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   299篇
  免费   10篇
  309篇
  2021年   3篇
  2020年   2篇
  2017年   2篇
  2016年   4篇
  2015年   13篇
  2014年   11篇
  2013年   9篇
  2012年   9篇
  2011年   7篇
  2010年   8篇
  2009年   9篇
  2008年   10篇
  2007年   12篇
  2006年   7篇
  2005年   11篇
  2004年   9篇
  2003年   8篇
  2002年   8篇
  2001年   11篇
  2000年   14篇
  1999年   8篇
  1998年   12篇
  1997年   4篇
  1996年   4篇
  1995年   8篇
  1994年   3篇
  1993年   7篇
  1992年   7篇
  1990年   3篇
  1989年   4篇
  1986年   3篇
  1984年   4篇
  1983年   5篇
  1982年   4篇
  1981年   3篇
  1977年   3篇
  1975年   2篇
  1973年   3篇
  1970年   2篇
  1969年   2篇
  1937年   2篇
  1918年   2篇
  1910年   2篇
  1908年   2篇
  1904年   2篇
  1902年   3篇
  1900年   2篇
  1887年   2篇
  1882年   1篇
  1879年   1篇
排序方式: 共有309条查询结果,搜索用时 31 毫秒
91.
We investigated the relationships between fungal community composition, latitude and a range of physicochemical parameters in 58 soils sampled from a 2370 km latitudinal gradient between South Georgia (54°S, 38°W) in the sub-Antarctic and Mars Oasis (72°S, 68°W) on Alexander Island in the southern maritime Antarctic. Our study, which is based on approximately ten times the number of samples used in previous similar studies, indicates that latitude and its associated environmental parameters are not related to fungal community composition. Significant changes in the composition of soil fungal communities were observed in relation to gradients of the ratio of total organic carbon to nitrogen, and, to a lesser extent, soil pH.  相似文献   
92.
We have optimised an indirect immunoperoxidase technique demonstrating bromodeoxyuridine (BrdU) incorporation into dividing cells for cerebellar tissue sections of four-day-old rats injected with this marker. This permits confident identification of granule-cell precursors engaged in DNA synthesis in the external granular layer of the developing cerebellum. Preservation of BrdU immunoreactivity is attained using methanol/acetic acid fixation and different pretreatments before immunostaining, while unlabeled nuclei can be recognized clearly after Feulgen or hematoxylin counterstaining. We established conditions to ensure satisfactory BrdU uptake without affecting cell-cycle progression during the postlabeling time period. The dose of BrdU employed provides saturation S-phase labeling from at least 1 h after BrdU delivery. Various kinetic parameters and phase durations have been determined in experiments involving a single injection or cumulative labeling sequences, and the cycle time was calculated based on two models of generative behavior: steady-state and exponential growth. The working hypothesis of steadystate kinetics can be adopted successfully if the existence of neuroblasts with different proliferation rates is taken into account.  相似文献   
93.

Background  

Cowpea [Vigna unguiculata (L.) Walp.] is one of the most important food and forage legumes in the semi-arid tropics because of its ability to tolerate drought and grow on poor soils. It is cultivated mostly by poor farmers in developing countries, with 80% of production taking place in the dry savannah of tropical West and Central Africa. Cowpea is largely an underexploited crop with relatively little genomic information available for use in applied plant breeding. The goal of the Cowpea Genomics Initiative (CGI), funded by the Kirkhouse Trust, a UK-based charitable organization, is to leverage modern molecular genetic tools for gene discovery and cowpea improvement. One aspect of the initiative is the sequencing of the gene-rich region of the cowpea genome (termed the genespace) recovered using methylation filtration technology and providing annotation and analysis of the sequence data.  相似文献   
94.
A new genus and species of aglaspidid arthropod, Tremaglaspis unite , is described from the well-known upper Tremadoc locality of Y Garth in North Wales, and compared with other Cambrian and Ordovician taxa. It is remarkable in having a large hypostome, a character shared with other primitive arachnomorphs.  相似文献   
95.
96.
Gobbling activity of Eastern wild turkeys (Meleagris gallopavo silvestris; hereafter, turkeys) has been widely studied, focusing on drivers of daily variation. Weather variables are widely believed to influence gobbling activity, but results across studies are contradictory and often equivocal, leading to uncertainty in the relative contribution of weather variables to daily fluctuations in gobbling activity. Previous works relied on road‐based auditory surveys to collect gobbling data, which limits data consistency, duration, and quantity due to logistical difficulties associated with human observers and restricted sampling frames. Development of new methods using autonomous recording units (ARUs) allows researchers to collect continuous data in more locations for longer periods of time, providing the opportunity to delve into factors influencing daily gobbling activity. We used ARUs from 1 March to 31 May to detail gobbling activity across multiple study sites in the southeastern United States during 2014–2018. We used state‐space modeling to investigate the effects of weather variables on daily gobbling activity. Our findings suggest rainfall, greater wind speeds, and greater temperatures negatively affected gobbling activity, whereas increasing barometric pressure positively affected gobbling activity. Therefore, when using daily gobbling activity to make inferences relative to gobbling chronology, reproductive phenology, and hunting season frameworks, stakeholders should recognize and consider the potential influences of extended periods of inclement weather.  相似文献   
97.
98.
99.
100.
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