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   888篇
  免费   75篇
  国内免费   106篇
林业   38篇
农学   37篇
基础科学   122篇
  246篇
综合类   296篇
农作物   17篇
水产渔业   136篇
畜牧兽医   60篇
园艺   32篇
植物保护   85篇
  2024年   2篇
  2023年   18篇
  2022年   23篇
  2021年   31篇
  2020年   35篇
  2019年   30篇
  2018年   39篇
  2017年   46篇
  2016年   58篇
  2015年   29篇
  2014年   37篇
  2013年   42篇
  2012年   55篇
  2011年   68篇
  2010年   47篇
  2009年   51篇
  2008年   55篇
  2007年   59篇
  2006年   47篇
  2005年   48篇
  2004年   38篇
  2003年   29篇
  2002年   29篇
  2001年   20篇
  2000年   23篇
  1999年   17篇
  1998年   14篇
  1997年   14篇
  1996年   10篇
  1995年   9篇
  1994年   9篇
  1993年   11篇
  1992年   6篇
  1991年   1篇
  1990年   4篇
  1989年   9篇
  1988年   4篇
  1982年   1篇
  1955年   1篇
排序方式: 共有1069条查询结果,搜索用时 46 毫秒
71.
在对寿光市花卉大棚土壤进行取样分析测定的基础上 ,通过对大棚栽培条件的积盐分析 ,进一步对大棚积盐规律、土壤内可溶性的盐分组成进行了分析  相似文献   
72.
73.
The Ionic and Osmotic Factors Controlling Motility of Fish Spermatozoa   总被引:4,自引:0,他引:4  
This review presents actual knowledge about energetic, ionic, osmotic and gaseous control of fish sperm motility and its duration. Right after they are activated, fish spermatozoa of most species swim for a short period of time, in the range of one to several minutes. What determines the activation process? Is it due to the new ionic, gaseous and/or osmotic environment? Why is the duration of motility so short? Is it resulting from a fast exhaustion of energy stores (ATP, ADP, AMP, PCr) combined with the above-mentioned ionic/osmotic stress leading to morphological alterations? The motility criteria (flagellar beat frequency, head displacement velocity, flagellar waves morphology, etc.) used to characterize fish sperm movement and sperm flagella will be described. Most parameters change very rapidly during the brief motility period of fish sperm. Then will be considered the main environmental factors, ionic and/or osmotic signals, responsible of the activation of fish sperm motility. Then the metabolic compounds involved in cell energetics will be considered as their concentrations also rapidly change during the motility phase. An additional feature will then be discussed concerning the mechanisms by which fish sperm cell can be revived for a second motility round at the end of the first motility period. A model is proposed to explain how external osmolarity can control internal ionic composition, the latter being the key factor controlling flagellar activity.  相似文献   
74.
ABSTRACT:   Post-release movement and diel activity patterns of hatchery-reared and wild black-spot tuskfish were examined using ultrasonic telemetry. Five hatchery-reared and four wild fish were released in the sandy bottom of Urasoko Bay, Ishigaki Island, Okinawa, Japan, and monitored using automated monitoring receivers from November 2005 to February 2006. Both hatchery-reared and wild fish tended to stay near the release site for over two weeks, before leaving the release site. Both hatchery-reared and wild tuskfish showed diurnal rhythm intermittently; signals were recorded more frequently in the daytime and less frequently in the nighttime, suggesting that the fish of both origins were active during the day and inactive during the night. These findings indicate that the one-year-old hatchery-reared tuskfish have some consistent behavioral characteristics with those of the wild.  相似文献   
75.
76.
77.
78.
79.
Yong-Hae  KIM 《Fisheries Science》2003,69(3):535-546
ABSTRACT:   The presented model involves the application of chaos theory to generate fish movements resulting from environmental stimuli. The model uses three steps within a model neural network such as input stimuli, central decision making and response output resulting in fish movements. The stimuli in the first step include the main abiotic and biotic factors, which could be quantified as an intensity parameter that was then normalized as a ratio between 0 and 1. The decision-making process can be generated using chaos dynamics with the stimuli parameters. The response of fish movements from the output signal representing movement speed and direction of fish can be re-regulated as main movement pattern depending on physiological state or life cycle by third response filtering. The simulation results seen as a movement pattern for sea bream and flounder using this neural chaotic model fitted very well to the observations of fish tracked in the sea by ultrasonic tracking methods. It was also revealed that the fish movement components generated as movement velocity and direction when in tidal flow had similar patterns to those patterns seen in field observations with similar irregular and chaotic variations with time.  相似文献   
80.
通过非饱和带地下水中溶质运移的理论基础的研究,介绍了非饱和带内溶质运移模拟的研究方法及影响因素,在此基础上建立了非饱和带水流模型以及溶质运移的数学模型,并介绍了模型的求解方法有限差分法及其求解原理。同时介绍了影响非饱和带内溶质运移的相关参数及多种边界条件,研究了典型特征参数的设定及影响因素。指出了非饱和带溶质运移的理论研究意义,并举例提出了相关建议措施。  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