全文获取类型
收费全文 | 4787篇 |
免费 | 495篇 |
国内免费 | 1篇 |
专业分类
5283篇 |
出版年
2023年 | 24篇 |
2022年 | 53篇 |
2021年 | 127篇 |
2020年 | 66篇 |
2019年 | 84篇 |
2018年 | 100篇 |
2017年 | 89篇 |
2016年 | 155篇 |
2015年 | 239篇 |
2014年 | 238篇 |
2013年 | 299篇 |
2012年 | 379篇 |
2011年 | 376篇 |
2010年 | 233篇 |
2009年 | 205篇 |
2008年 | 311篇 |
2007年 | 324篇 |
2006年 | 291篇 |
2005年 | 252篇 |
2004年 | 240篇 |
2003年 | 232篇 |
2002年 | 215篇 |
2001年 | 55篇 |
2000年 | 39篇 |
1999年 | 47篇 |
1998年 | 63篇 |
1997年 | 39篇 |
1996年 | 33篇 |
1995年 | 32篇 |
1994年 | 29篇 |
1993年 | 19篇 |
1992年 | 19篇 |
1991年 | 21篇 |
1990年 | 18篇 |
1989年 | 23篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 23篇 |
1984年 | 20篇 |
1983年 | 18篇 |
1982年 | 17篇 |
1981年 | 16篇 |
1980年 | 16篇 |
1979年 | 14篇 |
1978年 | 16篇 |
1976年 | 11篇 |
1973年 | 11篇 |
1971年 | 10篇 |
1968年 | 10篇 |
排序方式: 共有5283条查询结果,搜索用时 15 毫秒
1.
Kathryn M. Denkowski George C. Denkowski 《Applied psychophysiology and biofeedback》1984,9(3):353-364
This study examined whether group progressive relaxation training was as effective as individual EMG biofeedback training in facilitating the academic achievement and self-control of 45 hyperactive elementary school children. Academic achievement was assessed with the Gates-MacGinities Reading Tests, and self-control was measured with the Nowicki-Strickland and the Teacher Rating scales. Eight sessions were scheduled at weekly intervals. Progressive relaxation was conducted in groups of seven or eight and was induced with a commercial audiocassette program. EMG training augmented frontalis biofeedback with those taped exercises. A placebo group listened to taped children's stories. Multivariate analysis of variance indicated no significant differences among the three contrast groups when all dependent variables were considered together. However, univariate F values and discriminant analysis disclosed locus of control to be significantly more internal for the progressive relaxation condition. Also, differences between the two relaxation and the placebo groups, though not statistically significant, were all in the expected direction. While the relative efficacy of group progressive relaxation could not be established conclusively, the data appeared sufficiently positive to warrant further investigation of this cost-effective prospective intervention.The opinions expressed by the authors are not necessarily those of their respective institutions. 相似文献
2.
3.
F prostaglandins function as potent olfactory stimulants that comprise the postovulatory female sex pheromone in goldfish 总被引:4,自引:0,他引:4
This study establishes that ovulated female goldfish release F type prostaglandins (PGFs) to the water where they stimulate male spawning behavior and comprise the goldfish postovulatory pheromone. We first demonstrated that ovulated and prostaglandin-injected female goldfish release immunoreactive PGFs to the water. Next, using electro-olfactogram recording (EOG), we determined that waterborne prostaglandins function as potent olfactory stimulants for mature male goldfish. Prostaglandin F2 alpha (PGF2 alpha) and its metabolite 15-keto-prostaglandin F2 alpha (15K-PGF2 alpha) were the most potent prostaglandins; the former had a detection threshold of 10(-10) M and the latter a detection threshold of 10(-12) M. Studies of prostaglandin-injected fish indicated that PGF metabolites are an important component of the pheromone. Cross-adaptation experiments using the EOG demonstrated that goldfish have separate olfactory receptor sites for PGF2 alpha and 15K-PGF2 alpha that are independent from those that detect other olfactory stimulants. Finally, we established that male goldfish exposed to low concentrations of waterborne PGFs exhibit reproductive behaviors similar to those elicited by exposure to the odor of ovulated fish. Together with our recent discovery that a steroidal maturational hormone functions as a preovulatory "priming" pheromone for goldfish, these findings suggest that hormones and their metabolites may commonly serve as reproductive pheromones in fish. 相似文献
4.
