全文获取类型
收费全文 | 71929篇 |
免费 | 4096篇 |
国内免费 | 41篇 |
专业分类
林业 | 3365篇 |
农学 | 2590篇 |
基础科学 | 483篇 |
8998篇 | |
综合类 | 11474篇 |
农作物 | 2667篇 |
水产渔业 | 3726篇 |
畜牧兽医 | 36983篇 |
园艺 | 935篇 |
植物保护 | 4845篇 |
出版年
2018年 | 1139篇 |
2017年 | 1203篇 |
2016年 | 1139篇 |
2015年 | 959篇 |
2014年 | 1192篇 |
2013年 | 2708篇 |
2012年 | 2128篇 |
2011年 | 2560篇 |
2010年 | 1727篇 |
2009年 | 1782篇 |
2008年 | 2630篇 |
2007年 | 2416篇 |
2006年 | 2366篇 |
2005年 | 2171篇 |
2004年 | 2222篇 |
2003年 | 2196篇 |
2002年 | 2063篇 |
2001年 | 2384篇 |
2000年 | 2386篇 |
1999年 | 1934篇 |
1998年 | 921篇 |
1997年 | 831篇 |
1996年 | 788篇 |
1995年 | 869篇 |
1994年 | 846篇 |
1993年 | 772篇 |
1992年 | 1525篇 |
1991年 | 1545篇 |
1990年 | 1668篇 |
1989年 | 1511篇 |
1988年 | 1450篇 |
1987年 | 1340篇 |
1986年 | 1386篇 |
1985年 | 1336篇 |
1984年 | 1130篇 |
1983年 | 1026篇 |
1982年 | 630篇 |
1979年 | 1004篇 |
1978年 | 797篇 |
1977年 | 719篇 |
1976年 | 682篇 |
1975年 | 722篇 |
1974年 | 766篇 |
1973年 | 835篇 |
1972年 | 804篇 |
1971年 | 751篇 |
1970年 | 730篇 |
1969年 | 745篇 |
1967年 | 659篇 |
1966年 | 634篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Previous experiments demonstrated that treatment of longan trees with potassium chlorate (KClO3) induces “off season floral induction” (FI) even in the absence of the naturally required cool temperature [Manochai, P., Sruamsiri, P., Wiriya-alongkorn, W., Naphrom, D., Hegele, M., Bangerth, F., 2005. Year around off season flower induction in longan (Dimocarpus longan, Lour.) trees by KClO3 applications: potentials and problems. Sci. Hortic. 104, 379–390]. Potassium chlorate, however, cannot replace the presence of functional mature leaves and sufficient light intensity. Here we examined in more detail the effect of shade (about 10% of natural sunlight) on KClO3 affected hormone concentrations/transport of leaves and shoot apical buds (SAB) and their interactions with FI. 相似文献
992.
- 1. River lamprey (Lampetra fluviatilis) and sea lamprey (Petromyzon marinus) are designated features of the River Derwent Special Area of Conservation (SAC) and the Humber Estuary (a possible SAC). This study determined the condition of lamprey populations in the Yorkshire Ouse catchment by assessing the species composition, distribution, abundance and size‐structure of larval (ammocoete) populations in five major tributary rivers (Derwent, Swale, Ure, Nidd and Wharfe).
- 2. According to EU condition assessment criteria, Lampetra populations (assumed to be mostly river lamprey) are at present in favourable condition, with site mean (±SE) densities ranging from 2.7 (±1.2) to 160.3 (±50.5) individuals m?2 (all river means >2 individuals m?2), and at least two size (≈age) classes present in optimal microhabitats. By contrast, no sea lamprey larvae were recorded, suggesting that populations of this species are in unfavourable condition.
- 3. Actions to protect and enhance nationally or internationally important stocks must be implemented from at least a catchment perspective, because many of the issues affecting such species are not localized. With respect to lampreys, particular attention should be given to protecting spawning and nursery habitats, improving water quality, reducing impingement at abstraction points, preventing exploitation at spawning grounds and increasing passage at potential physical obstructions.
993.
James R. Guest Peter A. Todd Eugene Goh Balasubramaniam S. Sivalonganathan Konda P. Reddy 《水产资源保护:海洋与淡水生态系统》2008,18(5):570-579
- 1. Giant clams have been a sustainable resource for millennia, but unregulated harvesting has led to local extinctions within the Indo‐Pacific region. Giant clam mariculture can produce large numbers of juveniles for restocking wild populations where natural recruitment is low or absent.
- 2. Singapore is surrounded by more than 60 small islands, many with fringing reefs. These reefs, however, experience increased turbidity and sedimentation resulting from massive coastal development projects and regular dredging of shipping lanes.
