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   1241篇
  免费   129篇
  国内免费   91篇
  2024年   5篇
  2023年   34篇
  2022年   24篇
  2021年   25篇
  2020年   30篇
  2019年   58篇
  2018年   46篇
  2017年   56篇
  2016年   53篇
  2015年   39篇
  2014年   66篇
  2013年   83篇
  2012年   41篇
  2011年   52篇
  2010年   41篇
  2009年   68篇
  2008年   77篇
  2007年   75篇
  2006年   78篇
  2005年   63篇
  2004年   43篇
  2003年   45篇
  2002年   34篇
  2001年   34篇
  2000年   26篇
  1999年   23篇
  1998年   17篇
  1997年   24篇
  1996年   20篇
  1995年   12篇
  1994年   16篇
  1993年   8篇
  1992年   16篇
  1991年   13篇
  1990年   17篇
  1989年   8篇
  1988年   5篇
  1987年   6篇
  1986年   7篇
  1985年   16篇
  1984年   13篇
  1983年   8篇
  1982年   10篇
  1981年   7篇
  1980年   4篇
  1977年   2篇
  1976年   2篇
  1975年   3篇
  1974年   2篇
  1973年   3篇
排序方式: 共有1461条查询结果,搜索用时 31 毫秒
871.
1. Growth rates of immature invertebrates can be of profound importance to subsequent reproductive success in the adult stage. If diet selection in herbivores is a principal means of maximising growth and rate of development, individuals should select food of the highest quality. 2. If disparities are found between performance and preference, these may be the result of individuals balancing nutritional gain against other demands on their behaviour, such as a need for shelter. The responses of the crane‐fly herbivore Tipula montana Curtis (Diptera: Tipulidae) to a range of moss genera from an upland environment were investigated to determine whether diet selection was determined by food quality alone, as measured by its effect on larval growth and survival. 3. Larvae were reared individually on single moss genus diets, and their growth, development, and survival were measured. Each of the mosses supported growth and development although the mean weights achieved differed by a factor of two between food genera. In two‐choice preference experiments, individuals broadly chose bryophyte foods as expected from their quality. One notable exception was the moss Pleurozium schreberi, which gave the best growth performance but was among the least preferred. 4. The two methods of assessing preference gave different results: observations of larvae showed relatively greater selection for dense mosses than faecal pellet analysis. A strong dietary preference for an angiosperm, the sedge Carex bigelowii, over all mosses supported the interpretation that crane‐fly larvae benefit from bryophytes as a refuge, and that this factor may override dietary quality.  相似文献   
872.
Summary   Volunteers play an important role in recovery efforts on threatened Australian birds. We surveyed the demography, motivation and preferences of people registered with the Threatened Bird Network, a network dedicated to placing volunteers in recovery efforts. Most were employed or studying and were members of conservation or natural history groups. The main reason for volunteering was an interest in conservation, and most volunteers considered habitat conservation of primary importance. They preferred outdoor projects on highly threatened species and where organizers set clear goals, provided feedback and supervised in a friendly and helpful manner. We also surveyed the recovery projects to determine the extent of volunteer involvement required, and the expectations recovery teams had of volunteers. Volunteers made a major contribution to recovery efforts, with $3.4 million being directed to 32 recovery efforts in the period 1996−2000 − a contribution that has been increasing in recent years. Most activities involved surveying and searching and most activities were available in spring, which coincides with the breeding season of the taxa involved. Volunteers travelled considerable distances, yet only a few projects provided financial support.  相似文献   
873.
