全文获取类型
收费全文 | 2247篇 |
免费 | 186篇 |
国内免费 | 3篇 |
出版年
2021年 | 26篇 |
2019年 | 23篇 |
2017年 | 26篇 |
2016年 | 34篇 |
2015年 | 56篇 |
2014年 | 65篇 |
2013年 | 102篇 |
2012年 | 114篇 |
2011年 | 128篇 |
2010年 | 73篇 |
2009年 | 63篇 |
2008年 | 88篇 |
2007年 | 109篇 |
2006年 | 84篇 |
2005年 | 89篇 |
2004年 | 103篇 |
2003年 | 92篇 |
2002年 | 98篇 |
2001年 | 25篇 |
2000年 | 22篇 |
1999年 | 40篇 |
1998年 | 39篇 |
1997年 | 27篇 |
1996年 | 20篇 |
1995年 | 22篇 |
1994年 | 20篇 |
1993年 | 24篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1990年 | 20篇 |
1989年 | 23篇 |
1988年 | 23篇 |
1987年 | 16篇 |
1985年 | 22篇 |
1984年 | 26篇 |
1983年 | 14篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 31篇 |
1979年 | 16篇 |
1978年 | 27篇 |
1977年 | 19篇 |
1976年 | 26篇 |
1974年 | 24篇 |
1973年 | 21篇 |
1972年 | 17篇 |
1963年 | 14篇 |
1961年 | 17篇 |
1959年 | 15篇 |
1957年 | 16篇 |
排序方式: 共有2436条查询结果,搜索用时 15 毫秒
81.
Chrysopa downesi reproduces only in the spring and the resulting adults enter an aestival-autumnal-hibernal diapause which is primarily controlled by photoperiod. In the laboratory, constant photoperiods result in diapause induction and maintenance, whereas a series of short days followed by long days prevents or terminates diapause and promotes reproduction. The stages most sensitive to the diapause-averting stimulus are the free-living third instar, the third instar within the cocoon, and the pupa.C. downesi responds in different ways to three aspects of photoperiod: (a) an all-or-none response (diapause prevention or induction) to a sequence of two critical photoperiods, (b) an all-or-none response (diapause prevention or induction) to the difference between the long and short daylengths (a 4 hr difference is sufficient to avert diapause but a 2 hr difference is not), and (c) a quantitative response to the absolute duration of day (or night) length (after the short day requirement is fulfilled the rate of diapause termination is related to daylength).Differences and similarities in phenological adaptations and in photoperiodic responses of C. downesi, C. carnea, and C. harrisii reflect the degree of phylogenetic relationship between these closely related species. 相似文献
82.
Bacteriophage crosses using density-labeled parents have been carried out under conditions restricting DNA synthesis. The parental material and genetic contributions to progeny manifesting recombination within a genetic interval sufficiently short to exhibit high negative interference have been examined. The unreplicated products of recombination isolated as phage particles appear to contain long continuous heteroduplex regions which are heterozygous for the closely linked markers. Recombination between closely linked markers seems to be the consequence of the removal of base-pair mismatches that are present within the heteroduplex regions. This localized reduction of heterozygosity within the heteroduplex regions that join the parental components of recombinant DNA molecules can account for high negative interference. 相似文献
83.
84.
85.
Maurice Jay Bernard Voirin Aurangzeb Hasan Jean-François Gonnet Marie-Rose Viricel 《Biochemical Systematics and Ecology》1980,8(2):127-132
A survey of the leaves and flowers of 62 representatives of the tribe Loteae (Leguminosae) showed the presence of several classes of flavonoids: flavonol 7-methyl ethers (rhamnocitrin, rhamnetin), 8-O-substituted flavonols (gossypetin, limocitrin, sexangularetin, corniculatusin), 3′,4′,5′-tri-O-substituted flavonols (myricetin, mearnsetin, syringetin, laricitrin), proanthocyanidins and flavone-C-glycosides. The trisubstitution of the B-ring and the 8-O-substitution of the A-ring allow the definition of a major group including the genera Dorycnium, Bonjeania, Lotus and Tetragonolobus. The presence of proanthocyanidins and 7-O-methylation determine a second group consisting of the genus Anthyllis. Finally, Securigera, on the basis of its flavonoid chemistry, appears to be rather remote from other members of the tribe. 相似文献
86.
Maurice Ellis 《BMJ (Clinical research ed.)》1959,1(5117):299-300
87.
88.
89.
On the Mechanism of Integration of Transforming Deoxyribonucleate 总被引:12,自引:0,他引:12
Maurice S. Fox 《The Journal of general physiology》1966,49(6):183-196
The characteristics of the intermediates in the reaction, between DNA and pneumococcus, that results in genetic transformation are described in so far as they have been characterized. Transformation with DNA isolated from bacteria carrying in addition to genetic markers 32P as a radioactive label and 2H and 15N as density labels has permitted the characterization of the product of recombination between the newly introduced DNA and the DNA of a recipient bacterium. The evidence for a single strand displacement mechanism producing a hybrid structure in the DNA of the recipient bacteria is presented. Progeny of single transformants have been examined. The results of these segregation studies permit the further characterization of this hybrid product of transformation as a genetically heterozygous structure. 相似文献
90.