全文获取类型
收费全文 | 39648篇 |
免费 | 116篇 |
国内免费 | 186篇 |
专业分类
系统科学 | 203篇 |
丛书文集 | 587篇 |
教育与普及 | 69篇 |
理论与方法论 | 148篇 |
现状及发展 | 17477篇 |
研究方法 | 1595篇 |
综合类 | 19202篇 |
自然研究 | 669篇 |
出版年
2013年 | 366篇 |
2012年 | 536篇 |
2011年 | 1220篇 |
2010年 | 241篇 |
2008年 | 657篇 |
2007年 | 770篇 |
2006年 | 788篇 |
2005年 | 720篇 |
2004年 | 736篇 |
2003年 | 681篇 |
2002年 | 701篇 |
2001年 | 1344篇 |
2000年 | 1254篇 |
1999年 | 807篇 |
1992年 | 773篇 |
1991年 | 571篇 |
1990年 | 670篇 |
1989年 | 685篇 |
1988年 | 634篇 |
1987年 | 679篇 |
1986年 | 660篇 |
1985年 | 792篇 |
1984年 | 621篇 |
1983年 | 530篇 |
1982年 | 481篇 |
1981年 | 491篇 |
1980年 | 590篇 |
1979年 | 1241篇 |
1978年 | 1029篇 |
1977年 | 985篇 |
1976年 | 819篇 |
1975年 | 871篇 |
1974年 | 1202篇 |
1973年 | 1017篇 |
1972年 | 1053篇 |
1971年 | 1211篇 |
1970年 | 1524篇 |
1969年 | 1116篇 |
1968年 | 1143篇 |
1967年 | 1070篇 |
1966年 | 966篇 |
1965年 | 674篇 |
1964年 | 184篇 |
1959年 | 366篇 |
1958年 | 625篇 |
1957年 | 424篇 |
1956年 | 349篇 |
1955年 | 338篇 |
1954年 | 328篇 |
1948年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
991.
Angela M. Ortega-León Maricela Villagrán-Santa Cruz J. Jamie Zú?iga-Vega Raúl Cueva-del Castillo Fausto R. Méndez-de la Cruz 《西北部美国博物学家》2011,69(1)
Asynchronous reproduction is a common phenomenon in high-elevation populations of lizards from Central México. Sperm storage in the reproductive tract of females is the mechanism for making oocyte fertilization possible. Our study addresses questions related to functional oviductal sperm storage of females mating on different dates throughout the reproductive season. A population of Sceloporus mucronatus with copulation in the summer and ovulation in the fall was chosen for this experiment. Eleven females that copulated in the field during June and 13 females that copulated in captivity during August were maintained in the laboratory until parturition. The number of pregnant females and the litter sizes produced in each experimental group were indicative of the viability of the stored sperm. Sperm stored in the reproductive tract of females were able to fertilize eggs after 4 months. No significant differences were found in the number of pregnant females between the 2 experimental groups nor in the litter sizes that they produced. We found that the amount of time sperm were held in the female reproductive tract (ca. 3 months) had no effect on the capacity of sperm to fertilize eggs. Histological examination of 8 oviducts collected before the mating season eliminated the possibility of sperm storage from one year to the next. In this system, sperm retention could have evolved as a response mechanism to deal with the asynchrony between sexes in the reproductive cycles. However, we cannot rule out alternative hypotheses. 相似文献
992.
Small mammal populations inhabiting radioactive waste disposal areas could be important vectors of contaminant redistribution, given sufficiently high numbers. Earlier studies conducted at the Subsurface Disposal Area (SDA) in southeastern Idaho found small mammal densities equaling or exceeding densities in native habitat. Our live-trapping study was conducted in 1988 and 1989 to assess the role of edge habitat (where SDA crested wheatgrass [ Agropyron cristatum ] plantings and native sagebrush habitat are separated by an earthen dike) in facilitating use of this highly modified site by small mammals. Small mammals had a significantly greater density in SDA edge habitat than in the interior. Total density of small mammals and immediately around the SDA appeared to be less variable over time than density in native sagebrush habitat for years when data were available. This phenomenon was largely attributable to steady or increasing SDA population densities of the most common species, Peromyscus maniculatus and Perognathus parvus , during 1988-89, when most small mammal species had below-average densities in surrounding areas. The variety of foraging options in edge habitat may have allowed these relatively opportunistic species to avoid widespread population declines associated with drought years in 1988-89. Movements by P. manicualtus across the boundary were common, suggesting that this species did indeed utilize both habitat types. Preferences for edge habitat could potentially be used to formulate strategies that reduce use of waste site by small mammals. 相似文献
993.
