收费全文 | 64503篇 |
免费 | 4225篇 |
国内免费 | 5篇 |
2023年 | 464篇 |
2022年 | 453篇 |
2021年 | 944篇 |
2020年 | 848篇 |
2019年 | 878篇 |
2018年 | 1990篇 |
2017年 | 1854篇 |
2016年 | 2415篇 |
2015年 | 3213篇 |
2014年 | 3170篇 |
2013年 | 4276篇 |
2012年 | 5125篇 |
2011年 | 4567篇 |
2010年 | 2824篇 |
2009年 | 2240篇 |
2008年 | 3536篇 |
2007年 | 3329篇 |
2006年 | 3150篇 |
2005年 | 2690篇 |
2004年 | 2633篇 |
2003年 | 2356篇 |
2002年 | 2160篇 |
2001年 | 1473篇 |
2000年 | 1446篇 |
1999年 | 1131篇 |
1998年 | 490篇 |
1997年 | 361篇 |
1996年 | 366篇 |
1995年 | 331篇 |
1994年 | 264篇 |
1993年 | 245篇 |
1992年 | 572篇 |
1991年 | 483篇 |
1990年 | 439篇 |
1989年 | 433篇 |
1988年 | 429篇 |
1987年 | 373篇 |
1986年 | 364篇 |
1985年 | 350篇 |
1984年 | 348篇 |
1983年 | 242篇 |
1982年 | 227篇 |
1981年 | 174篇 |
1980年 | 178篇 |
1979年 | 216篇 |
1978年 | 209篇 |
1975年 | 196篇 |
1974年 | 218篇 |
1973年 | 222篇 |
1972年 | 186篇 |
Myrtaceae, Lauraceae and Fabaceae are regarded as essential floristic elements of Atlantic forests due to their outstanding species richness, endemism levels, and ecological functions. Nonetheless, Atlantic forests are being subjected to multiple human disturbances that compromise the conservation of their flora. This study, therefore, intended to address whether there exist potential areas for conservation with great richness of tree/shrub species of the aforementioned families in different forest types within the subtropical Atlantic Forest. For this, data collected systematically across?~?23% of the subtropical Brazilian Atlantic Forest were employed. The univariate Local Moran I statistic was used to search for clusters of sample plots with great richness of species of the studied families. Six clusters were found throughout the evergreen rainforest (ERF) and Araucaria forest (AF), and most of them contained more than half of all species of these families observed on the sample plots, besides many others belonging to different families. A cluster of Myrtaceae and a cluster of Lauraceae in the ERF were the only ones that overlapped protected areas. The clusters of Lauraceae in the AF, located in ecotone zones with the ERF, had?~?50% of native forest cover, whereas the clusters of Myrtaceae and Fabaceae had?~?10% of forest cover. Inasmuch as forests in the study area have been heavily exploited, the clusters have relevant conservation value. Protected areas could be expanded or converted into more restrictive conservation categories to enhance the conservation of populations of key elements of the Atlantic Forest. Yet, non-protected areas deserve attention regarding the management of forest resources and conservation-by-use strategies.
相似文献Location Dry vegetation enclaves within the limits of the Cerrado domain in eastern Brazil.
Methods We determined the mitochondrial DNA haplotypes of 55 individuals (representing 12 populations) based on sequence data of a 483-bp fragment from the cytochrome c oxidase subunit II (COII) gene. Phylogenetic and coalescent analyses were used to test for the occurrence of demographic events and to infer the time of divergence amongst genetically independent groups.
Results Our analyses revealed the existence of two divergent subclades (G1 and G2) plus an introgressed clade restricted to the southernmost range of D. gouveai . Subclades G1 and G2 displayed genetic footprints of range expansion and segregated geographical distributions in south-eastern and some central highland regions, east and west of the Paraná River valley. Molecular dating indicated that the main demographic and diversification events occurred in the late to middle Pleistocene.
Main conclusions The phylogeographical and genetic patterns observed for D. gouveai in this study are consistent with changes in the distribution of dry vegetation in eastern Brazil. All of the estimates obtained by molecular dating indicate that range expansion and isolation pre-dated the Last Glacial Maximum, occurring during the late to middle Pleistocene, and were probably triggered by climatic changes during the Pleistocene. The current patchy geographical distribution and population subdivision in D. gouveai is apparently closely linked to these past events. 相似文献
The littoral zone of shallow water bodies in the Czech Republic has been studied quite consistently at several fishponds. The use of algae, especially diatoms, for the monitoring of the state of lotic freshwater also has a long tradition. The main objective of the presented paper is to validate the feasibility of the use of littoral periphyton comunities for the biomonitoring of standing waters. At the investigated sites, littoral periphytic diatoms were studied together with selected enviromental variables (pH, conductivity, nutrients – especially total phosphorus) on three types of natural substrates (epilithon, epiphyton, epipelon). The evaluation of the diatom community was performed on the basis of the checklists of algal indicator species published by authors from the Czech Republic, Austria and the Netherlands. The data were subjected to statistical software NCCS 2000 (GLM Anova and ``Ward's minimum'' variance cluster analysis). Littoral periphytic diatoms appear to be good indicators of the fishpond water quality. The selected substrates show non-significant differences therefore the average values from all substrates were used. The best indicatory system for evaluation of Czech fishponds was van Dam's index.
相似文献