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   32418篇
  免费   151篇
  国内免费   305篇
系统科学   454篇
丛书文集   495篇
教育与普及   109篇
理论与方法论   97篇
现状及发展   12641篇
研究方法   1297篇
综合类   17118篇
自然研究   663篇
  2013年   407篇
  2012年   667篇
  2011年   1333篇
  2010年   451篇
  2009年   442篇
  2008年   782篇
  2007年   907篇
  2006年   908篇
  2005年   832篇
  2004年   645篇
  2003年   603篇
  2002年   563篇
  2001年   1010篇
  2000年   1036篇
  1999年   690篇
  1994年   340篇
  1992年   578篇
  1991年   463篇
  1990年   511篇
  1989年   466篇
  1988年   426篇
  1987年   477篇
  1986年   462篇
  1985年   585篇
  1984年   454篇
  1983年   380篇
  1982年   382篇
  1981年   347篇
  1980年   386篇
  1979年   952篇
  1978年   697篇
  1977年   708篇
  1976年   575篇
  1975年   615篇
  1974年   794篇
  1973年   704篇
  1972年   665篇
  1971年   765篇
  1970年   991篇
  1969年   795篇
  1968年   791篇
  1967年   787篇
  1966年   717篇
  1965年   539篇
  1959年   274篇
  1958年   455篇
  1957年   316篇
  1956年   283篇
  1955年   284篇
  1954年   262篇
排序方式: 共有10000条查询结果,搜索用时 646 毫秒
821.
Native mushrooms play an important, though often underestimated, role in deer, elk, and caribou diets in North America. Mushrooms are often noted as an unusual or anomalous food in the diets of cervids; yet they often dominate diets in the late summer and fall in forested areas of western North America and throughout the year in the southeastern U.S. Mushrooms are particularly high in protein (16-19%), phosphorus (average 0.75%), and potassium (average 2%). Also, mushroom production is generally greatest in fall. Therefore, they are a highly nutritious food in late season when other native forages may marginally meet basal nutrient requirements of ungulates.  相似文献   
822.
Cottus greenei, a potentially threatened species endemic to Idaho, was collected from 49 localities in 25 springs/streams in south central Idaho. Most localities were along the north bank of the Snake River in waters of the Thousand Springs formation, Gooding County. One population was found in a spring in the main Snake River. Another sculpin, Cottus bairdi, was collected with C. greenei at 23 locations in 16 springs/streams. Confusion concerning the type locality of Cottus greenei is discussed.     相似文献   
823.
Data were gathered concerning the seasonal activity pattern of a population of Columbian ground squirrels ( Spermophilus columbianus ) in the Idaho Primitive Area. Adult females were significantly more active in June of all years than were adult males. A relationship between ground squirrel activity and temperature is postulated in which the squirrels alter their activity so as to avoid high temperatures and possible heat stress.  相似文献   
824.
最近提出了一种采用标准符号数二进制码(canonic signed-digit binary representation,CSDBR)来计算AXBY(modN)的快速双重指数模算法.该算法声明当指数的长度为k时,该算法平均仅需要1.306k次模乘.由于已知的此类算法至少需要1.503k次模乘,该算法具有明显的性能优势.然而,无论是该算法的提出者还是其他研究者均没有给出正确的复杂度分析.本文通过利用马尔科夫链模型对该算法进行正式的复杂度研究并进行一定规模的统计实验后证实,实际上该算法平均需要1.556k次模乘.这项研究的意义在于揭示到目前为止,基于标准符号数位码的双重指数模算法的最高性能仍然无法降低到1.5k次模乘以下.  相似文献   
825.
Lysozymes are antibacterial effectors of the innate immune system in animals that hydrolyze peptidoglycan. Bacteria have evolved protective mechanisms that contribute to lysozyme tolerance such as the production of lysozyme inhibitors, but only inhibitors of chicken (c-) and invertebrate (i-) type lysozyme have been identified. We here report the discovery of a novel Escherichia coli inhibitor specific for goose (g-) type lysozymes, which we designate PliG (periplasmic lysozyme inhibitor of g-type lysozyme). Although it does not inhibit c- or i-type lysozymes, PliG shares a structural sequence motif with the previously described PliI and MliC/PliC lysozyme inhibitor families, suggesting a common ancestry and mode of action. Deletion of pliG increased the sensitivity of E. coli to g-type lysozyme. The existence of inhibitors against all major types of animal lysozyme and their contribution to lysozyme tolerance suggest that lysozyme inhibitors may play a role in bacterial interactions with animal hosts.  相似文献   
826.
提出了一种基于契约的计划生育策略. 依据该策略, 公民可在结婚时通过契约形式自由选择成功生育一个男孩, 一个女孩, 或者两个孩子作为终止生育的条件. 用概率分析和随机过程分析的手段从数学上论证了这项策略的可行性. 结果表明, 该策略在满足公民对生育子女性别或数量的基本选择权基础上, 使得一对夫妇的平均生育数量接近2, 可以总体上保证人口数量的平稳, 也不会造成男女性别比例的失衡.  相似文献   
827.
对北京丰台区公园式温泉度假中心的设计方案进行了简要分析。该方案遵循了"北国南园"的设计理念,保留并改造了现状绿化、现有建筑和水域,使之能融入新的规划中。  相似文献   
828.
Described is Sclerocactus blainei Welsh & Thorne, sp. nov. from Nye County, Nevada.     相似文献   
829.
Ponderosa pine cones from 10 areas in Arizona were collected prior to natural seed dispersal and dissected to determine the number of sound, hollow, and insect-damaged seeds in each cone. Total and sound seed yields per cone did not vary significantly among areas but did vary significantly among trees within each area. Numbers of hollow and Megastigmus -infested (Hymenoptera: Torymidae) seeds varied significantly among areas and trees within areas. Numbers of sound seed increased significantly with increasing cone length but did not change with increasing numbers of cones per cluster. The percentages of Megastigmus -infested seed did not change significantly with increasing cone length or number of cones per cluster.  相似文献   
830.
Two roadside surveys were conducted for dwarf mistletoes parasitizing lodgepole pine and Douglas-fir on the Sawtooth National Forest, Idaho. One survey used variable-radius plots located less than 150 m from roads. The 2nd survey used variable-radius plots established at 200-m intervals along 1600-m transects run perpendicular to the same roads. Estimates of the incidence (percentage of trees infected and percentage of plots infested) and severity (average dwarf mistletoe rating) for both lodgepole pine and Douglas-fir dwarf mistletoes were not significantly different for the 2 survey methods. These findings are further evidence that roadside-plot surveys and transect-plot surveys conducted away from roads provide similar estimates of the incidence of dwarf mistletoes for large forested areas.  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