全文获取类型
收费全文 | 5771篇 |
免费 | 574篇 |
国内免费 | 1篇 |
出版年
2021年 | 80篇 |
2020年 | 57篇 |
2019年 | 52篇 |
2018年 | 95篇 |
2017年 | 77篇 |
2016年 | 107篇 |
2015年 | 200篇 |
2014年 | 225篇 |
2013年 | 262篇 |
2012年 | 325篇 |
2011年 | 328篇 |
2010年 | 220篇 |
2009年 | 168篇 |
2008年 | 275篇 |
2007年 | 283篇 |
2006年 | 268篇 |
2005年 | 258篇 |
2004年 | 276篇 |
2003年 | 254篇 |
2002年 | 242篇 |
2001年 | 119篇 |
2000年 | 97篇 |
1999年 | 93篇 |
1998年 | 73篇 |
1997年 | 56篇 |
1996年 | 67篇 |
1995年 | 68篇 |
1994年 | 56篇 |
1993年 | 48篇 |
1992年 | 77篇 |
1991年 | 86篇 |
1990年 | 68篇 |
1989年 | 70篇 |
1988年 | 54篇 |
1987年 | 67篇 |
1986年 | 59篇 |
1985年 | 74篇 |
1984年 | 59篇 |
1983年 | 52篇 |
1982年 | 54篇 |
1981年 | 62篇 |
1980年 | 47篇 |
1979年 | 68篇 |
1978年 | 76篇 |
1977年 | 61篇 |
1976年 | 65篇 |
1975年 | 45篇 |
1974年 | 58篇 |
1973年 | 52篇 |
1972年 | 41篇 |
排序方式: 共有6346条查询结果,搜索用时 15 毫秒
1.
Phenylobacterium immobile, a bacterium which is able to degrade the herbicide chloridazon, utilizes for L-tyrosine synthesis arogenate as an obligatory intermediate which is converted in the final biosynthetic step by a dehydrogenase to tyrosine. This enzyme, the arogenate dehydrogenase, has been purified for the first time in a 5-step procedure to homogeneity as confirmed by electrophoresis. The Mr of the enzyme that consists of two identical subunits amounts to 69000 as established by gel electrophoresis after cross-linking the enzyme with dimethylsuberimidate. The Km values were 0.09 mM for arogenate and 0.02 mM for NAD+. The enzyme has a high specificity with respect to its substrate arogenate. 相似文献
2.
Lens includes L.
culinaris subsp. culinaris (the cultivated lentil) and several wild species distributed from the Mediterranean region to western Asia. We compared sequence
variation in the ITS region among species of Lens in an effort to end persisting uncertainty regarding the phylogeny of the genus. The parsimony analysis revealed a single
minimum-length tree with a topology congruent with patterns derived by previous studies of nuclear and chloroplast DNA RFLPs.
The basal and highly divergent status of the L. nigricans clade is depicted, and the progenitor-derivative relationship between L. culinaris subsp. orientalis and L. culinaris subsp. culinaris is reaffirmed. Resolution in the tree was improved by combining the ITS data set with a pre-existing set of chloroplast DNA
restriction site data obtained from the same group of samples.
Received May 8, 2000 Accepted October 26, 2001 相似文献
3.
John A. Schneider Rakhi Dandona Shravani Pasupneti Vemu Lakshmi Chuanhong Liao Vijay Yeldandi Kenneth H. Mayer 《PloS one》2010,5(7)
Studies of HIV prevention interventions such as pre-exposure prophylaxis (PREP) and circumcision in India are limited. The present study sought to investigate Indian truck-drivers initial commitment to PREP and circumcision utilizing the AIDS Risk Reduction Model. Ninety truck-drivers completed an in-depth qualitative interview and provided a blood sample for HIV and HSV-2 testing. Truck-drivers exhibited low levels of initial commitment towards PREP and even lower for circumcision. However, potential leverage points for increasing commitment were realized in fear of infecting family rather than self, self-perceptions of risk, and for PREP focusing on cultural beliefs towards medication and physicians. Cost was a major barrier to both HIV prevention interventions. Despite these barriers, our findings suggest that the ARRM may be useful in identifying several leverage points that may be used by peers, health care providers and public health field workers to enhance initial commitment to novel HIV prevention interventions in India. 相似文献
4.
Mycotoxins are fungal metabolite which may in some cases exhibit a high health hazard potential. Mycotoxins can show carcinogenic, mutagenic, toxic, teratogenic or immunotoxic effects. Mycotoxin exposure in the workplace may occur through inhalation and skin contact,e.g. during occupational handling of organic matter such as livestock feed, food products, or waste. Various studies suggest that both acute and chronic effects can occur, depending at least on the exposure level. The magnitude of the potential health risks associated with a respiratory or dermal intake of mycotoxins has largely remained unclear to date. However, according to the directive 2000/54/EC on biological agents and the corresponding German Biological Agents Ordinance, employers are also required to consider the potential hazards posed by toxic effects of biological agents when assessing workplace risks. The aim of this article, therefore, is to present some basis information that should facilitate an evaluation of the significance of mycotoxins in the context of assessing workplace risks. It also provides suggestions for occupational health and safety measures. 相似文献
5.
Halasz Norbert; Nowycky Martha C.; Shepherd Gordon M.; Hofelt Tomas 《Chemical senses》1985,10(2):203-218
aftographic exeperiments on the localization of radiolabelednoradrenaline, dopamine and dopa, as well as immunohistochemicalstudies on hydroxylase-like activity, are summarized and comparedin both rat and turtle olfactory bulbs. Evoked field potentialstudies on effects of dopamine are also discussed. The histochemicalstudies suggest that dopaminergic periglomerular neurons arethe most significant cellular component of the catecholaminergicsystem in the olfactory bulb of both species. Scattered fluorescentcell group was also present in the internal plexiform layerand superficial granule cell layer of the turtle olfactory bulb.Other fibres, not related to intrinsic bulbar neuronal cellbodies, were also labeled, mostly in the granule cell layerbut also in the external plexiform layer. These might belongto a centrifugal catecholaminergic system from brain stem neurons.In the in vitro turtle olfactory bulb, dopamine and apomorphinedepressed the amplitude of field potentials evoked by a singlevolley in the olfactory nerve or lateral olfactory tract, andreduced the depression and latency of reponses when paired volleywere delivered. It is suggested that catecholaminergic systemsplay a key role in modulating mitral cell activity through actionsin both superficial (glomerular) and deep (granule) layers.This may involve direct actions, or other, non-catecholaminergicinterneurons. 相似文献
6.
Martha Macintyre 《The Australian journal of anthropology》2012,23(1):121-122
7.
8.
9.
10.
The growth and development of pods and seeds of Phaseolus vulgaris are described by the following parameters: fresh and dry weight, length, ratio of fresh weight to length and nitrogen content. Pod and seed development are independent and sequential. Pod photosynthesis contributes only marginally to pod development, but may be significant in refixing CO2 released in respiration. Pods and seeds do not seem to act simply as competing sinks. Nitrogen accumulation in the seeds begins only when it has ceased in the pods. Ways in which regulation may be achieved are discussed. Manipulation of nitrogen fertilization to enhance seed growth is suggested. 相似文献