全文获取类型
收费全文 | 795篇 |
免费 | 36篇 |
国内免费 | 87篇 |
出版年
2024年 | 4篇 |
2023年 | 14篇 |
2022年 | 14篇 |
2021年 | 13篇 |
2020年 | 16篇 |
2019年 | 23篇 |
2018年 | 20篇 |
2017年 | 17篇 |
2016年 | 24篇 |
2015年 | 19篇 |
2014年 | 28篇 |
2013年 | 44篇 |
2012年 | 18篇 |
2011年 | 28篇 |
2010年 | 28篇 |
2009年 | 36篇 |
2008年 | 47篇 |
2007年 | 44篇 |
2006年 | 48篇 |
2005年 | 51篇 |
2004年 | 39篇 |
2003年 | 38篇 |
2002年 | 34篇 |
2001年 | 23篇 |
2000年 | 16篇 |
1999年 | 27篇 |
1998年 | 20篇 |
1997年 | 19篇 |
1996年 | 19篇 |
1995年 | 24篇 |
1994年 | 19篇 |
1993年 | 19篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1972年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有918条查询结果,搜索用时 31 毫秒
1.
Classification and ordination of plant communities along an altitudinal gradient on the Presidential Range,New Hampshire,USA 总被引:1,自引:0,他引:1
An analysis of vegetation along an altitudinal gradient on the Presidential Range, New Hampshire, USA, using the Braun–Blanquet approach followed by multivariate data analysis is presented. Twelve main plant communities have been distinguished. Floristic information is presented in twelve tables and one appendix. The relationships of the communities to complex environmental gradients are analyzed using Correspondence Analysis. Floristic composition and community structure are controlled primarily by the altitudinal gradient (temperature, precipitation), and by mesotopographic conditions (snow accumulation, exposure and cryoturbation, slope position, and soil moisture). 相似文献
2.
Over the past two decades, the dendroclimate community has produced various annually resolved, warm season temperature reconstructions for the extratropical Northern Hemisphere. Here we compare these tree-ring based reconstructions back to 831 CE and present a set of basic metrics to provide guidance for non-specialists on their interpretation and use. We specifically draw attention to (i) the imbalance between (numerous) short and (few) long site chronologies incorporated into the hemispheric means, (ii) the beneficial effects of including maximum latewood density chronologies in the recently published reconstructions, (iii) a decrease in reconstruction covariance prior to 1400 CE, and (iv) the varying amplitudes and trends of reconstructed temperatures over the past 1100 years. Whereas the reconstructions agree on several important features, such as warmth during medieval times and cooler temperatures in the 17th and 19th centuries, they still exhibit substantial differences during 13th and 14th centuries. We caution users who might consider combining the reconstructions through simple averaging that all reconstructions share some of the same underlying tree-ring data, and provide four recommendations to guide future efforts to better understand past millennium temperature variability. 相似文献
3.
4.
George A. Antonelis Jr. Mark S. Lowry Douglas P. DeMaster Clifford H. Fiscus 《Marine Mammal Science》1987,3(4):308-322
Stomach lavaging was used to study the feeding habits of northern elephant seals ( Mirounga angustirostris ) found on San Miguel Island, California, during the spring of 1984. Fifty-nine elephant seals were chemically immobilized with an intramuscular injection of ketamine hydrochloride. Once immobilized, an animal's stomach was intubated, filled with 3–4 liters of water to create a slurry of the undigested food items, and evacuated into a collection device. The stomachs of 57 (96.6%) of the animals lavaged contained identifiable parts of prey. Twenty-nine different food items were identified, 12 of which have not been previously reported as prey of the northern elephant seal: two teleost fish, Coryphaenoides acrolepis (Pacific rattail) and another unidentified macrourid; two crustaceans, Pasiphaea pacifica (glass shrimp) and Euphausia sp.; six squid, Abraliopsis felis, Gonatus berryi, Histioteuthis dofleini, Cranchia scabra, Taonius pavo, and Galiteuthis sp. and two octopi, Octopus dofleini and Octopus rubescens. 相似文献
5.
