全文获取类型
收费全文 | 17994篇 |
免费 | 1307篇 |
国内免费 | 1篇 |
出版年
2021年 | 121篇 |
2020年 | 111篇 |
2019年 | 107篇 |
2018年 | 292篇 |
2017年 | 259篇 |
2016年 | 450篇 |
2015年 | 808篇 |
2014年 | 716篇 |
2013年 | 1062篇 |
2012年 | 1263篇 |
2011年 | 1244篇 |
2010年 | 732篇 |
2009年 | 569篇 |
2008年 | 1108篇 |
2007年 | 1085篇 |
2006年 | 1049篇 |
2005年 | 988篇 |
2004年 | 909篇 |
2003年 | 856篇 |
2002年 | 789篇 |
2001年 | 386篇 |
2000年 | 404篇 |
1999年 | 348篇 |
1998年 | 142篇 |
1997年 | 129篇 |
1996年 | 106篇 |
1995年 | 119篇 |
1994年 | 107篇 |
1993年 | 84篇 |
1992年 | 210篇 |
1991年 | 215篇 |
1990年 | 192篇 |
1989年 | 148篇 |
1988年 | 163篇 |
1987年 | 136篇 |
1986年 | 131篇 |
1985年 | 115篇 |
1984年 | 108篇 |
1983年 | 88篇 |
1982年 | 79篇 |
1981年 | 104篇 |
1980年 | 72篇 |
1979年 | 107篇 |
1978年 | 100篇 |
1977年 | 81篇 |
1976年 | 92篇 |
1975年 | 76篇 |
1974年 | 89篇 |
1973年 | 69篇 |
1971年 | 75篇 |
排序方式: 共有10000条查询结果,搜索用时 396 毫秒
161.
J Lefèvre 《Journal of theoretical biology》1983,102(2):225-248
Modeling the pulmonary arterial tree (PAT) is considered here as an optimal synthesis problem. Firstly, a class of candidate models is specified: the three-dimensional symmetric dichotomous fractal trees of elastic tubes described by Womersley's equations. Secondly, the parameters are shown to be constrained by interactions of PAT with the rest of the body; these constraints are used to limit the volume of the parametric space to which attention will be directed in the synthesis step. Thirdly, a teleonomical hypothesis is proposed: a naturally selected PAT must have a minimal input impedance under conditions keeping total arterial volume and distensibility as small as possible. This hypothesis is translated in mathematical terms and the resulting cost-function minimized in the limited parametric volume. The optimal model has parameter values and an impedance spectrum corresponding satisfactorily with real data. Moreover this model gives a clear picture of the internal hemodynamic behavior of PAT as an impedance matching device. 相似文献
162.
At least nine monoclonal antibodies against phytochrome from Pisum sativum L. and 20 against phytochrome from Avena sativa L. have been obtained from mouse hybridomas that were produced by fusion of spleen cells with SP 2/O-Ag14 myeloma cells. Hybridomas were selected and cloned in a single step by plating on a semisolid methylcellulose medium. Eight antibodies to Pisum and one to Avena phytochrome were immunopurified from hybridoma medium or ascitic fluid. When necessary, secreted antibodies were verified to be against phytochrome by demonstrating to be against phytochrome by demonstrating immunoadsorption of phytochrome, detected as loss of photoactivity and-or by appearance of the approx. 120,000-dalton phytochrome band upon sodium dodecyl sulfate polyacrylamide gel electrophoresis. 相似文献
163.
Cognitive schema were used to explain health and illness behaviors among Chinese students. University students in Hong Kong were asked to attribute causes and suggest solutions to five health/mental health problems: Weakness/Fatigue, Tension/ Anxiety, Sleep Difficulty, Hollow/Emptiness, and Headache. The patterns of endorsement on the causal and solution categories used for the five problems were compared using a new asymptotic chi-squared test. The response patterns were found to be significantly different across the five problems. Each problem was attributed to multiple causes including psychological, social, situational, somatic, and existential factors. The intended solutions were often related to the nature of the causal attributions especially when the problems were mild. In lay help-seeking, the Hong Kong students would attempt a variety of self-help measures. However, for professional consultation, the medical doctor would be the primary care professional the students would turn to for most of the problems except in the case of Hollow/Emptiness. 相似文献
164.
We have developed and used a sensitive and specific radioimmunoassay to demonstrate the presence of CRF-like immunoreactivity in extra-hypothalamic areas of ovine brain. Synthetic CRF displaced antibody bound tracer at an ED50 value of 200 pg and there was no cross-reactivity with LHRH, TRH, ACTH, beta-endorphin and several other peptides. Displacement of bound 125I-CRF by brain extracts exhibited curves parallel to synthetic CRF standards. Highest concentrations (1 ng/mg tissue) of CRF-like immunoreactivity were found in the median eminence but surprisingly, high concentrations of CRF-like immunoreactivity were found in frontal, parietal, occipital and particularly temporal areas of cerebral cortex. Much lower concentrations were found in other brain areas including the basal ganglia, limbic system and brain stem. 相似文献
165.
166.
167.
168.
Summary We have described some characteristics of Lolium multiflorum cultivated under very short photoperiods (2 hours and 1 hour). The estimations of leaf growth were based on dry weight, surface measurements, and chlorophyll content. The pigment analyses were carried out by column chromatography; chloroplast ultrastructure was observed after chemical fixation.These measurements have permitted us to note a sharp drop in the growth curve of plants grown under different day-lengths: the limiting photoperiod lies between 1 hour and 2 hours of daily illumination.Pigment analyses and chloroplast ultrastructure observations show that there is a greater difference between plants cultivated under 1 hour and 2 hours of daily illumination than between plants cultivated under 2 hours and 12 hours.A decrease in day-length causes a deficit in the chlorophyll b content as well as a poor development of the grana.We have attempted to correlate these structural anomalies with the abnormal chlorophyll a/chlorophyll b ratio. 相似文献
169.
170.