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   55928篇
  免费   263篇
  国内免费   549篇
系统科学   1243篇
丛书文集   190篇
教育与普及   288篇
理论与方法论   521篇
现状及发展   31711篇
研究方法   868篇
综合类   19728篇
自然研究   2191篇
  2013年   809篇
  2012年   616篇
  2011年   2650篇
  2009年   615篇
  2008年   827篇
  2007年   935篇
  2006年   1029篇
  2005年   1182篇
  2004年   2272篇
  2003年   1933篇
  2002年   1578篇
  2001年   1511篇
  2000年   1053篇
  1999年   1020篇
  1998年   646篇
  1997年   776篇
  1996年   533篇
  1994年   687篇
  1993年   693篇
  1992年   946篇
  1991年   805篇
  1990年   900篇
  1989年   690篇
  1988年   651篇
  1987年   684篇
  1986年   758篇
  1985年   896篇
  1984年   784篇
  1983年   687篇
  1982年   808篇
  1981年   829篇
  1980年   904篇
  1979年   1417篇
  1978年   1251篇
  1977年   1231篇
  1976年   1118篇
  1975年   1080篇
  1974年   929篇
  1973年   1179篇
  1972年   1204篇
  1971年   1241篇
  1970年   1322篇
  1969年   1203篇
  1968年   1184篇
  1967年   1049篇
  1966年   888篇
  1965年   737篇
  1964年   485篇
  1958年   526篇
  1957年   435篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
The diet of Long - billed Curlew chicks is described for the first time. Five insect orders and one arachnid order were identified from nine stomach contents samples. Grasshoppers and carabid beetles were dominant prey items.   相似文献   
992.
Techniques for locating, capturing, and monitoring activities and movements of Flammulated Owls were developed during four years of study. Adults responded to imitated territorial song throughout the nesting season. Nests were found by noting vocalizations and behaviors associated with courtship-feeding and food transfers between the sexes. After fledging, broods were difficult to follow, unless the young or adults were radio-tagged. Adults and young were captured with noose poles, hoop nets, and mist nets. Backpack harnesses and tail-mounted transmitters were used on adults and backpacks on fledglings. Transmitter life was 30–40 nights.      相似文献   
993.
994.
Thirty - nine Utah streams were sampled for cutthroat trout. Of these, 31 contain cutthroat or cutthroat / rainbow hybrid populations. By using starch gel electrophoresis, these populations were segregated into three groups. One group consisted predominately of fish from the Sevier River (of the Bonneville Basin) and Colorado drainages. A second was primarily populations from the Bear River Drainage (Bonneville Basin) as well as some scattered populations along the Wasatch Front (Bonneville Basin). The third consisted of Wasatch Front populations and populations that have hybridized with rainbow trout. Since different subspecies of cutthroat trout are native to the Colorado and Bonneville drainages, one would expect the populations from within the Bonneville Basin to be more similar to one another and less similar to the Colorado River populations. That this did not occur raises questions concerning the evolutionary relationships of the subspecies and the populations. It is clear that at least a northern (Bear River) and southern (Sevier River) form of the Bonneville cutthroat exists. The Wasatch Front may represent an intermediate zone where these two forms intergrade.      相似文献   
995.
The utilization of the stinger and the predatory technique of the scorpion, Paruroctonus boreus, was studied under laboratory conditions. During the study, 83 feedings were observed. Age of the scorpions and the percentage of prey stung by them were used to classify the scorpions into groups. The scorpions aged 13–61 days always stung prey. After 62 days the scorpions began to selectively utilize the stinger. Utilization declined until it reached 30 percent in the adult stage. The stinger is apparently necessary for prey capture only in the early life stages.    相似文献   
996.
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   
997.
The height of oak ( Quercus gambelii Nutt.) stems was measured on several fire scars within the Uinta National Forest and vicinity and compared with the height of oak stems on adjacent, nonburned areas. A significant relationship exists between the recovery rate of oak after fire and elevation, with the recovery rate being greatest at low elevations. A trend also exists showing that recovery tends to be greater on south to westerly exposures than on north to easterly exposures.  相似文献   
998.
A pallid bat ( Antrozous pallidus ) was taken 20 km SE Warren, Carbon County, Montana. This represents a 410 km range extension and a new record for Montana.  相似文献   
999.
Diatoms epiphytic on Phragmites australis (Cav.) Trin. ex Steaded stems were collected from a single clone at the southern end of Provo Bay, Utah Lake, Utah. Diatom populations from both living and dead stem sections were analyzed. Species diversity in each sample was high, indicating that the stems provide a relatively stable habitat for diatom epiphytes. Of the 23 genera found, only Gomphonema and Navicula showed significant trends toward stem preference. The diatoms in this study support the current view that Utah Lake is a slightly saline, eutrophic system.  相似文献   
1000.
New distribution records are given for seven grass species now found in Arizona, New Mexico, and Texas.    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