An electroencephalographic study of the brain activity in the wakefulness-sleep cycle was carried out on rats of Krushinskii-Molodkina line (KM) with hereditary predisposition to audiogenic convulsions and on Wistar rats that were insensitive to the convulsiogenic sound effect, but with epileptiform manifestations appearing on the background of cadmium intoxication and administration of kainic acid into the caudate nucleus head. There were revealed several EEG patterns whose presence was an indicator of formation of disorders of the CNS activity of the paroxysmal character in the animals. It has been established that in the phase of the rat rapid-wave sleep, a high representation of episodes with predominance of α-diapason EEG oscillations can be considered a specific non-paroxysmal abnormality due to the presence of convulsive syndrome in these animals. There was shown a long steady decrease of sensitivity of KM rats to the convulsiogenic sound effect, which appeared after multiple audiogenic generalized tonicoclonic convulsive attacks, correlated with a decrease of the degree of ?-diapason oscillations and with an increase of representation of α-diapason waves on EEG in the state of the animal quiet wakefulness. The role of disintegration in activity of the ascending activating brain systems in the animal and human paroxysmal syndromes is discussed.  相似文献   
874.
Atopic eczema (AE) is a chronic inflammatory disease in which genetic predisposition and environmental factors such as microorganisms contribute to the symptoms. The yeast Malassezia Sympodialis, part of the normal human cutaneous flora, can act as an allergen eliciting specific IgE and T-cell reactivity in patients with AE. The major M. sympodialis allergen Mala s 1 is localized mainly in the yeast cell wall and exposed on the cell surface. Interestingly, Mala s 1 does not exhibit any significant sequence homology to known proteins. Here we present the crystal structure of Mala s 1 determined by single-wavelength anomalous dispersion techniques using selenomethionine-substituted Mala s 1. Mala s 1 folds into a 6-fold beta-propeller, a novel fold among allergens. The putative active site of Mala s 1 overlaps structurally to putative active sites in potential homologues, Q4P4P8 and Tri 14, from the plant parasites Ustilago maydis and Gibberella zeae, respectively. This resemblance suggests that Mala s 1 and the parasite proteins may have similar functions. In addition, we show that Mala s 1 binds to the phosphoinositides (PI) PI(3)P, PI(4)P, and PI(5)P, lipids possibly playing a role in the localization of Mala s 1 to the cell surface. The crystal structure of Mala s 1 will provide insights into the role of this major allergen in the host-microbe interactions and induction of an allergic response in AE.  相似文献   
875.
d-Glutamic acid is a required biosynthetic building block for peptidoglycan, and the enzyme glutamate racemase (GluR) catalyzes the inter-conversion of D and L-glutamate enantiomers. Therefore, GluR is considered as an attractive target for the design of new antibacterial drugs. Here, we report the crystal structures of GluR from Streptococcus pyogenes in both inhibitor-free and inhibitor-bound forms. The inhibitor free GluR crystallized in two different forms, which diffracted to 2.25 Å and 2.5 Å resolution, while the inhibitor-bound crystal diffracted to 2.5 Å resolution. GluR is composed of two domains of α/β protein that are related by pseudo-2-fold symmetry and the active site is located at the domain interface. The inhibitor, γ-2-naphthylmethyl-d-glutamate, which was reported earlier as a novel potent competitive inhibitor, makes several hydrogen bonds with protein atoms, and the naphthyl moiety is located in the hydrophobic pocket. The inhibitor binding induces a disorder in one of the loops near the active site. In both crystal forms, GluR exists as a dimer and the interactions seen at the dimer interface are almost identical. This agrees well with the results from gel filtration and dynamic light-scattering studies.  相似文献   
876.
877.