The present study was carried out on the non-tidal reaches of the River Ely, South Wales, from October 1979 to October 1980. The seasonal variations of the chlorophyll-a content of the phytoplankton was unimodal with one maximum in May and a minimum in December. The chlorophyll-a content varied from 0.277 to 41.259 mg m?3. The primary productivity showed a bimodal seasonal distribution with two maxima in May and September and lower values throughout the remainder of the year, particularly in winter. The value for the primary productivity varied from 0.269 to 24.302 mg C m?3 h?1. A positive correlation was obtained between chlorophyll-a content and primary productivity. The seasonal distribution of the dominant algal species and the saprobity of the River Ely were also studied. The diatom species almost showed a similar seasonal periodicity. Their concentrations were low during the winter months and high during most of the spring and summer months. Many of the dominant diatom species found in the phytoplankton populations were either considered by the other authors as saprobic (Nitzchia palea) or as inhabitants of eutrophic waters (Gomphonema parvulum, Navicula cryptocephala and Synedra ulna). Chlamydomonas spp. were the most abundant green algae followed by Chlorella vulgaris. The effect of sewage effluent disposal and cattle excreta at three of the sampling sites might partly explain the presence of high Chlamydomonas spp. populations at those sites. Physical factors (low flow rates, high transparency and water temperature) and organic pollution at some sampling sites seemed to play an important role in increasing the number of algal species during spring and summer. The non-tidal reaches of the River Ely were found to be β-mesosaprobic above and below Station 5 and α-mesosaprobic at the latter station and therefore, the river can be considered as polluted at Station 5 and mildly polluted at the others. 相似文献
5.
Studies on C'3 polymorphism in Denmark 总被引:1,自引:0,他引:1
6.
Kathryn E. Stecke 《Flexible Services and Manufacturing Journal》1989,1(4):287-324
This article is a detailed case study of a particular FMS that will be operational in 1989. It describes the daily planning and operating problems that will need to be addressed. The algorithms that will operate this system are presented. Given the daily changing production requirements, the algorithms begin with an aggregate planning feasibility check. Then planning, scheduling, inventory management, and breakdowns are addressed. The key problems in operating this system are tool management problems. Detailed tooling data and their analysis are presented in an appendix to address these problems. 相似文献
7.
Intraprotoplasmic feruloylation of arabinoxylans in Festuca arundinacea cell cultures 总被引:2,自引:2,他引:0
Graminaceous primary cell walls contain polysaccharides to which are esterified feruloyl residues. Ester biosynthesis is highly specific and the present experiments were performed to ascertain the likely site of feruloylation in living grass cell cultures. Cell cultures of tall fescue grass (Festuca arundinacea Schreber) incorporated exogenous l-[1-3H]arabinose into polymers at a linear rate after a short lag of approx. 1–3 min. Radiolabelled polymers did not start to accumulate in the culture medium until 20–35 min after [3H]arabinose was supplied. However, polymer-bound feruloyl-arabinose residues began to accumulate 3H after a lag of 1–3 min. Assuming that the onset of secretion of radiolabelled polymers into the medium indicates the time before which essentially all the radiolabel was internal to the plasma membrane, the results show that the polysaccharide-bound [3H]arabinose residues must have been feruloylated within the protoplast.Abbreviations AIR
alcohol-insoluble residue
- BAW
butan1-ol/acetic acid/water (12:3:5 by volume)
- BEW
butan-1-ol/ ethanol/water (20:5:11 by volume)
- EPW
ethyl acetate/pyridine/ water (8:2:1 by volume)
- RAra
Chromatographic mobility relative to that of l-arabinose
We are very grateful to Mr. Gundolf Wende for assistance with the characterisation of the feruloyl esters. K.E.M. is funded by a studentship from the Science and Engineering Research Council in collaboration with Zeneca Agrochemicals. 相似文献
8.
