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   16213篇
  免费   36篇
  国内免费   59篇
系统科学   89篇
丛书文集   182篇
教育与普及   33篇
理论与方法论   86篇
现状及发展   6390篇
研究方法   927篇
综合类   8270篇
自然研究   331篇
  2013年   123篇
  2012年   343篇
  2011年   682篇
  2010年   135篇
  2008年   399篇
  2007年   378篇
  2006年   396篇
  2005年   423篇
  2004年   394篇
  2003年   347篇
  2002年   335篇
  2001年   473篇
  2000年   454篇
  1999年   317篇
  1992年   269篇
  1991年   203篇
  1990年   234篇
  1989年   221篇
  1988年   226篇
  1987年   223篇
  1986年   210篇
  1985年   296篇
  1984年   235篇
  1983年   162篇
  1982年   172篇
  1981年   153篇
  1980年   184篇
  1979年   421篇
  1978年   315篇
  1977年   318篇
  1976年   295篇
  1975年   329篇
  1974年   396篇
  1973年   369篇
  1972年   384篇
  1971年   442篇
  1970年   561篇
  1969年   472篇
  1968年   477篇
  1967年   438篇
  1966年   391篇
  1965年   291篇
  1964年   87篇
  1959年   169篇
  1958年   298篇
  1957年   199篇
  1956年   185篇
  1955年   150篇
  1954年   185篇
  1948年   138篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
161.
162.
Soil physical and chemical properties were studied to identify habitat differences among 15 sagebrush dominated plant community types of the Great Basin. The sagebrush taxa studied followed an apparent gradient of increasing soil fertility, with Artemisia nova commonly occupying the lowest fertility sites. Sites of moderate soil fertility tended to support A. arbuscula , A. longiloba , and A. tridentata ssp. Wyomingensis . Artemisia tridentata ssp. Tridentata and A. tridentata ssp. Vaseyana were commonly found on sites with the highest soil fertility levels as indicated by maximum organic carbon, nitrogen, and phosphorus levels within such soils. Mollic epipedon depth, total depth, and water-holding capacity of the soil also increased as the dominant sagebrush taxon of a community type changed from A. nova to A. tridentata ssp. Vaseyana . Data from this study provide information concerning soil relationships within relatively undisturbed rangeland communities. Such data are required if future soil sampling efforts within the Great Basin are to have a basis for comparison.  相似文献   
163.
Thirty-eight accessions of 20 taxa (species and subspecies) of Chrysothamnus from a range of environments were grown in a uniform garden. During mid-June, terminal and lateral leaves were removed, fixed, and processed for light-microscopy studies. Area and perimeter measurements of ink tracings of midleaf vein cross sections were measured using computer-scan techniques. Leaf area and perimeter were compared with vein perimeter and area measurements. Area and perimeter of these Chrysothamnus leaves varied in response to the environment at the collection location. Plants native to hot, dry conditions had small, thick leaves, whereas plants native to cooler, more mesic conditions had relatively large, thin leaves. Similarly, leaves with round veins were adapted to hot, dry environments and leaves with elliptical veins were adapted to cooler, more mesic environments. Both terminal (young) and lateral (older) leaves were sampled. Terminal leaves showed significant (p < .05) correlational responses to environmental parameters, whereas the lateral leaves, while fronding in the predicted morphological directions, did not generally show significant responses. The different responses of terminal and lateral leaves may have been due to canopy position and uniform environmental conditions of the garden. Leaves with large cross sections characteristically had large veins, and leaves with small cross sections characteristically had small veins. Large leaf cross sections were normally flat. Small leaf cross sections were normally round. Decreasing leaf size and increasing leaf thickness were correlated with aridity.  相似文献   
164.
Wyoming lacks a rich molluscan fauna due to its considerable elevation, extremes of temperature, the short period between killing frosts, scanty, moisture, and intermittent creeks. Recent mollusks found in the state total 42 genera (109 species), including 6 genera (23 species) of bivalves, 15 genera (42 species) of freshwater gastropods, and 21 genera (44 species) of land gastropods. Synonymy, the counties in which each species was collected, and habitat notes are given.  相似文献   
165.
