首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   696篇
  免费   66篇
  2024年   1篇
  2023年   3篇
  2022年   4篇
  2021年   15篇
  2020年   10篇
  2019年   11篇
  2018年   19篇
  2017年   17篇
  2016年   29篇
  2015年   27篇
  2014年   43篇
  2013年   34篇
  2012年   65篇
  2011年   52篇
  2010年   42篇
  2009年   36篇
  2008年   46篇
  2007年   62篇
  2006年   37篇
  2005年   29篇
  2004年   31篇
  2003年   28篇
  2002年   26篇
  2001年   8篇
  2000年   3篇
  1999年   10篇
  1998年   7篇
  1997年   7篇
  1996年   3篇
  1995年   2篇
  1994年   7篇
  1993年   7篇
  1992年   10篇
  1991年   5篇
  1990年   2篇
  1989年   2篇
  1988年   7篇
  1987年   2篇
  1986年   3篇
  1985年   4篇
  1984年   3篇
  1980年   1篇
  1971年   1篇
  1958年   1篇
排序方式: 共有762条查询结果,搜索用时 250 毫秒
751.
Summary Standard spider frames were continuously rotated in a vertical klinostat at different speeds, changed each day, in a rising sequence through 0.3, 0.7, 2.3, 4.9, 10, 20, 30, and 60 rpm. They were also rotated at various speeds between 100 and 150 rpm. Speed of rotation did have a marked effect on a number of web parameters. It is concluded that gravity is an important compass reference for orientation during web construction. The effect was greatest at medium speeds, and less pronounced not only at slower but also at higher speeds. The most striking effect was the complete disorientation of the spider during construction of the capture spiral at 5 and 10 rpm. Surprisingly no comparable disorientation was found in the auxiliary spiral. This suggests different orientation mechanisms for the two spirals. The ability to orient the capture spiral at high speeds may be due to the effect of centripetal forces, but it may also be attributed to an evolutionary adaptation to building in windy environments where the spider might experience linear accelerations of severalg.Abbreviation rpm rotations per minute  相似文献   
752.
Middle-aged men with abdominal obesity were treated in a double-blind study with moderate doses of transdermal preparations of testosterone (T), dihydrotestosterone (DHT), or placebo. This resulted in moderately elevated T concentrations and marked decreases in follicle stimulating and luteinizing hormones in the group treated with T, while the DHT group showed elevated DHT, markedly lower T values, and less diminution of gonadotropin concentrations. In the group treated with T visceral fat mass decreased (measured by computerized tomography) without significant changes in other depot fat regions. Lean body mass did not change. In the group treated with T, glucose disposal rate, measured with the euglycemic hyperinsulinemic clamp method, was markedly augmented. Plasma triglycerides, cholesterol, and fasting blood glucose concentrations as well as diastolic blood pressure decreased. There were no such changes in the DHT or placebo treatment groups. The men treated with T reported increased well-being and energy. In none of the groups did prostate volume, specific prostate antigen concentration, genitourinary history, or urinary flow measurement change. It is suggested that supplementation of abdominal obese men with moderate doses of T might have several beneficial effects. (OBESITY RESEARCH 1993;1:245–251)  相似文献   
753.

Background  

The cytoskeletal mechanisms that underlie organelle transport in plants are intimately linked to acto-myosin function. This function is mediated by the attachment of myosin heads to F-actin and the binding of cargo to the tails. Acto-myosin also powers vigorous cytoplasmic streaming in plant cells. Class XI myosins exhibit strikingly fast velocities and may have extraordinary roles in cellular motility. Studies of the structural basis of organelle transport have focused on the cargo-binding tails of myosin XI, revealing a close relationship with the transport of peroxisomes, mitochondria, and Golgi-vesicles. Links between myosin heads and F-actin-based motility have been less investigated. To address this function, we performed localization studies using the head-neck domain of AtMYA2, a myosin XI from Arabidopsis.  相似文献   
754.
Recent studies revealed a high diversity of reproductive structures in heterotreme brachyurans, while those of Thoracotremata seem rather uniform. Yet, there still is a huge lack of data in this group as only few species have been studied with respect to their reproductive system. The phylogenetic position of Percnidae is ambiguous. Recent molecular studies place it within polyphyletic grapsoids. We herein study the reproductive morphology of Percnon gibbesi using histology, scanning electron microscopy, micro‐computed tomography and 3D‐reconstructions to test whether this species shows the characteristic thoracotreme pattern. Our results reveal that the male copulatory system conforms to other thoracotremes. It is composed of two pairs of pleopods (gonopods) and likewise paired penes. The first gonopod is relatively long. It possesses a bent terminal process with a distal opening of the ejaculatory canal, a character also present in other grapsoids. The second gonopod is short and terminates in an apical girdle. The female reproductive system reveals a combination of characters, so far unknown for thoracotremes. The paired oviducts do not lead into the seminal receptacles, but run into separate cuticular ducts joined with the vaginae. Accessory sperm storage organs, the bursae, are also connected to the vaginae. Bursae have previously only been described in heterotreme crabs. The data presented in this study reveals a higher diversity of thoracotreme reproductive systems than anticipated.  相似文献   
755.
756.
Genome editing by RNA-guided nucleases, such as SpCas9, has been used in numerous different plant species. However, to what extent multiple independent loci can be targeted simultaneously by multiplexing has not been well documented. Here, we developed a toolkit, based on a highly intron-optimized zCas9i gene, which allows assembly of nuclease constructs expressing up to 32 single guide RNAs (sgRNAs). We used this toolkit to explore the limits of multiplexing in two major model species, and report on the isolation of transgene-free octuple (8×) Nicotiana benthamiana and duodecuple (12×) Arabidopsis thaliana mutant lines in a single generation (T1 and T2, respectively). We developed novel counter-selection markers for N. benthamiana, most importantly Sl-FAST2, comparable to the well-established Arabidopsis seed fluorescence marker, and FCY-UPP, based on the production of toxic 5-fluorouracil in the presence of a precursor. Targeting eight genes with an array of nine different sgRNAs and relying on FCY-UPP for selection of non-transgenic T1, we identified N. benthamiana mutant lines with astonishingly high efficiencies: All analyzed plants carried mutations in all genes (approximately 112/116 target sites edited). Furthermore, we targeted 12 genes by an array of 24 sgRNAs in A. thaliana. Efficiency was significantly lower in A. thaliana, and our results indicate Cas9 availability is the limiting factor in such higher-order multiplexing applications. We identified a duodecuple mutant line by a combination of phenotypic screening and amplicon sequencing. The resources and results presented provide new perspectives for how multiplexing can be used to generate complex genotypes or to functionally interrogate groups of candidate genes.  相似文献   
757.