William D. Skinner 《西北部美国博物学家》2011,45(2)
The mean size of food and amount of food consumed by cutthroat trout from Palisades Creek in southeastern Idaho increased with trout length. Number of organisms of terrestrial origin, number of aquatic larvae, number of ants, and number of berries from redstem dogwood were related to trout length. The size range and number of taxa consumed increased with trout size, indicating that as trout get larger, they broaden their feeding menu. The minimum size of food consumed was relatively constant for all trout, but larger trout appeared to feed more from the stream bottom. Trout may have a minimum length of food, below which items cannot be detected as food. Other possible factors affecting the feeding of cutthroat trout are mentioned. 相似文献
994.
We translocated 196 Sage Grouse ( Centrocercus urophasianus into Sawtooth Valley, Idaho, during March-April 1986-87 to augment a small resident population. Forty-four grouse equipped with radio transmitters were monitored through spring and summer. Nest sites ( n = 6) had greater ( P = .032) horizontal cover than did independent random plots ( n = 7). During summer, grouse used sites ( n = 50) with taller live and dead shrub heights, greater shrub canopy cover, and more ground litter ( P n = 50) 50-300 m from use sites. Distance to edge and mountain sagebrush ( Artemisia tridentata vaseyana ) density best separated use sites from independent random plots in logistic regression analysis and correctly classified 64% of the use sites and 78% of the independent random plots. Sage Grouse used sties that had narrower frequency distributions for many variables than did independent plots ( P < .04), suggesting selection for uniform habitat. 相似文献
995.
Use of interstate highway overpasses and billboards for nesting by the common raven ( Corvus corax )
Common ravens are usually rather shy birds around their nests and avoid nesting too near human activity. We here report the use of overpasses along heavily traveled Interstate Highway 84 in Idaho and Utah for nest placement. Nests are within 6 m of passing vehicles. 相似文献
996.
997.
Lewisia kelloggii has been understood as a rare plant with a disjunct range in California and Idaho. Examination of herbarium specimens and analysis of isozymes in 6 Idaho and 7 California populations revealed consistent differences between plants of the 2 states. Fixed differences in alleles at 2 loci (AAT2 and PGI1) distinguished Idaho from California plants. Genetic identities based on isozymes between Idaho and California populations averaged 0.58, lower than the average for congeneric plant species. Idaho plants were smaller than most California plants, but California plants were variable. The most consistent morphological difference between Idaho and California specimens was the difference in the number of glands on the margins of bracts and sepals. Idaho plants had 0 (-5) pink glands on each margin of these organs, all on teeth near the tips. In California plants these organs had 12-25 glands on each margin, the distal ones elevated on teeth and the proximal ones sessile. We recognize the Idaho plants as a new species, L. sacajaweana , and retain the name L. kelloggii for the California populations. 相似文献
998.
999.
Lekelia D. Jenkins 《Studies in history and philosophy of science》2010,41(1):75-85
This paper explores the evolution of a trading zone by organizing the case study of turtle excluder devices within the model proposed by Collins et al. (2007). The case study offers evidence that trading zones do evolve and that the concepts of enforced and fractionated trading zones hold practical utility for describing and defining the complexities of actual exchanges. In this case a trading zone evolved from enforced to fractionated and ultimately diverged into two trading zones. For each step of the evolution I describe the forces that drove these transitions. Finally, I present an adapted trading zone model that is conceptually a better fit for the turtle excluder device case study. 相似文献
1000.
In higher vertebrates, sulfatases belong to a conserved family of enzymes that are involved in the regulation of cell metabolism
and in developmental cell signaling. They cleave the sulfate from sulfate esters contained in hormones, proteins, and complex
macromolecules. A highly conserved cysteine in their active site is post-translationally converted into formylglycine by the
formylglycine-generating enzyme encoded by SUMF1 (sulfatase modifying factor 1). This post-translational modification activates all sulfatases. Sulfatases are extensively
glycosylated proteins and some of them follow trafficking pathways through cells, being secreted and taken up by distant cells.
Many proteoglycans, glycoproteins, and glycolipids contain sulfated carbohydrates, which are sulfatase substrates. Indeed,
sulfatases operate as decoding factors for a large amount of biological information contained in the structures of the sulfated
sugar chains that are covalently linked to proteins and lipids. Modifications to these sulfate groups have pivotal roles in
modulating specific signaling pathways and cell metabolism in mammals. 相似文献