At peak standing crop, rillscale ( Atriplex suckleyi ) foliage grown on amended bentonite mine spoil contained adequate digestible energy, crude protein, and all mineral elements except phosphorus necessary for cattle, sheep, antelope, and deer. Amendments (sawdust, NPK, gypsum) generally did not affect forage quality. Iron, manganese, aluminum, sodium, and potassium concentrations were high and may have adversely affected forage quality. Forage utility would be limited to a few months during the growing season.  相似文献   
166.
167.
Soil samples from the Al horizon and dominant subsoil horizon at 190 sites were analyzed for Ca, Mg, K, P, and organic matter contents in conjunction with a soil resource inventory of the Caribou National Forest. Vegetative composition and production data were compared to the edaphic factors to derive relationships useful to the land manager. Organic matter was effective in distinguishing between soil orders and was positively correlated to vegetative production. Vegetative cycling of the nutrients P and K was most pronounced in soils of the order Mollisols, to a lesser degree in the Alfisols, and not at all in the Entisols. The K/Mg ratio of the soil showed a negative correlation to grass production and a positive correlation with shrub production. An interpretative table is provided to aid determinations of high versus low values for the edaphic factors studied.  相似文献   
168.
Thirty-nine previously unpublished reports of the endangered black-footed ferret from Wyoming are listed with dates, locations, number of animals, sources, and comments.  相似文献   
169.
Spatial heterogeneity in visually similar sites under Utah juniper canopy in Colorado National Monument was examined. Sample sites were arranged in a transect 24 m long, such that distances between samples were 0.013 m, 0.03 m, 1.0 m, 12 m, and 24 m. Twenty-five taxa of algae were observed, mostly belonging to Cyanophyta. Algal density varied by more than an order of magnitude within the 46 samples examined. The coefficients of variation for each distance class were very similar, demonstrating that algal patchiness can be as significant on a scale less than 0.013 m as it is on a scale of 24 m. Goodall's random pairing analysis of spatial pattern supported this conclusion by indicating that the minimal area for sampling soil algal crust populations at this site was equal to or less than 0.013 m. Because of the microscale heterogeneity in algal communities in this study, we recommend that future researchers take composite samples if they wish to quantify algae of microbiotic crusts.  相似文献   
170.
From October 2003 through April 2006, we collected 565 fleas incidental to a distribution survey of mammals in southwestern South Dakota. Sixty-one specimens, representing 18 species of mammals, possessed 20 species of fleas. The geographic distributions of these flea species revealed 8 new records for the Black Hills and its adjacent grasslands. Four species&mdash; Megarthroglossus divisus, Stenoponia americana, Odontopsyllus dentatus, and Amaradix euphorbi &mdash;constitute new records for South Dakota, thus increasing the state&rsquo;s known flea fauna to 42 species. Hunters, trappers, and field biologists should be aware that serosurveillance during the 1990s revealed the presence of sylvatic plague and tularemia in the Black Hills area. Desde octubre de 2003 hasta abril de 2006, colectamos 565 pulgas de manera incidental durante un estudio de la distribuci&oacute;n de mam&iacute;feros en el suroeste del estado de Dakota del Sur. Los 61 espec&iacute;menes, que representaban 18 especies de mam&iacute;feros, albergaban 20 especies de pulgas. Sus distribuciones geogr&aacute;ficas revelaron 8 nuevos registros para las Colinas Negras (Black Hills) y las praderas adyacentes. Cuatro de estas especies ( Megarthroglossus divisus, Stenoponia americana, Odontopsyllus dentatus y Amaradix euphorbi ) constituyen nuevos registros para Dakota del Sur. Esto aumenta el n&uacute;mero de especies de pulgas conocidas del estado a 42 especies. Los cazadores, tramperos y bi&oacute;logos de campo deben estar conscientes de que la vigilancia serol&oacute;gica durante la d&eacute;cada de los 1990 revel&oacute; la presencia de la plaga silv&aacute;tica y la tularemia en el &aacute;rea de las Colinas Negras.  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