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   1305篇
  免费   76篇
  国内免费   3篇
林业   29篇
农学   15篇
基础科学   11篇
  103篇
综合类   237篇
农作物   30篇
水产渔业   93篇
畜牧兽医   705篇
园艺   9篇
植物保护   152篇
  2023年   10篇
  2022年   23篇
  2021年   29篇
  2020年   24篇
  2019年   23篇
  2018年   18篇
  2017年   31篇
  2016年   22篇
  2015年   22篇
  2014年   16篇
  2013年   53篇
  2012年   54篇
  2011年   47篇
  2010年   45篇
  2009年   44篇
  2008年   58篇
  2007年   56篇
  2006年   45篇
  2005年   34篇
  2004年   33篇
  2003年   37篇
  2002年   35篇
  2001年   28篇
  2000年   20篇
  1999年   26篇
  1998年   22篇
  1997年   15篇
  1996年   23篇
  1995年   17篇
  1994年   14篇
  1993年   23篇
  1992年   21篇
  1991年   22篇
  1990年   20篇
  1989年   22篇
  1988年   14篇
  1987年   20篇
  1986年   11篇
  1985年   16篇
  1984年   13篇
  1981年   15篇
  1980年   15篇
  1979年   18篇
  1978年   12篇
  1976年   16篇
  1974年   9篇
  1973年   9篇
  1972年   11篇
  1971年   12篇
  1969年   9篇
排序方式: 共有1384条查询结果,搜索用时 15 毫秒
101.
102.
103.
104.
105.
106.
The kinematics of the outer parts of three intermediate-luminosity elliptical galaxies were studied with the Planetary Nebula Spectrograph. The galaxies' velocity-dispersion profiles were found to decline with the radius, and dynamical modeling of the data indicates the presence of little if any dark matter in these galaxies' halos. This unexpected result conflicts with findings in other galaxy types and poses a challenge to current galaxy formation theories.  相似文献   
107.
Cloning of complementary DNA for GAP-43, a neuronal growth-related protein   总被引:33,自引:0,他引:33  
GAP-43 is one of a small subset of cellular proteins selectively transported by a neuron to its terminals. Its enrichment in growth cones and its increased levels in developing or regenerating neurons suggest that it has an important role in neurite growth. A complementary DNA (cDNA) that encodes rat GAP-43 has been isolated to study its structural characteristics and regulation. The predicted molecular size is 24 kilodaltons, although its migration in SDS-polyacrylamide gels is anomalously retarded. Expression of GAP-43 is limited to the nervous system, where its levels are highest during periods of neurite outgrowth. Nerve growth factor or adenosine 3',5'-monophosphate induction of neurites from PC12 cells is accompanied by increased GAP-43 expression. GAP-43 RNA is easily detectable, although at diminished levels, in the adult rat nervous system. This regulation of GAP-43 is concordant with a role in growth-related processes of the neuron, processes that may continue in the mature animal.  相似文献   
108.
Growth cones are specialized structures that form the distal tips of growing axons. During both normal development of the nervous system and regeneration of injured nerves, growth cones are essential for elongation and guidance of growing axons. Developmental and regenerative axon growth is frequently accompanied by elevated synthesis of a protein designated GAP-43. GAP-43 has now been found to be a major component of growth-cone membranes in developing rat brains. Relative to total protein, GAP-43 is approximately 12 times as abundant in growth-cone membranes as in synaptic membranes from adult brains. Immunohistochemical localization of GAP-43 in frozen sections of developing brain indicates that the protein is specifically associated with neuropil areas containing growth cones and immature synaptic terminals. The results support the proposal that GAP-43 plays a role in axon growth.  相似文献   
109.
Comparative Earth History and Late Permian Mass Extinction   总被引:6,自引:0,他引:6  
The repeated association during the late Neoproterozoic Era of large carbon-isotopic excursions, continental glaciation, and stratigraphically anomalous carbonate precipitation provides a framework for interpreting the reprise of these conditions on the Late Permian Earth. A paleoceanographic model that was developed to explain these stratigraphically linked phenomena suggests that the overturn of anoxic deep oceans during the Late Permian introduced high concentrations of carbon dioxide into surficial environments. The predicted physiological and climatic consequences for marine and terrestrial organisms are in good accord with the observed timing and selectivity of Late Permian mass extinction.  相似文献   
110.
Although C4 plant expansions have been recognized in the late Miocene, identification of the underlying causes is complicated by the uncertainties associated with estimates of ancient precipitation, temperature, and partial pressure of atmospheric carbon dioxide (PCO2). Here we report the carbon isotopic compositions of leaf wax n-alkanes in lake sediment cores from two sites in Mesoamerica that have experienced contrasting moisture variations since the last glacial maximum. Opposite isotopic trends obtained from these two sites indicate that regional climate exerts a strong control on the relative abundance of C3 and C4 plants and that in the absence of favorable moisture and temperature conditions, low PCO2 alone is insufficient to drive an expansion of C4 plants.  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