全文获取类型
收费全文 | 5024篇 |
免费 | 564篇 |
专业分类
5588篇 |
出版年
2022年 | 41篇 |
2021年 | 73篇 |
2020年 | 46篇 |
2019年 | 49篇 |
2018年 | 62篇 |
2017年 | 69篇 |
2016年 | 112篇 |
2015年 | 172篇 |
2014年 | 176篇 |
2013年 | 269篇 |
2012年 | 280篇 |
2011年 | 321篇 |
2010年 | 200篇 |
2009年 | 170篇 |
2008年 | 241篇 |
2007年 | 230篇 |
2006年 | 210篇 |
2005年 | 197篇 |
2004年 | 195篇 |
2003年 | 184篇 |
2002年 | 200篇 |
2001年 | 115篇 |
2000年 | 103篇 |
1999年 | 116篇 |
1998年 | 60篇 |
1997年 | 49篇 |
1996年 | 67篇 |
1995年 | 54篇 |
1994年 | 58篇 |
1993年 | 61篇 |
1992年 | 65篇 |
1991年 | 55篇 |
1990年 | 70篇 |
1989年 | 87篇 |
1988年 | 56篇 |
1987年 | 41篇 |
1986年 | 46篇 |
1985年 | 49篇 |
1984年 | 50篇 |
1983年 | 41篇 |
1982年 | 50篇 |
1981年 | 40篇 |
1980年 | 37篇 |
1979年 | 49篇 |
1977年 | 35篇 |
1976年 | 50篇 |
1975年 | 33篇 |
1974年 | 52篇 |
1973年 | 42篇 |
1971年 | 33篇 |
排序方式: 共有5588条查询结果,搜索用时 15 毫秒
71.
72.
Governing equations of motion for the otolith organs and their response to a step change in velocity of the skull 总被引:2,自引:0,他引:2
The otolith organs are the linear motion sensors of the inner ear. They comprise an overdamped second-order system and respond to gravity and skull acceleration. The governing equations of motion which describe the relative displacement of the mass with respect to the skull are developed. When these equations are non-dimensionalized they indicate that the elastic term is almost negligible with respect to the viscous and inertial terms. For a step change in skull velocity an analytic solution is given for the elastic term equal to zero and numeric solutions are given for small values of the elastic term. 相似文献
73.
The involvement of polyamines in cytokinesis has been examined in eggs of the amphibian X. laevis. Microinjection of spermine or spermidine into unfertilized eggs induced a shortening of the first division cycle and early formation of cleavage-like constrictions. Eggs were activated by injection and developed furrows about 45 min later, whereas the first division normally occurred around 120 min after activation. In terms of concentration, spermine was slightly more effective than spermidine, but putrescine had no influence on the division cycle. 相似文献
74.
Transformation of Neurospora crassa with recombinant plasmids containing the cloned glutamate dehydrogenase (am) gene: evidence for autonomous replication of the transforming plasmid. 总被引:10,自引:6,他引:4 下载免费PDF全文
D M Grant A M Lambowitz J A Rambosek J A Kinsey 《Molecular and cellular biology》1984,4(10):2041-2051
We have characterized Neurospora crassa transformants obtained with plasmid pJR2, which consists of the Neurospora glutamate dehydrogenase (am) gene cloned in pUC8 and an am132 host strain which contains a deletion encompassing the cloned fragment. Every one of 33 transformants tested showed extreme meiotic instability: less than 1 or 2% am+ progeny were obtained in initial or successive backcrosses between am+ transformants and am132 or in intercrosses between am+ progeny. Furthermore, am+ progeny from backcrosses gave a high proportion of auxotrophic (am) mitotic segregants during vegetative growth. These results indicate that the am+ character is not stably integrated into chromosomal DNA in any of the transformants tested. Nuclear DNAs from six transformants were analyzed by Southern hybridization. All six transformants contained sequences homologous to pJR2. Four showed restriction fragments expected for unmodified pJR2, but most showed additional bands. Southern blots of undigested DNAs showed that the plasmid sequences are present predominantly in high-molecular-weight form (larger than 20 kilobases). Southern blots showed that auxotrophic (am) progeny from a backcross to am132 had lost restriction bands corresponding to free plasmid but retained additional bands, apparently integrated into chromosomal DNA in a nonfunctional manner. Considered together, these results are most reasonably interpreted as follows: recombinant plasmids containing the am+ gene can replicate autonomously in N. crassa, the free plasmids are present in oligomeric or modified form or both, and plasmid sequences also integrate at multiple sites in the deletion host but in a nonfunctional manner. An alternate interpretation--that tandem repeats of the plasmid are integrated into chromosomal DNA but eliminated during meiosis--cannot be completely excluded. However, stable integration of the am gene can be obtained under a variety of other conditions, viz., using the am gene cloned in a phage lambda vector (J. A. Kinsey and J. A. Rambosek, Mol. Cell. Biol. 4:117-122, 1984), using derivatives of pJR2, or using pJR2 to transform a frameshift mutant. 相似文献
75.
Jan E Grant 《Plant Cell, Tissue and Organ Culture》1984,3(2):169-173
Cotyledonary tissue from immature embryos of Glycine canescens was induced to callus and then form embryo-like structures. These structures could be cultured into whole plants and grown in soil. 相似文献
76.
Selected aspects of collared peccary (Dicotyles tajacu) reproductive biology in a captive texas herd
Collared peccaries (Dicotyles tajacu) from 32 litters were born in captivity during the period from September, 1981 through July, 1983. Incidence of singleton, twin, and triplet litters was 28%, 66%, and 6%, respectively. Mean (± SE) weight of young from singleton, twin, and triplet litters was 751 (± 35) gm, 698 (± 17) gm, and 612 (± 74) gm, respectively. Sex ratio among all litters was 56:44 (M:F). Neonatal mortality amounted to 9% of the young born. Mean duration of estrus was 2.60 days and mean length of gestation was 145.6 days. Evidence indicated that multiple copulations increased ovulation rate and the chance for conception. 相似文献
77.
78.
79.
80.
Miss S. Dobrofsky Dr. W. F. Grant 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1980,58(2):157-160
Summary A study of seed position in the pod ofLotus corniculatus L. cv. Mirabel (Fabaceae) suggested that reduced seed set after self-pollination is not due to an inability of the self-pollen tube to reach the end of the ovary. As in other cultivars, it has been demonstrated that cultivar Mirabel produced less seed per pod and shorter pods after self- than after cross-pollination. No differences were noted for percent germination of seeds produced by both types of pollination indicating that the number of seeds per pod is a reliable index of the ultimate productive potential of the pollination. 相似文献