全文获取类型
收费全文 | 4665篇 |
免费 | 282篇 |
国内免费 | 892篇 |
出版年
2024年 | 4篇 |
2023年 | 63篇 |
2022年 | 106篇 |
2021年 | 121篇 |
2020年 | 101篇 |
2019年 | 159篇 |
2018年 | 199篇 |
2017年 | 117篇 |
2016年 | 141篇 |
2015年 | 144篇 |
2014年 | 380篇 |
2013年 | 437篇 |
2012年 | 273篇 |
2011年 | 257篇 |
2010年 | 249篇 |
2009年 | 271篇 |
2008年 | 298篇 |
2007年 | 348篇 |
2006年 | 243篇 |
2005年 | 272篇 |
2004年 | 207篇 |
2003年 | 192篇 |
2002年 | 181篇 |
2001年 | 138篇 |
2000年 | 117篇 |
1999年 | 103篇 |
1998年 | 88篇 |
1997年 | 62篇 |
1996年 | 72篇 |
1995年 | 61篇 |
1994年 | 60篇 |
1993年 | 45篇 |
1992年 | 28篇 |
1991年 | 40篇 |
1990年 | 33篇 |
1989年 | 31篇 |
1988年 | 18篇 |
1987年 | 20篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 25篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1972年 | 4篇 |
排序方式: 共有5839条查询结果,搜索用时 15 毫秒
71.
72.
晋陕蒙交界地区生态退化与恢复的研究现状和展望 总被引:3,自引:1,他引:2
晋陕蒙交界地区是我国生态环境破坏最严重的地区之一,当地所存在的植被退化、水土流失和土地沙化等一系列生态问题,不但威胁该地区居民的生存与发展,而且也影响我国东部地区的生态环境质量与经济发展。因此,开展针对该地区的生态恢复理论和恢复技术研究就显得非常重要。本文对该地区数十年来在生态恢复方面的理论研究和治理实践工作的历史和现状进行了分析,提出了今后应该加强对植被退化机理、植被恢复目标群落的确定和如何在大尺度地域上开展生态恢复等问题进行研究的建议,以便为更好地开展该地区的生态恢复工作提供帮助。 相似文献
73.
横断山北部高山区不同生态条件下土壤微生物数量及其与生态因子的相关性 总被引:8,自引:0,他引:8
采用全球定位系统(GPS)采集了横断山北部高山区35个样点的土样,用稀释平板法对该区细菌、真菌和放线菌进行分离计数,并分析了微生物数量与土壤养分、土壤水分、土地利用方式和pH等因子的相关性。结果表明,该区域3大类土壤微生物数量表现为细菌>放线菌>真菌。土壤微生物数量在不同区域表现为松林口>雀儿山>折多山;林地微生物比草地丰富,但不及草地微生物均衡。微生物数量与土壤理化因子间的相关分析表明,放线菌数量与pH呈显著正相关,真菌数量与土壤全钾含量呈极显著正相关。 相似文献
74.
75.
The human ventral premotor cortex overlaps, at least in part, with Broca's region in the dominant cerebral hemisphere, that is known to mediate the production of language and contributes to language comprehension. This region is constituted of Brodmann's areas 44 and 45 in the inferior frontal gyrus. We summarize the evidence that the motor related part of Broca's region is localized in the opercular portion of the inferior frontal cortex, mainly in area 44 of Brodmann. According to our own data, there seems to be a homology between Brodmann area 44 in humans and the monkey area F5. The non-language related motor functions of Broca's region comprise complex hand movements, associative sensorimotor learning and sensorimotor integration. Brodmann's area 44 is also a part of a specialized parieto-premotor network and interacts significantly with the neighbouring premotor areas. In the ventral premotor area F5 of monkeys, the so called mirror neurons have been found which discharge both when the animal performs a goal-directed hand action and when it observes another individual performing the same or a similar action. More recently, in the same area mirror neurons responding not only to the observation of mouth actions, but also to sounds characteristic to actions have been found. In humans, through an fMRI study, it has been shown that the observation of actions performed with the hand, the mouth and the foot leads to the activation of different sectors of Broca's area and premotor cortex, according to the effector involved in the observed action, following a somatotopic pattern which resembles the classical motor cortex homunculus. On the other hand the evidence is growing that human ventral premotor cortex, especially Brodmann's area 44, is involved in polymodal action processing. These results strongly support the existence of an execution-observation matching system (mirror neuron system). It has been proposed that this system is involved in polymodal action recognition and might represent a precursor of language processing. Experimental evidence in favour of this hypothesis both in the monkey and humans is shortly reviewed. 相似文献
76.
Explaining the heterogeneous distribution of biodiversity across the Earth has long been a challenge to ecologists and biogeographers. Here, we document the patterns of plant species richness for different taxonomic groups in China's nature reserves, and discuss their possible explanations at national and regional scales, using vascular plant richness data coupled with information on climate and topographical variables. We found that water deficit, energy and elevation range (a surrogate of habitat heterogeneity) represent the primary explanations for variation in plant species richness of the nature reserves across China. There are consistent relationships between species richness and climate and habitat heterogeneity for different taxonomic vascular plant groups at the national scale. Habitat heterogeneity is strongly associated with plant richness in all regions, whereas climatic constraints to plant diversity vary regionally. In the regions where energy is abundant or water is scarce, plant richness patterns were determined by water and habitat heterogeneity, whereas in the region with low energy inputs, water interacting with energy, and habitat heterogeneity determined its species richness pattern. Our results also suggest that energy variables alone do not represent the primary predictor of plant richness. 相似文献
77.
We studied avian frugivory and seed dispersal in a dioecious shrub, Rhamnus alaternus, focusing on the quantitative and qualitative components of effectiveness. The study took place at three locations in the northeast of the Iberian Peninsula, and examined bird behaviour, intensity of feeding, and the consequences for seedling emergence. The coincidence between the bird breeding season and fruit ripening of R. alaternus in the absence of other ripe fruit, generates a monospecific interaction. The extant frugivorous species were mainly legitimate seed dispersers and their abundance was low. Sylvia melanocephala and S. undata were the most important at one site whereas S. atricapilla, Erithacus rubecula and Turdus merula predominated at the other two sites. Fruit handling took place directly on the branches. Bird species used microhabitats differently as first post-feeding perch, which usually was a short distance away. The low density of frugivorous birds in all localities, among others factors, resulted in satiation of the disperser community and many mature fruits unconsumed. Both adults and juveniles feed upon the plants and their foraging patterns are similar. Adults of S. melanocephala were observed to feed fruit to nestlings and consequently a second phase of dispersal potentially arises from the transport of fecal sacs. Pulp removal and passage through the digestive tract increased the probability of seedling emergence. This plant-dispersal interaction has important consequences, both positive and negative for the plant. Positively, the fruiting of R. alaternus at a time when other ripe fruits are not available avoids interspecific competition for seed dispersers. In addition, a low density of seed rain may reduce intraspecific competition. Negatively, the low density and small size of the breeding frugivorous bird community limit fruit handling and removal away from the parent plants, while the territorial behaviour of birds at that time of the year reduces the potential distances of seed dispersal. 相似文献
78.
79.
80.