Acclimation of hyperaccumulators to heavy metal-induced stress is crucial for phytoremediation and was investigated using the hyperaccumulator Thlaspi caerulescens and the nonaccumulators T. fendleri and T. ochroleucum. Spatially and spectrally resolved kinetics of in vivo absorbance and fluorescence were measured with a novel fluorescence kinetic microscope. At the beginning of growth on cadmium (Cd), all species suffered from toxicity, but T. caerulescens subsequently recovered completely. During stress, a few mesophyll cells in T. caerulescens became more inhibited and accumulated more Cd than the majority; this heterogeneity disappeared during acclimation. Chlorophyll fluorescence parameters related to photochemistry were more strongly affected by Cd stress than nonphotochemical parameters, and only photochemistry showed acclimation. Cd acclimation in T. caerulescens shows that part of its Cd tolerance is inducible and involves transient physiological heterogeneity as an emergency defence mechanism. Differential effects of Cd stress on photochemical vs nonphotochemical parameters indicate that Cd inhibits the photosynthetic light reactions more than the Calvin-Benson cycle. Differential spectral distribution of Cd effects on photochemical vs nonphotochemical quenching shows that Cd inhibits at least two different targets in/around photosystem II (PSII). Spectrally homogeneous maximal PSII efficiency (F(v)/F(m)) suggests that in healthy T. caerulescens all chlorophylls fluorescing at room temperature are PSII-associated.  相似文献   
878.
The conservation status of all the amphibians in China is analyzed,and the country is shown to be a global priority for conservation in comparison to many other countries of the world.Three Chinese regions are particularly rich in amphibian diversity:Hengduan,Nanling,and Wuyi mountains.Sala-manders are more threatened than frogs and toads.Several smaller families show a high propensity to become seriously threatened:Bombinatoridae,Cryptobranchidae,Hynobiidae and Salamandridae.Like other parts of the world,stream-breeding,high-elevation forest amphibians have a much higher likeli-hood of being seriously threatened.Habitat loss,pollution,and over-harvesting are the most serious threats to Chinese amphibians.Over-harvesting is a less pervasive threat than habitat loss,but it is more likely to drive a species into rapid decline.Five conservation challenges are mentioned with recommendations for the highest priority research and conservation actions.  相似文献   
879.
The extent to which welfare needs of breeding dogs are met in commercial dog-breeding kennels is a potential point of controversy. This analysis sought to understand US residents’ perceptions and priorities related to dog welfare : by investigating (a) perceptions of breeding-dog welfare needs and (b) perceptions of various nonhuman animal welfare information sources. Using best/worst-choice experiments conducted in an online survey, respondents’ choices for most and least important breeding-dog welfare needs (n = 508) and most/least trusted canine welfare information sources (n = 508) were analyzed. The survey sample was targeted to be representative of the US population in terms of gender, age, region of residence, income, and education. The largest preference shares (relatively most important) for breeding-dog welfare needs were for “availability of food and water” (39.2%) and health/veterinary care (18.1%). The largest preference shares (relatively most trusted sources) for welfare information were American Society for the Prevention of Cruelty to Animals, a veterinarian, and American Veterinary Medical Association, with 25.1%, 16.4%, and 14.1% shares, respectively.  相似文献   
880.
The United Nations (UN) Convention on the Rights of Persons with Disabilities (CRPD) is the most up‐to‐date international legal instrument concerning the rights of persons with disabilities. Such persons are taken to include those with serious mental disorders. According to an authoritative interpretation of a crucial Article (Article 12 ‐ Equal recognition before the law) by the UN CRPD Committee, involuntary detention and treatment of people with mental health disabilities are prohibited under the Convention. Both conventional mental health law and “capacity‐based” law are deemed to violate the Convention. However, some other UN bodies are not in full agreement (for example, the UN Human Rights Committee and the Subcommittee on Prevention of Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment), while others are less explicitly absolutist (for example, the Human Rights Council). Furthermore, strong criticisms of the position of the CRPD Committee have been mounted from a number of academic quarters. These criticisms center on whether the role of a person's ability to make a decision can be ignored, no matter the circumstances. Much of the above debate turns on the concept of “legal capacity” and the now often‐repeated precept that one must always respect the “will and preferences” of the person with a disability. However, “will and preferences” remains undefined. In this paper, I offer an analysis of “will and preferences” that can clarify interventions that may be acceptable or non‐acceptable under the terms of the UN Convention.  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