全文获取类型
收费全文 | 563197篇 |
免费 | 56719篇 |
国内免费 | 1076篇 |
专业分类
620992篇 |
出版年
2018年 | 17242篇 |
2017年 | 16154篇 |
2016年 | 13769篇 |
2015年 | 8725篇 |
2014年 | 9501篇 |
2013年 | 13381篇 |
2012年 | 19668篇 |
2011年 | 29776篇 |
2010年 | 23826篇 |
2009年 | 19329篇 |
2008年 | 24647篇 |
2007年 | 27144篇 |
2006年 | 12864篇 |
2005年 | 13690篇 |
2004年 | 13216篇 |
2003年 | 13278篇 |
2002年 | 12568篇 |
2001年 | 20446篇 |
2000年 | 20531篇 |
1999年 | 16196篇 |
1998年 | 5997篇 |
1997年 | 6470篇 |
1996年 | 6121篇 |
1995年 | 5718篇 |
1994年 | 5565篇 |
1993年 | 5703篇 |
1992年 | 13703篇 |
1991年 | 13517篇 |
1990年 | 12796篇 |
1989年 | 12611篇 |
1988年 | 11528篇 |
1987年 | 11160篇 |
1986年 | 10417篇 |
1985年 | 10429篇 |
1984年 | 8793篇 |
1983年 | 7595篇 |
1982年 | 5981篇 |
1981年 | 5267篇 |
1980年 | 5075篇 |
1979年 | 8164篇 |
1978年 | 6595篇 |
1977年 | 6007篇 |
1976年 | 5656篇 |
1975年 | 6170篇 |
1974年 | 6379篇 |
1973年 | 6323篇 |
1972年 | 6263篇 |
1971年 | 5776篇 |
1970年 | 4350篇 |
1969年 | 4182篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed
of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited
by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures
arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc.
Natl. Acad. Sci. 95, 7215–7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the
direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal
wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations.
The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether
the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain,
(ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions
do change, we find that in all cases the overall character of the helicoidal solutions is preserved. 相似文献
42.
The role of DNA sequence in determining nucleosome positions in vivo was investigated by comparing the positions adopted by nucleosomes reconstituted on a yeast plasmid in vitro using purified core histones with those in native chromatin containing the same DNA, described previously. Nucleosomes were reconstituted on a 2.5 kilobase pair DNA sequence containing the yeast TRP1ARS1 plasmid with CUP1 as an insert (TAC-DNA). Multiple, alternative, overlapping nucleosome positions were mapped on TAC-DNA. For the 58 positioned nucleosomes identified, the relative positioning strengths and the stabilities to salt and temperature were determined. These positions were, with a few exceptions, identical to those observed in native, remodeled TAC chromatin containing an activated CUP1 gene. Only some of these positions are utilized in native, unremodeled chromatin. These observations suggest that DNA sequence is likely to play a very important role in positioning nucleosomes in vivo. We suggest that events occurring in yeast CUP1 chromatin determine which positions are occupied in vivo and when they are occupied. 相似文献
43.
Hostile intercommunity relations, including attacking and killing extra-community infants of both sexes have occurred at most
wild chimpanzee sites. We describe three recent cases of intercommunity attacks on infants committed by members of the Ngogo
chimpanzee community in Kibale National Park, Uganda. Two of the attacks resulted in confirmed infanticides while a third
attack probably resulted in the infant's death. In common with previous accounts of chimpanzee infanticides, the attacks described
here occurred during boundary patrols outside the Ngogo community's usual range, adult and adolescent males were the main
participants, one infant was cannibalized after being killed, and the victims’ mothers did not accompany the attacking party
back to the Ngogo range. However, the patrol parties during each infanticide were larger than before and included females
from the Ngogo community. Our observations indirectly support both the range expansion and imbalance of power hypotheses,
which address why and under which conditions chimpanzee intercommunity encounters lead to aggression. These cases of intercommunity
infanticide add to the growing database of the phenomenon in wild chimpanzees. 相似文献
44.
D R Garrod 《Experimental cell research》1972,72(2):588-591
45.
Transcriptive expression during sea urchin embryogenesis 总被引:2,自引:0,他引:2
46.
47.
48.
A finite element based method to determine the incremental elastic material properties of planar membranes was developed and evaluated. The method is applicable to tissues that exhibit inhomogeneity, geometric and material nonlinearity, and anisotropy. Markers are placed on the tissue to form a four-node quadrilateral element. The specimen is loaded to an initial reference state, then three incremental loading sets are applied and the nodal displacements recorded. One of these loadings must include shear. These data are used to solve an over-determined system of equations for the tangent stiffness matrix. The method was first verified using analytical data. Next, data obtained from a latex rubber sheet were used to evaluate experimental procedures. Finally, experiments conducted on preconditioned rat skin revealed nonlinear orthotropic behavior. The vector norm comparing the applied and calculated nodal force vectors was used to evaluate the accuracy of the solutions. 相似文献
49.
50.