- 3. Seven reefs off Singapore's southern islands were surveyed (9670 m2) for giant clams. Also, an experiment was conducted to determine the growth of Tridacna squamosa reared in aquaria under three light treatments: ~50% ambient photosynthetically active radiation (PAR); ~25% ambient PAR; and ~12% ambient PAR. Finally, 144 clams (T. squamosa) were transplanted to four reefs around Singapore to study survival and growth in a heavily impacted environment.
- 4. A total of 23 adult clams from three species were found during the survey, representing a mean density of 0.24 per 100 m2. Most clams were found at Raffles Lighthouse, Singapore's ‘best’ reef. No juvenile clams were encountered. In the aquarium experiment, clam growth was significantly different among the three light treatments, with growth greatest in the ~50% ambient PAR treatment. Of the 144 transplanted clams, 116 (80.6%) were recovered after 7 months. All specimens had increased in size, with growth rates among reefs ranging from 3.3 mm month?1 (SD=1.3 mm) to 4.8 mm month?1 (SD=1.6 mm).
- 5. Results suggest that, despite high levels of sedimentation and turbidity on Singapore's reefs, giant clams can survive and grow well. Restocking efforts using maricultured clams may be effective in enhancing the dwindling local populations. It is not clear, however, whether a self‐sustaining community can be established as high sedimentation may hinder larval settlement and survival.
994.
- 1. To conserve biodiversity in a human‐dominated landscape, a science‐based inventory and monitoring plan is needed that quantifies existing resources, isolates drivers that maintain natural communities, determines harmful stressors, and links ecological drivers and human stressors. A tactical approach is proposed for conservation planning using freshwater fish at the Cape Cod National Seashore.
- 2. Freshwater fish are well studied and occur across environmental gradients. The lentic systems at the national park are relatively pristine yet are enveloped by a region of high population density. Using fish community data, three steps were taken for tracking anthropogenic impacts in a human dominated landscape. First, fish and potential drivers were sampled intensively along a gradient to determine which fish metrics reflect natural communities and which abiotic and biotic factors structure them. Second, emerging and existing regional human threats were identified. Third, these human threats were linked to the potential drivers that maintain natural communities to identify the most informative metrics to monitor and track change.
- 3. Fish communities, water quality, habitat, and food resources were sampled concurrently in 18 ponds in 1999 and 2000. Three common fish species explained 98% of variation in numbers across systems. Based on ecological relationships, pH, depth, vegetation, prey, and community complexity were determined to maintain biodiversity of freshwater fish communities.
- 4. The primary human threats here included: development‐related, land‐use changes; non‐point source pollution; eutrophication from septic systems; and introduced species that are a byproduct of high human visitation. These are common threats in many rapidly urbanized areas and are likely to have relevance to many sites.
- 5. To track the impact of emerging threats to freshwater ponds related to increased human population, monitoring changes in water quality, vegetated habitat, fish diversity, and trophic interactions are recommended.
995.
S. CHATZIFOTIS F. KOKOU K. AMPATZIS I.E. PAPADAKIS P. DIVANACH C.R. DERMON 《Aquaculture Nutrition》2008,14(5):405-415
This study aims at examining the effect of caffeine administration on growth, feed efficiency, and consumption of sea bream (Sparus aurata), reared in winter temperatures. Moreover, it is questioned whether caffeine has a central action in the brain and its effects are partly mediated via central brain mechanisms. For this, we studied the influences of caffeine treatment on the cerebral pattern of the cholinergic neurotransmission and the novel neuromodulator nitric oxide (NO), by means of acetyl‐cholinesterase (AchE) and nitric oxide synthase (NOS) histochemistry. Five different diets containing 0.0, 0.1, 1.0, 2.0 and 5.0 g caffeine kg?1 of diet were administrated to five groups of fish. Caffeine adversely affected sea‐bream growth at a concentration higher than 1 g kg?1 diet and increased feed conversion ratio in the treatments of 2 and 5 g kg?1 (P < 0.05). The daily consumption of feeds was similar to all groups, indicating that caffeine did not influence diet palatability. AChE‐ and NADPH‐diaphorase histochemistry showed densely labeled cells and fibers mainly in dorsal telencephalon, preoptic, pretectal, hypothalamic areas, optic tectum, reticular formation, cerebellum and motor nuclei. When compared with matched caffeine‐treated animals, no differences in the histochemical pattern and cell densities of cerebral AChE and NADPH‐diaphorase were found. 相似文献
996.