Total RNA from post mortem human caudate nucleus, cerebellum, cerebral cortex and pheochromocytoma tissues has been prepared. Northern blot analysis, using a single-stranded human proenkephalin A antisense probe (cRNA), revealed the existence of two different proenkephalin A-like sequences in the human caudate nucleus and pheochromocytoma RNA extracts of approximately 1400 and 1000 nucleotides in length respectively, whereas no specific RNA bands could be detected in the cortex and only the 1400 nucleotide band was present in the cerebellum. Under highly stringent hybridization conditions, the proenkephalin A-like RNA bands still appear, indicating that the detected RNA species have either identical or a closely related sequence to that of the wellcharacterized human proenkephalin A mRNA sequence. 相似文献
6.
Expression of c-myc protooncogene in rat lens cells during development,maturation and reversal of galactose cataracts 总被引:2,自引:0,他引:2
Yi Wen Songtao Shu Nalin J. Unakar Isaac Bekhor 《Molecular and cellular biochemistry》1992,112(1):73-79
It is well established that normal patterns of epithelial cell proliferation and metabolism, and of fiber cell differentiation and maturation are essential for the maintenance of transparency in the ocular lens. Several factors, including exposure to high levels of sugars, have been known to result in the compromise of lens transparency. For example, initiation of lens cell damage by galactose induces lens epithelial cells to proliferate. Elevated levels of c-myc mRNA have usually been correlated with rapid cell growth and increased entry of cells into the S phase. Therefore, changes in c-myc mRNA levels may provide an early indication of the stimulation of lens epithelial cells to proliferate and differentiate, which has been postulated to be an early and important event in response to lens cell injury by galactose. By Northern blot hybridization analysis we quantitated c-myc mRNA levels in the lens capsule epithelia of rats (1) exposed to galactose, and (2) undergoing a partial recovery from the galactose-induced cell damage. At the onset of lens cell damage, we find c-myc mRNA to elevate to 6-fold by 24 hr, and by 48 hr decreases to about 3-fold the normal levels. During recovery, c-myc mRNA continues to be expressed at high levels approaching a 10-fold increase by day 12, then decreasing to levels of about 8-fold the control by day 30. The 24 h transitory elevation in c-myc mRNA in lens epithelial cells is in accord with our previous observations on the 24 h increase in MP26, crystallin and aldose reductase mRNAs following a high influx of galactose. Therefore, the elevation in c-myc mRNA as well suggest that galactose appears to cause lens cells to undergo an early transitory period of gene induction following the exposure of lens cells to galactose. 相似文献
7.
8.
Kaijiro Anzai Shunsuke Kobayashi Narumi Kitamura Yuri Kanai Hiromichi Nakajima Yoshioki Suehiro Sataro Goto 《Journal of neurochemistry》1986,47(3):673-677
We isolated a mouse genomic clone that hybridized with small RNA present in the cytoplasm of the brain. The RNA was about 150 nucleotides long. This RNA seemed to be specific to the brain, since it was not found in the liver or kidney. The clone DNA contained a sequence homologous to 82-nucleotide "identifier" core sequence of cDNA clones of rat. The sequence contained a split promoter for RNA polymerase III and was flanked by a 12-nucleotide direct repeat (ATAAATAATTTA). 相似文献
9.
Molecular Genetic Analysis of Myelin-Deficient Mice: Shiverer Mutant Mice Show Deletion in Gene(s) Coding for Myelin Basic Protein 总被引:17,自引:6,他引:11
10.
Multiple discriminant analysis is a useful multivariate technique in vegetation studies that can be employed for several purposes, even if the underlying statistical assumptions are not satisfied. An example of application of this method is discussed: multiple discriminant analysis was successfully used for evaluating the predictivity of forest types defined by numerical classification of vegetation data with respect to soil variables.Species nomenclature follows Pignatti (1982) for vascular plants and Augier (1966) for mosses.The research has been supported by the IDROSER (Bologna, Italy) and by the Italian C. N. R. (Grant no. 83.02548.04, resp. Prof. A. Pirola). We wish to thank Prof. E. Feoli, Prof. E. van der Maarel, Prof. L. Orlóci and Prof. A. Pirola for suggestions. We are also indebted to Dr N. Filippi who analyzed the soil samples. 相似文献