全文获取类型
收费全文 | 5003篇 |
免费 | 816篇 |
国内免费 | 1346篇 |
出版年
2024年 | 33篇 |
2023年 | 174篇 |
2022年 | 153篇 |
2021年 | 239篇 |
2020年 | 303篇 |
2019年 | 344篇 |
2018年 | 280篇 |
2017年 | 357篇 |
2016年 | 356篇 |
2015年 | 299篇 |
2014年 | 299篇 |
2013年 | 369篇 |
2012年 | 238篇 |
2011年 | 289篇 |
2010年 | 251篇 |
2009年 | 330篇 |
2008年 | 322篇 |
2007年 | 340篇 |
2006年 | 265篇 |
2005年 | 269篇 |
2004年 | 206篇 |
2003年 | 196篇 |
2002年 | 167篇 |
2001年 | 141篇 |
2000年 | 114篇 |
1999年 | 106篇 |
1998年 | 93篇 |
1997年 | 89篇 |
1996年 | 57篇 |
1995年 | 51篇 |
1994年 | 51篇 |
1993年 | 37篇 |
1992年 | 36篇 |
1991年 | 38篇 |
1990年 | 39篇 |
1989年 | 17篇 |
1988年 | 26篇 |
1987年 | 33篇 |
1986年 | 15篇 |
1985年 | 29篇 |
1984年 | 15篇 |
1983年 | 7篇 |
1982年 | 19篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1958年 | 4篇 |
排序方式: 共有7165条查询结果,搜索用时 600 毫秒
991.
Denture-related stomatitis managed with tissue conditioner and hard autopolymerising reline material
doi: 10.1111/j.1741–2358.2010.00398.x Denture related stomatitis managed with tissue conditioner and hard autopolymerising reline material. Objective: To compare the response of denture‐related stomatitis (DS) under management with a tissue conditioner (TC) and autopolymerising hard reline material (AHRM). Background data: Denture‐related stomatitis affects up to 75% of denture wearers; not wearing the denture at night, using TC or prescribing topical or systemic antifungal agents could reduce its incidence. Materials and methods: This was a double‐blind study consisting of 44 participants with DS who wear denture; they were randomly divided into two unmatched groups according to the material used for the management of DS. The TC was replaced weekly, and the AHRM was placed at the beginning of the study and was not changed for 4 weeks. A dentist performed an initial and a weekly clinical diagnosis for DS; the clinical situation was recorded by means of photographs for each week. Results: Both TC and AHRM were effective in the management of DS. Significant differences were found in the DS resolution time (p < 0.001), taking longer for the TC. Conclusions: Both the tissue conditioner and AHRM are effective for the management of DS, but AHRM requires less time for recovery and as a result fewer appointments are required for the patient. 相似文献
992.
Trevors JT 《Microbiological research》2003,158(1):1-6
DNA and RNA are nucleic acids that cells and viruses use to produce copies of themselves. However, there is an immense paucity of knowledge on how these nucleic acids originated and changed as early bacteria became capable of growth and cell division. One possibility is that parallel evolution of the genetic code and protein synthesis was required for assembly of the first cells capable of growth and division. It is also possible that DNA-RNA duplices were intermediate genetic material in the early assembly of the first cells. These ideas will be discussed as well as other aspects of the assembly of the first cells on the Earth. 相似文献
993.
The use of the photochemical reflectance index (PRI) as a promising proxy of light use efficiency (LUE) has been extensively studied, and some issues have been identified, notably the sensitivity of PRI to leaf pigment composition and the variability in PRI response to LUE because of stress. In this study, we introduce a method that enables us to track the short‐term PRI response to LUE changes because of photosynthetically active radiation (PAR) changes. The analysis of these short‐term relationships between PRI and LUE throughout the growing season in two species (Quercus robur L. and Fagus sylvatica L.) under two different soil water statuses showed a clear change in PRI response to LUE, which is related to leaf pigment content. The use of an estimated or approximated PRI0, defined as the PRI of perfectly dark‐adapted leaves, allowed us to separate the PRI variability due to leaf pigment content changes and the physiologically related PRI variability over both daily (PAR‐related) and seasonal (soil water content‐related) scales. The corrected PRI obtained by subtracting PRI0 from the PRI measurements showed a good correlation with the LUE over both of the species, soil water statuses and over the entire growing season. 相似文献
994.
995.
996.
