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   846258篇
  免费   96354篇
  国内免费   470篇
  2016年   10824篇
  2015年   17836篇
  2014年   19466篇
  2013年   25125篇
  2012年   27566篇
  2011年   25289篇
  2010年   17641篇
  2009年   16799篇
  2008年   22139篇
  2007年   22666篇
  2006年   21281篇
  2005年   25800篇
  2004年   24490篇
  2003年   22300篇
  2002年   19620篇
  2001年   34958篇
  2000年   34928篇
  1999年   28851篇
  1998年   10972篇
  1997年   11468篇
  1996年   10969篇
  1995年   10435篇
  1994年   10281篇
  1993年   10073篇
  1992年   24829篇
  1991年   24178篇
  1990年   23903篇
  1989年   23181篇
  1988年   21485篇
  1987年   21061篇
  1986年   19267篇
  1985年   19402篇
  1984年   15928篇
  1983年   13833篇
  1982年   10905篇
  1981年   9711篇
  1980年   9228篇
  1979年   15332篇
  1978年   12270篇
  1977年   11043篇
  1976年   10466篇
  1975年   11506篇
  1974年   12415篇
  1973年   12235篇
  1972年   11016篇
  1971年   10243篇
  1970年   8794篇
  1969年   8346篇
  1968年   7458篇
  1967年   6820篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Fixation with a saturated aqueous solution of potassium pyroantimonate produces electron-opaque antimonate deposits in the nucleoli of lutheinic and hepatic cells. The comparative analysis between normal and actinomycin D segregated nucleoli fixed either with glutaraldehyde-osmium or potassium pyroantimonate facilitates location of the ring-shaped precipitates in the fibrillar centers and the fine dense ones in the pars fibrosa.  相似文献   
952.
953.
954.
955.
956.
As the climate warms, species that cannot tolerate changing conditions will only persist if they undergo range shifts. Redistribution ability may be particularly variable for benthic marine species that disperse as pelagic larvae in ocean currents. The blue mussel, Mytilus edulis, has recently experienced a warming-related range contraction in the southeastern USA and may face limitations to northward range shifts within the Gulf of Maine where dominant coastal currents flow southward. Thus, blue mussels might be especially vulnerable to warming, and understanding dispersal patterns is crucial given the species'' relatively long planktonic larval period (>1 month). To determine whether trace elemental “fingerprints” incorporated in mussel shells could be used to identify population sources (i.e. collection locations), we assessed the geographic variation in shell chemistry of blue mussels collected from seven populations between Cape Cod, Massachusetts and northern Maine. Across this ∼500 km of coastline, we were able to successfully predict population sources for over two-thirds of juvenile individuals, with almost 80% of juveniles classified within one site of their collection location and 97% correctly classified to region. These results indicate that significant differences in elemental signatures of mussel shells exist between open-coast sites separated by ∼50 km throughout the Gulf of Maine. Our findings suggest that elemental “fingerprinting” is a promising approach for predicting redistribution potential of the blue mussel, an ecologically and economically important species in the region.  相似文献   
957.
958.
The mountain pine beetle (MPB) is a native bark beetle of western North America that attacks pine tree species, particularly lodgepole pine. It is closely associated with the ophiostomatoid ascomycetes Grosmannia clavigera, Leptographium longiclavatum, Ophiostoma montium, and Ceratocystiopsis sp.1, with which it is symbiotically associated. To develop a better understanding of interactions between beetles, fungi, and host trees, we used target-specific DNA primers with qPCR to assess the changes in fungal associate abundance over the stages of the MPB life cycle that occur in galleries under the bark of pine trees. Multivariate analysis of covariance identified statistically significant changes in the relative abundance of the fungi over the life cycle of the MPB. Univariate analysis of covariance identified a statistically significant increase in the abundance of Ceratocystiopsis sp.1 through the beetle life cycle, and pair-wise analysis showed that this increase occurs after the larval stage. In contrast, the abundance of O. montium and Leptographium species (G. clavigera, L. longiclavatum) did not change significantly through the MPB life cycle. From these results, the only fungus showing a significant increase in relative abundance has not been formally described and has been largely ignored by other MPB studies. Although our results were from only one site, in previous studies we have shown that the fungi described were all present in at least ten sites in British Columbia. We suggest that the role of Ceratocystiopsis sp.1 in the MPB system should be explored, particularly its potential as a source of nutrients for teneral adults.  相似文献   
959.
Fast, efficient and selective deprotection of the tert-butoxycarbonyl (Boc) group of various amino acids and peptides was achieved by using hydrogen chloride (4 m) in anhydrous dioxane solution for 30 min at room temperature. In the cases studied in our laboratory, this protocol provided superior selectivity to deprotect Nalpha-Boc groups in the presence of tert-butyl esters and tert-butyl ethers, including thio-tert-butyl ethers, but not phenolic tert-butyl ethers.  相似文献   
960.
The presence of litter has the potential to alter the population dynamics of plants. In this paper, we explore the effects of litter on population dynamics using a simple experimental laboratory system with populations of the annual crucifer, Cardamine pensylvanica. Using a factorial experiment with four densities and three litter levels, we determined the effect of litter on biomass and plant fecundity, and the life stages responsible for these changes in yield. Although litter had significant effects on seed germination and on seedling survivorship, we show, using a population dynamics model, that these effects were not demographically significant. Rather, the potential effect of litter on population dynamics resulted almost entirely from its effect on biomass. Persistent litter suppressed plant biomass and apparently removed the direct density effect present in the absence of litter. Thus, litter changed the shape of the recruitment curve from slightly humped to asymptotic. In addition to changing the shape of the recruitment curve, litter reduced the carrying capacity of the populations. Thus, the population dynamics model indicated that not all statistically significant responses were dynamically significant. Given the potential complexity of litter effects, simple population models provide a powerful tool for understanding the potential consequences of short-term responses. Received: 8 September 1999 / Accepted: 5 April 2000  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