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   2295篇
  免费   11篇
  国内免费   21篇
系统科学   56篇
教育与普及   11篇
理论与方法论   42篇
现状及发展   268篇
研究方法   320篇
综合类   1401篇
自然研究   229篇
  2021年   10篇
  2020年   9篇
  2019年   13篇
  2018年   19篇
  2017年   17篇
  2016年   28篇
  2015年   14篇
  2014年   30篇
  2013年   32篇
  2012年   163篇
  2011年   394篇
  2010年   55篇
  2009年   17篇
  2008年   165篇
  2007年   169篇
  2006年   156篇
  2005年   214篇
  2004年   195篇
  2003年   186篇
  2002年   167篇
  2001年   15篇
  2000年   15篇
  1999年   19篇
  1998年   8篇
  1997年   5篇
  1996年   6篇
  1995年   7篇
  1994年   5篇
  1993年   5篇
  1992年   15篇
  1991年   8篇
  1990年   9篇
  1989年   9篇
  1988年   6篇
  1987年   4篇
  1985年   9篇
  1984年   4篇
  1983年   6篇
  1979年   11篇
  1978年   6篇
  1977年   4篇
  1975年   7篇
  1974年   4篇
  1972年   5篇
  1971年   7篇
  1970年   8篇
  1969年   7篇
  1968年   7篇
  1967年   9篇
  1966年   7篇
排序方式: 共有2327条查询结果,搜索用时 15 毫秒
101.
Reported are 105 species of Scolytidae (Coleoptera) from Idaho. About one-third of these are rarely collected, of which 22 species are known from a single locality each. Twelve species reported from Idaho for the first time are: Carphoborus carri Swaine, C. sansoni Swaine, Phloeosinus hoferi Blackman, Conophthorus monophyllae Hopkins, Dryocoetes betulae Hopkins, Ips confuses (LeConte), Pityophthorus absonus Blackman, P. aquilus Blackman , P. blandus Blackman, P. deletus LeConte, P. sculptor Blackman, and Xyleborinus saxeseni (Ratzeburg). Significant extensions of the known distributions in Idaho are reported for seven other scolytids; Alniphagus aspericollis (LeConte), Dendroctonus murrayanae Hopkins, Phloeotribus lecontei Schedl, Procryphalus mucronatus (LeConte), Trypophloeus populi Hopkins, Xyleborus dispar (Fabricius), and X. intrusus Blandford. Xyleborus dispar especially needs study in anticipation that it may become increasingly important in Idaho fruit trees and other woody plants including ornamentals and shade trees.  相似文献   
102.
Mapping units were developed based on landform, soils, and plant communities for the 40,000 ha of salt desert vegetation located below the maximum level of pluvial Lake Gilbert in Grass Valley, Nevada. Lake plain, beach, and bar features of the pluvial lake provide the dominant landforms. Fine textured lake sediments have produced salt- and Na-affected soils. The distribution and structure of plant communities are related to depth of the groundwater table, fluctuations in this depth, and the salt content of the groundwater. Wind and water erosion combine to continually evolve new environments for colonization by plants in this ecosystem.    相似文献   
103.
Two canids, Vulpes vulpes and Canis lupus , are recorded from shoreline deposits of Lake Bonneville in north central Utah. Both species are new records in the Pleistocene sediments of Utah and add to our scarce knowledge of the large carnivores that inhabited the shoreline environments of Lake Bonneville.  相似文献   
104.
Crayfish distribution and composition in Utah are poorly documented. Based upon limited collections, the native Pacifastacus gambelii is widespread and often abundant in the Bear and Weber river drainages and is occasionally represented in smaller tributaries to the Great Salt Lake and to the Raft River. Pacifastacus leniusculus was collected from Utah County and Procambarus clarkii from Tooele County; the origin of these populations is not known. The nonnative crayfish Orconectes virilis is currently abundant in the Virgin, Price, and Duchesne river basins and the Glen Canyon, Flaming Gorge, Deer Creek, and Starvation reservoirs; it is expanding in Huntington North, Scofield, and Willard reservoirs. Further work is required to develop a more complete inventory and monitor the impacts on aquatic ecosystems of expanding nonnative crayfish populations. Surreptitious stockings can be dealt with only if the public is made aware of the adverse consequences of ill-conceived introductions.  相似文献   
105.
Migraine is a common, heterogeneous and heritable neurological disorder. Its pathophysiology is incompletely understood, and its genetic influences at the population level are unknown. In a population-based genome-wide analysis including 5,122 migraineurs and 18,108 non-migraineurs, rs2651899 (1p36.32, PRDM16), rs10166942 (2q37.1, TRPM8) and rs11172113 (12q13.3, LRP1) were among the top seven associations (P < 5 × 10(-6)) with migraine. These SNPs were significant in a meta-analysis among three replication cohorts and met genome-wide significance in a meta-analysis combining the discovery and replication cohorts (rs2651899, odds ratio (OR) = 1.11, P = 3.8 × 10(-9); rs10166942, OR = 0.85, P = 5.5 × 10(-12); and rs11172113, OR = 0.90, P = 4.3 × 10(-9)). The associations at rs2651899 and rs10166942 were specific for migraine compared with non-migraine headache. None of the three SNP associations was preferential for migraine with aura or without aura, nor were any associations specific for migraine features. TRPM8 has been the focus of neuropathic pain models, whereas LRP1 modulates neuronal glutamate signaling, plausibly linking both genes to migraine pathophysiology.  相似文献   
106.