1α,25-Dihydroxyvitamin D3 (1α, 25-(OH)2D3) has been shown to increase cytosolic calcium and inositol trophosphate levels in rat osteosarcoma cells (ROS 17/2.8) and to increase nuclear calcium in these cells. To determine the mechanism(s) of 1α, (OH)2D3-induced changes in the calcium, the effect of the hormone on phospholipid metabolism in isolated osteoblast nuclei wa assessed. 1α,25 (OH)2D3, 20 nM, increased inositol triphosphate levels in the nuclei after 5 min of treatment. The biologically inactive epimer, 1β,25-(OH)2D3, had no significant effect on inositol triphosphate levels. ATP, 1 mM, also increased inositol triphosphate levels in the isolated nuclei after 5 min. 1α,25-(OH)2D3, 20 nM, increased calcium in the isolated nuclei in the presence but not in the absence of extranuclear calcium with 5 min. Nuclear calcium was also increased within 5 min by ATP, 1 mM, and inositol triphosphate, 1 mM. The effects of ATP on nuclear calcium was not additive with 1α, 25-(OH)2D3, suggesting that these two agents increase nuclear calcium in these osteoblast-like cells by similar mechanisms. In summary, 1α,25-(OH)2D3 amd ATP rapidly increase inositol triphosphate levels in isolated from ROS 17/2.8 cells. The hormone, the nucleotide, and the inositol phospholipid nuclear calcium. Thus, the 1α,25-(OH)2D3 and ATP effects of nuclear calcium may be mediated by changes in phospholipid metabolism in the nuclei of these osteoblastlike cells. © Wiley-Liss, Inc. 相似文献
9.
The molecular weight of infectious pancreatic necrosis virus (IPNV) has been determined by analytical ultracentrifugation and dynamic light scattering. The sedimentation coefficient of the virus was found to be 435S. The average value for molecular weight is (55 +/- 7) x 106. The virus genome consists of two segments of double-stranded RNA (molecular weights, 2.5 x 106 and 2.3 x 106), which represents 8.7% of the virion mass. The capsid protein moiety of IPNV consists of four species of polypeptides, as determined by polyacrylamide gel electrophoresis. The number of molecules of each polypeptide in the virion has been determined. There are 22 molecules of the internal polypeptide alpha (molecular weight, 90,000), 544 molecules of the outer capsid polypeptide beta (molecular weight, 57,000), and 550 and 122 molecules, respectively, of the internal polypeptides gamma1 (molecular weight, 29,000) and gamma2 (molecular weight, 27,000). IPNV top component contains only the beta polypeptide species, and its molecular weight is estimated to be 31 x 106. The hydrodynamic diameter and electron microscopic diameter (calculated by catalase crystal-calibrated electron microscopy) of IPNV was compared with those of reovirus and encephalomyocarditis virus. Due to the swelling of the outer capsid, reovirus particles were found to be much larger when hydrated (96-nm diameter) than when dehydrated (76-nm diameter), having a large water content content and low average density. In contrast, IPNV particles are more rigid, having nearly the same average diameter under hydrous (64 nm) as under anhydrous conditions (59.3 nm). Encephalomyocarditis virus has a very low water content and does not shrink at all when prepared for electron microscopy. 相似文献
10.
Paula Sobenko Hatum Kathryn McMahon Kerrie Mengersen Paul PaoYen Wu 《Ecology and evolution》2022,12(8)
In general, it is not feasible to collect enough empirical data to capture the entire range of processes that define a complex system, either intrinsically or when viewing the system from a different geographical or temporal perspective. In this context, an alternative approach is to consider model transferability, which is the act of translating a model built for one environment to another less well‐known situation. Model transferability and adaptability may be extremely beneficial—approaches that aid in the reuse and adaption of models, particularly for sites with limited data, would benefit from widespread model uptake. Besides the reduced effort required to develop a model, data collection can be simplified when transferring a model to a different application context. The research presented in this paper focused on a case study to identify and implement guidelines for model adaptation. Our study adapted a general Dynamic Bayesian Networks (DBN) of a seagrass ecosystem to a new location where nodes were similar, but the conditional probability tables varied. We focused on two species of seagrass (Zostera noltei and Zostera marina) located in Arcachon Bay, France. Expert knowledge was used to complement peer‐reviewed literature to identify which components needed adjustment including parameterization and quantification of the model and desired outcomes. We adopted both linguistic labels and scenario‐based elicitation to elicit from experts the conditional probabilities used to quantify the DBN. Following the proposed guidelines, the model structure of the general DBN was retained, but the conditional probability tables were adapted for nodes that characterized the growth dynamics in Zostera spp. population located in Arcachon Bay, as well as the seasonal variation on their reproduction. Particular attention was paid to the light variable as it is a crucial driver of growth and physiology for seagrasses. Our guidelines provide a way to adapt a general DBN to specific ecosystems to maximize model reuse and minimize re‐development effort. Especially important from a transferability perspective are guidelines for ecosystems with limited data, and how simulation and prior predictive approaches can be used in these contexts. 相似文献