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   40641篇
  免费   123篇
  国内免费   216篇
系统科学   199篇
丛书文集   600篇
教育与普及   83篇
理论与方法论   135篇
现状及发展   18807篇
研究方法   1611篇
综合类   18999篇
自然研究   546篇
  2013年   360篇
  2012年   561篇
  2011年   1102篇
  2010年   241篇
  2008年   732篇
  2007年   808篇
  2006年   816篇
  2005年   763篇
  2004年   744篇
  2003年   726篇
  2002年   693篇
  2001年   1280篇
  2000年   1198篇
  1999年   799篇
  1994年   387篇
  1992年   783篇
  1991年   574篇
  1990年   694篇
  1989年   651篇
  1988年   618篇
  1987年   718篇
  1986年   650篇
  1985年   826篇
  1984年   650篇
  1983年   543篇
  1982年   511篇
  1981年   520篇
  1980年   602篇
  1979年   1309篇
  1978年   1059篇
  1977年   1033篇
  1976年   853篇
  1975年   888篇
  1974年   1172篇
  1973年   1046篇
  1972年   1060篇
  1971年   1149篇
  1970年   1503篇
  1969年   1144篇
  1968年   1146篇
  1967年   1106篇
  1966年   974篇
  1965年   695篇
  1959年   375篇
  1958年   645篇
  1957年   417篇
  1956年   385篇
  1955年   365篇
  1954年   357篇
  1948年   257篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Carebareia, n. gen., and its type - species, C. johnstoni, n. sp., are described. The genus Quadraseta Brennan, 1970, is redefined and a key to the 11 included species given. Four new species, Quadraseta tachirensis, holotype ex Akodon sp., Q. rotstieri, ex Proechimys guyannensis, Q. mirandae, holotype ex Oryzomys albigularis, and Q. falconensis, ex Sylvilagus floridanus, are described.  相似文献   
942.
Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4 st1\:*{behavior:url(#ieooui) } /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Prescribed burning of Texas wintergrass ( Stipa leucotricha Trin. & Rupr.) communities reduced soil water contents for two to six months, and additional reductions occurred when the subsequent crop of cool-season grasses and forbs began growth. These soil water reductions occurred despite reduced plant production following burning. Soil water reductions were greater at 15 to 45 cm depths than in the upper 15 cm and greater following spring burning. Soil temperatures were higher on burned plots for most of the first year following burning.     相似文献   
943.
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   
944.
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   
945.
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   
946.
AIM: To assess the impact of bolus volume on the characteristics of small intestinal (SI) impedance signals.METHODS: Concurrent SI manometry-impedance measurements were performed on 12 healthy volunteers to assess the pattern of proximal jejunal fluid bolus movement over a 14 cm-segment.Each subject was given 34 boluses of normal saline (volume from 1 to 30 mL) via the feeding tube placed immediately above the proximal margin of the studied segment.A bolus-induced impedance event occurred if there was > 12%...  相似文献   
947.
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   
948.
A bite by Chiracanthium mildei L. Koch is described.  相似文献   
949.
CFTR biogenesis starts with its co-translational insertion into the membrane of endoplasmic reticulum and folding of the cytosolic domains, towards the acquisition of a fully folded compact native structure. Efficiency of this process is assessed by the ER quality control system that allows the exit of folded proteins but targets unfolded/misfolded CFTR to degradation. If allowed to leave the ER, CFTR is modified at the Golgi and reaches the post-Golgi compartments to be delivered to the plasma membrane where it functions as a cAMP- and phosphorylation-regulated chloride/bicarbonate channel. CFTR residence at the membrane is a balance of membrane delivery, endocytosis, and recycling. Several adaptors, motor, and scaffold proteins contribute to the regulation of CFTR stability and are involved in continuously assessing its structure through peripheral quality control systems. Regulation of CFTR biogenesis and traffic (and its dysregulation by mutations, such as the most common F508del) determine its overall activity and thus contribute to the fine modulation of chloride secretion and hydration of epithelial surfaces. This review covers old and recent knowledge on CFTR folding and trafficking from its synthesis to the regulation of its stability at the plasma membrane and highlights how several of these steps can be modulated to promote the rescue of mutant CFTR.  相似文献   
950.
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