C. FERNANDES da SILVA E. BALLESTER J. MONSERRAT L. GERACITANO W. WASIELESKY JR P.C. ABREU 《Aquaculture Nutrition》2008,14(6):507-514
The nutritional quality of biofilm, a microbial community associated to an organic matrix, was evaluated in artificial substrate (polyethylene screen) in net cages during 30 days in the Patos Lagoon estuary, Southern Brazil. During this period, samples of biofilm were collected each 5 days for analysis of chlorophyll a, microorganisms abundance, dry weight, protein and lipid contents. During the study, chlorophyll a varied from 0.38 to 2.75 μg cm?2; dry weight between 7.16 and 17.63 mg cm?2; protein content from 0.43 to 1.76 mg cm?2 and lipid concentration between 1.21 and 4.23 mg cm?2. The variation of lipid in the biofilm was closely related to the abundance of free heterotrophic bacteria (34.25–56.54 × 106 cells cm?2), filamentous cyanobacteria (7.5–15.9 × 106 filaments cm?2), flagellates (6.92–12.89 × 106 cells cm?2) and mainly nematodes (29–1,414 organisms cm?2), while protein content varied similarly to the abundance of unicellular centric diatoms (52.10–179.81 × 103 cells cm?2), and nematodes. This information will allow a better management of food supply to raised aquatic organism with the utilization of natural productivity in the culture systems, with considerable decrease in production costs. 相似文献
997.
A.C.B. OLIVEIRA L.A. MARTINELLI M.Z. MOREIRA & J.E.P. CYRINO 《Aquaculture Nutrition》2008,14(1):10-13
Estimation of the apparent digestibility coefficient ADC(%) of C3 and C4 plants in experimental diets for fingerlings of tambaqui (Colossoma macropomum, Cuvier, 1818) was calculated by applying chromic oxide (Cr2O3) external marker methodology and by a proposed mathematical expression based on the isotopic composition (δ13C). A total of 240 tambaqui fingerlings each weighing ±48.2 g and measuring ±9.8 cm were maintained in eight 500‐L aquariums specially designed for faeces collection. The ADC(%) of the C3 and C4 diets did not differ significantly between the two methods, producing results of 75.6%; 76.2% and 74.4%; 72.8%, respectively. The ADC(%) results obtained by isotopic method presented less variation than by chromic oxide. The proposed mathematical expression for calculating the ADC(%) based on δ13C values offers an alternative methodology, which can reduce errors and diminish the effort required to collect biological material. However, it is important to note that this method is limited to analysis of diets or food items with distinct isotopic signals. 相似文献
998.
Using the fish silage to partially replace proteic feedstuff in aquafeeds is an alternative to mitigate sanitary and environmental problems caused by the lack of adequate destination for fisheries residues. It would also lower feed costs, consequently improving fish culture profitability. However, using fish silages in aquafeeds depends on determination of its apparent digestibility coefficients (ADC). This work aimed to determining the ADC of crude protein and amino acids of acid silage (AS), biological silage (BS) and enzymatic silage (ES) for juvenile Nile tilapia (94.5 ± 12.7 g). The ADCCP was: 92.0%, 89.1% and 93.7% for AS, BS and SE respectively. The average ADC of amino acids was: 91.8%, 90.8% and 94.6% for AS, BS and ES respectively. Results encourage the use of AS, BS and ES to partially replace protein sources in balanced diets for neotropical fish. 相似文献
999.
P. James K. Tzoulas M.D. Adams A. Barber J. Box J. Breuste T. Elmqvist M. Frith C. Gordon K.L. Greening J. Handley S. Haworth A.E. Kazmierczak M. Johnston K. Korpela M. Moretti J. Niemelä S. Pauleit M.H. Roe J.P. Sadler C. Ward Thompson 《Urban Forestry & Urban Greening》2009,8(2):65-75
In recent years social, economic and environmental considerations have led to a reevaluation of the factors that contribute to sustainable urban environments. Increasingly, urban green space is seen as an integral part of cities providing a range of services to both the people and the wildlife living in urban areas. With this recognition and resulting from the simultaneous provision of different services, there is a real need to identify a research framework in which to develop multidisciplinary and interdisciplinary research on urban green space. In order to address these needs, an iterative process based on the delphi technique was developed, which comprised email-mediated discussions and a two-day symposium involving experts from various disciplines. The two outputs of this iterative process were (i) an integrated framework for multidisciplinary and interdisciplinary research and (ii) a catalogue of key research questions in urban green space research. The integrated framework presented here includes relevant research areas (i.e. ecosystem services, drivers of change, pressures on urban green space, human processes and goals of provision of urban green space) and emergent research themes in urban green space studies (i.e. physicality, experience, valuation, management and governance). Collectively these two outputs have the potential to establish an international research agenda for urban green space, which can contribute to the better understanding of people's relationship with cities. 相似文献
1000.
Thomas H Hansen Kristian H Laursen Daniel P Persson Pai Pedas Søren Husted Jan K Schjoerring 《Plant methods》2009,5(1):12-11