107.
108.
Greasewood ( Sarcobatus ) is a succulent-leaved, halophytic shrub of North American origin. The genus comprises 2 species: Sarcobatus baileyi and Sarcobatus vermiculatus. Sarcobatus vermiculatus is common throughout much of western North America, but S. baileyi is much more limited in distribution and was previously thought to be endemic to Nevada. Here we document and describe a S. baileyi population in eastern California, comparing its morphology and ecology to an adjacent S. vermiculatus population. Morphologically, S. baileyi is smaller in stature but produces larger seeds; however, fewer S. baileyi seeds germinated and survived a 20-day laboratory incubation compared to seeds of S. vermiculatus. Sarcobatus baileyi has higher leaf Na concentrations and operates at much lower plant water potentials than S. vermiculatus under field conditions; however, no significant differences were observed between the 2 species in long-term water-use efficiency as measured by leaf &delta;13C. Leaf Na concentrations were very low in both species. Overall, these species differ greatly in a number of traits that are consistent with the upland, nonphreatophytic character of S. baileyi, which is in stark contrast to the phreatophytic character of S. vermiculatus. Both species, however, are very salt tolerant and have low leaf N concentrations, indicating the low nutrient availability and the potentially high salinity of their extreme habitats. Further investigation of comparable desert ridge environments should be conducted to determine the extent of S. baileyi in eastern California, and common garden comparisons of the 2 species should be conducted to compare their ecophysiological traits. El ";greasewood&rdquo; ( Sarcobatus ) es un arbusto halof&iacute;tico de origen norteamericano con hojas suculentas. El g&eacute;nero consta de dos especies: Sarcobatus baileyi y Sarcobatus vermiculatus. Aunque S. vermiculatus es com&uacute;n en gran parte del oeste de Norteamerica, S. baileyi tiene una distribuci&oacute;n mucho m&aacute;s limitada. Se pensaba anteriormente que S. baileyi era end&eacute;mico a Nevada, pero aqu&iacute; documentamos y describimos una poblaci&oacute;n en el este de California, comparando su morfolog&iacute;a y ecolog&iacute;a con las de una poblaci&oacute;n adyacente de S. vermiculatus. Morfol&oacute;gicamente, S. baileyi es m&aacute;s peque&ntilde;a de altura, pero produce semillas m&aacute;s grandes. Sin embargo, menos semillas de S. baileyi germinaron y sobrevivieron a una incubaci&oacute;n en laboratorio de 20 d&iacute;as, comparado con S. vermiculatus. Sarcobatus baileyi opera con potenciales de agua mucho m&aacute;s bajos que los de S. vermiculatus en condiciones de campo y tiene mayores concentraciones de Na en sus hojas. No obstante, no se observaron diferencias significativas en la eficiencia del uso de agua a largo plazo, medida en t&eacute;rminos de &delta;13C entre las 2 especies. Las concentraciones de N en las hojas fueron muy bajas en ambas especies. En general, estas especies difieren mucho en varios rasgos que son consistentes con el car&aacute;cter no freatof&iacute;tico de S. baileyi de tierras altas, el cual contrasta claramente con el car&aacute;cter freatof&iacute;tico de S. vermiculatus. Ambas especies, sin embargo, son bastante tolerantes a la sal y tienen concentraciones bajas de N en hojas, lo cual indica la baja disponibilidad de nutrientes y la potencial alta salinidad en sus h&aacute;bitats extremos. Investigaciones adicionales en ambientes comparables de bordes des&eacute;rticos deber&aacute;n llevarse a cabo para determinar la extensi&oacute;n S. baileyi en el este de California, adem&aacute;s de comparaciones de las dos especies en jard&iacute;n para comparar sus rasgos ecofisiol&oacute;gicos.  相似文献   
109.
The parasitic wasp Mymaromella pala Huber and Gibson (Hymenoptera: Mymarommatidae) was collected at 4 sites within a canyon reach of the Kootenai River in Lincoln County, Montana. This minute species has only recently been described, but it appears to have a large distribution throughout the United States and to be associated with upland and riparian forests. La avispa parasitaria Mymaromella pala Huber y Gibson (Hymenoptera: Mymarommatidae) se recolect&oacute; en cuatro sitios a lo largo de un ca&ntilde;&oacute;n del R&iacute;o Kootenai en Lincoln Co., MT. Hace poco que se describi&oacute; esta especie diminuta, pero parece tener una distribuci&oacute;n extensa en los EE.UU. y estar asociada con bosques altos y ribere&ntilde;os.  相似文献   
110.
Observations of changes in population density of native White River springfish ( Crenichthys baileyi ) in Pahranagat Valley led to the following hypothesis: introduced convict cichlids ( Cichlasoma nigrofasciatum ) cause reduced growth and recruitment; cover reduces the magnitude of the effect. The hypothesis was tested by establishing sympatric and allopatric groups of the two species in experimental aquaria with and without cover. Change in volume (= mass) and length of the two species over a three-month period in spring 1986 and 1987 was measured and analyzed using 2 × 2 factorial analyses. Convict cichlids caused reduced growth and eliminated recruitment of springfish under the experimental conditions. Cover did not influence growth but positively affected recruitment of springfish in allopatry. It is likely that a portion of the reduced springfish population densities in nature can be attributed to adverse effects from introduced cichlids.  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