We may expect butterflies as ectotherms to have particularly active life‐history stages that occur in the warmest and lightest times of the year; however, there are temperate species that are active when climatic conditions seem unfavourable and photoperiod short, such as the Taylor's checkerspot (Euphydryas editha taylori). For such species, studies suggest that even subtle changes to microclimate can potentially impact populations. Thus, understanding how in situ variations in microclimate influence the Taylor's checkerspot butterfly could provide much needed insights into more effective management. We conducted a series of surveys that explored (i) adult habitat use, (ii) final instar larval distribution and (iii) adult movement up to and across site boundaries at two sites in Oregon, USA, in 2010 and 2011. We found that in situ habitat use by the Taylor's checkerspot butterfly was strongly influenced by microclimate. Both adult activities and final instar larvae distribution were clustered within the warmest areas of the sites. Moreover, adults did not use up to 59% and larva up to 90% of their sites, despite vegetation structure and composition being uniform. More specifically, butterfly habitat use increased with increasing ground temperatures, and we found that areas with the highest ground temperatures were more exposed to direct sunlight. Similarly, we found that butterflies tended to only move through sunlit site boundaries. We conclude that the Taylor's checkerspot is sensitive to changes in its thermal environment at fine spatial scales. Our results highlight the importance of microclimate as an indicator of habitat quality, and establishing the thermal criteria in which species of concern exists may provide valuable insights into the implications of climate change. 相似文献
997.
To understand the mechanisms behind the diversification of herbivorous insects through insect–plant interactions, it is important to know how the insects change their diet breadth in response to environmental changes. In this study, we investigated the phylogeographical pattern of the leaf beetle Agelasa nigriceps to infer the evolutionary history of its host range. While this beetle commonly uses Actinidia arguta (Actinidiaceae) as a host plant, it has been recorded recently on Pterostyrax hispidus (Styracaceae), which is now increasing in abundance at some localities in Japan due to the indirect effects of high population size of a mammalian herbivore. Considerable variation among populations in the ability of Ag. nigriceps to use P. hispidus suggests that P. hispidus is a newly acquired host plant for this beetle. Phylogenetic analyses using mitochondrial DNA sequences and amplified fragment length polymorphism (AFLP) revealed a high degree of phylogeographical structure in Ag. nigriceps throughout Japan, which is consistent with the hypothesis that several glacial refugia existed in the Japanese archipelago. In contrast, no genetic structure associated with the host plants was detected. Both the mitochondrial DNA and AFLP analyses showed that populations that can use P. hispidus are polyphyletic. These results and geographical variation in host use suggest that the host range expansion to a novel host, P. hispidus, is a very recent and possibly ongoing phenomenon and has occurred independently in several regions. Our study illustrates that the host range of herbivorous insects can evolve repeatedly in response to similar environmental changes. 相似文献
998.
Michael Krützen Sina Kreicker Colin D. MacLeod Jennifer Learmonth Anna M. Kopps Pamela Walsham Simon J. Allen 《Proceedings. Biological sciences / The Royal Society》2014,281(1784)
Culturally transmitted tool use has important ecological and evolutionary consequences and has been proposed as a significant driver of human evolution. Such evidence is still scarce in other animals. In cetaceans, tool use has been inferred using indirect evidence in one population of Indo-Pacific bottlenose dolphins (Tursiops sp.), where particular dolphins (‘spongers’) use marine sponges during foraging. To date, evidence of whether this foraging tactic actually provides access to novel food items is lacking. We used fatty acid (FA) signature analysis to identify dietary differences between spongers and non-spongers, analysing data from 11 spongers and 27 non-spongers from two different study sites. Both univariate and multivariate analyses revealed significant differences in FA profiles between spongers and non-spongers between and within study sites. Moreover, FA profiles differed significantly between spongers and non-spongers foraging within the same deep channel habitat, whereas the profiles of non-spongers from deep channel and shallow habitats at this site could not be distinguished. Our results indicate that sponge use by bottlenose dolphins is linked to significant differences in diet. It appears that cultural transmission of tool use in dolphins, as in humans, allows the exploitation of an otherwise unused niche. 相似文献
999.
1000.
不同耕作措施对冬小麦-夏玉米复种连作系统土壤有机碳和水分利用效率的影响 总被引:2,自引:1,他引:1
在连续8年田间定位试验的基础上,分析了关中平原冬小麦 夏玉米复种连作系统2008—2009年连续两个生长季期间不同耕作措施(结合秸秆还田和不还田)对土壤有机碳和水分利用率的影响.结果表明: 相对于传统耕作,保护性耕作有利于土壤有机碳、水分利用效率和作物产量的提高,其中在“深松+秸秆还田”耕作模式下的增幅最高,土壤有机碳含量在0~30 cm土层增幅达到19.5%,水分利用效率和作物产量提高了16.9%和20.5%,而免耕模式则有效提高了0~10 cm土层有机碳含量.在该地区土壤和气候条件下,深松结合秸秆粉碎还田是最理想的耕作模式,最有利于土壤有机碳累积,并提高水分利用效率和作物产量. 相似文献