, an amphipod, four polychaete worms and a nemertine were tested for their ability to decimate nematodes, turbellarians, ostracods and benthic copepods. Only few macrobenthic species preyed heavily on permanent meiofauna: juveniles of the shore crabCarcinus maenas, the hermit crabPagurus bernhardus with the colonial hydrozoanHydractinia echinata on its shells, and the rag wormNereis diversicolor. When benthic infauna was protected from these predators with exclosures, juvenile macrofauna responded with a marked increase in number, while the permanent meiofauna remained unaltered except for a limited increase in nematodes. It is concluded that in the Wadden Sea abundance of permanent meiofauna (Nematoda, Turbellaria, Ostracoda, Copepoda) is only locally or temporarily regulated by macrobenthic predators.I acknowledge a grant from the Deutsche Forschungsgemeinschaft (DFG).  相似文献   
78.
The singlet oxygen traps, 2,5-diphenylfurane and 1,3-diphenylisobenzofurane were oxidized to cis-benzoylethylene and o-dibenzoylbenzene during the decomposition of diisopropyl-N-nitrosamine catalyzed by peroxidase. Singlet oxygen quenchers inhibited this conversion and also the chemiluminescence accompaying the catalyzed reaction. The chemiluminescence is enhanced by 1,4-diazobicyclo (2.2.2) octane, fluorescein, eosin rhodamine B and rose bengal but little effect was detected in the presence of 9,10-dibromoanthracene-2-sulfonate, 9,10-diphenylanthracene-2-sulfonate and anthracene-2-sulfonate. An emission spectrum of the unsensitized reaction in 560 – 600 nm region was observed. It is concluded that singlet oxygen is formed during peroxidase catalyzed degradation of diisopropyl-N-nitrosamine.  相似文献   
79.
Two new lecanid rotifers from India   总被引:5,自引:5,他引:0  
B. K. Sharma 《Hydrobiologia》1978,60(2):191-192
While working on the material collected from Calcutta and its environs, the author came across two new lecanid rotifers which have been described in this paper as a new species and a new variety.  相似文献   
80.
The fine structure of trophozoites and especially of merozoites of Babesia herpailuri is described before and after treatment with Imidocarb (Wellcome). The mostly piriform to oval merozoites possess an outer membrane and a supporting membrane below. The intratorium consists of a polar ring, rhoptries micronemes and the sperical body which lies beside the big nucleus and next to mitochondria. The endoplasmic reticulum and invaginations are not clearly formed. The cellular changes of Babesia herpailuri, observed one hour after drug treatment in trophozoites and six hours later in merozoites, concern the form and function of the parasite: widening of the subpellicular endoplasmic reticulum and of the perinuclear space; sporadic dilatation of the endoplasmic reticulum of the merozoites (9 fig.). Damaged membranes, dissolution of the cellular membrane, disintegration of the nuclei as are known effects of the Berenil treatment to Babesia herpailuri, are not noted results after the Imidocarb treatment. The original membrane systems of trophozoites as well as of merozoites, remain unaffected by the drug as long as investigations were carried on (24 h). The satisfying prophylactic effect of Imidocarb as well as the insignificant cellular damages on merozoites may be due to the small feeding of hemoglobin.  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