Background  

Since 2006, cases of haemorrhagic diathesis in young calves have been observed with a much higher incidence than previously known. The syndrome, now uniformly called Bovine Neonatal Pancytopenia (BNP), is characterized by multiple (external and internal) haemorrhages, thrombocytopenia, leukocytopenia, and bone marrow depletion. Although various infectious and toxicological causes of bleeding disorders in calves have been ruled out, the aetiology of BNP remains unknown. However, field observations have led to the hypothesis that the aetiological principle may be transmitted to calves via colostrum.  相似文献   
758.
Pseudomonas aeruginosa produces phenazine-1-carboxylic acid (PCA) and pyocyanin (PYO), which aid its anaerobic survival by mediating electron transfer to distant oxygen. These natural secondary metabolites are being explored in biotechnology to mediate electron transfer to the anode of bioelectrochemical systems. A major challenge is that only a small fraction of electrons from microbial substrate conversion is recovered. It remained unclear whether phenazines can re-enter the cell and thus, if the electrons accessed by the phenazines arise mainly from cytoplasmic or periplasmic pathways. Here, we prove that the periplasmic glucose dehydrogenase (Gcd) of P. aeruginosa and P. putida is involved in the reduction of natural phenazines. PYO displayed a 60-fold faster enzymatic reduction than PCA; PCA was, however, more stable for long-term electron shuttling to the anode. Evaluation of a Gcd knockout and overexpression strain showed that up to 9% of the anodic current can be designated to this enzymatic reaction. We further assessed phenazine uptake with the aid of two molecular biosensors, which experimentally confirm the phenazines’ ability to re-enter the cytoplasm. These findings significantly advance the understanding of the (electro) physiology of phenazines for future tailoring of phenazine electron discharge in biotechnological applications.  相似文献   
759.
  1. The hyporheic zone is an important habitat for benthic invertebrates and early-developmental stages of gravel spawning fish. However, the eutrophication of running waters and, in turn, the excessive periphyton biomass leads to its biological clogging. The result of these processes is oxygen depletion and a reduction in the habitat quality of the hyporheic zone.
  2. This study assessed whether top-down effects of two important European river fish species, the large herbivorous cypriniform common nase (Chondrostoma nasus, L.) and the large omnivorous cypriniform European chub (Squalius cephalus, L.), can reduce eutrophication effects in the hyporheic zone. A 4-week mesocosm-based field experiment in a eutrophic river was conducted using cage enclosures stocked or not with either nase or chub.
  3. The top-down control of periphyton was expected to reduce biological clogging and thereby increase oxygen availability in the hyporheic zone. Accordingly, we hypothesised that in enclosures stocked with either fish the concentrations of dissolved oxygen in the hyporheic zone would be higher and the periphyton biomass would be lower than in enclosures without fish stocking.
  4. Hyporheic oxygen concentrations were significantly higher in enclosures stocked with either nase or chub than in enclosures without fish stocking. However, periphyton ash-free dry mass was significantly reduced only in enclosures stocked with nase, not in those stocked with chub. Thus, the positive effects of nase and chub on hyporheic oxygen availability were caused by different mechanisms.
  5. Our results demonstrate that nase and chub can reduce eutrophication effects in the hyporheic zone of running waters. Hence, protecting and enhancing stocks of herbivorous and omnivorous fish will contribute to restoring the hyporheic zone in efforts to preserve biodiversity in eutrophic rivers.
  相似文献   
760.
Participatory Measurement, Reporting and Verification (PMRV), in the context of reducing emissions from deforestation and forest degradation with its co-benefits (REDD+) requires sustained monitoring and reporting by community members. This requirement appears challenging and has yet to be achieved. Other successful, long established, community self-monitoring and reporting systems may provide valuable lessons. The Indonesian integrated village healthcare program (Posyandu) was initiated in the 1980s and still provides effective and successful participatory measurement and reporting of child health status across the diverse, and often remote, communities of Indonesia. Posyandu activities focus on the growth and development of children under the age of five by recording their height and weight and reporting these monthly to the Ministry of Health. Here we focus on the local Posyandu personnel (kaders) and their motivations and incentives for contributing. While Posyandu and REDD+ measurement and reporting activities differ, there are sufficient commonalities to draw useful lessons. We find that the Posyandu kaders are motivated by their interests in health care, by their belief that it benefits the community, and by encouragement by local leaders. Recognition from the community, status within the system, training opportunities, competition among communities, and small payments provide incentives to sustain participation. We examine these lessons in the context of REDD+.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号