全文获取类型
收费全文 | 17919篇 |
免费 | 84篇 |
国内免费 | 2篇 |
专业分类
林业 | 3713篇 |
农学 | 1319篇 |
基础科学 | 140篇 |
2964篇 | |
综合类 | 880篇 |
农作物 | 2165篇 |
水产渔业 | 1861篇 |
畜牧兽医 | 1860篇 |
园艺 | 1139篇 |
植物保护 | 1964篇 |
出版年
2023年 | 17篇 |
2022年 | 9篇 |
2021年 | 26篇 |
2020年 | 41篇 |
2019年 | 47篇 |
2018年 | 2773篇 |
2017年 | 2729篇 |
2016年 | 1217篇 |
2015年 | 104篇 |
2014年 | 57篇 |
2013年 | 87篇 |
2012年 | 888篇 |
2011年 | 2249篇 |
2010年 | 2136篇 |
2009年 | 1309篇 |
2008年 | 1412篇 |
2007年 | 1684篇 |
2006年 | 118篇 |
2005年 | 190篇 |
2004年 | 182篇 |
2003年 | 235篇 |
2002年 | 120篇 |
2001年 | 20篇 |
2000年 | 60篇 |
1999年 | 20篇 |
1998年 | 15篇 |
1997年 | 17篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 12篇 |
1988年 | 15篇 |
1987年 | 6篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 6篇 |
1980年 | 4篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 9篇 |
1976年 | 7篇 |
1975年 | 18篇 |
1974年 | 4篇 |
1970年 | 4篇 |
1968年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Increased abundances of a number of seabird species have been found in areas of mussel longline aquaculture (compared to control
sites) in a recent study at Bantry Bay, southwest Ireland. The aim of this study was to investigate whether this form of mussel
aquaculture also affects the activity budgets and foraging behaviour of these seabirds. Scan-sampling was used to compare
seabird activity budgets between three areas of mussel longline aquaculture and three control sites. Foraging activities of
Great-Northern Divers Gavia immer, an Annex 1 species under the Euopean Union's Birds Directive, were also compared between sites using focal sampling. The
behaviour of gulls (Larus sp.), Cormorants Phalacrocorax carbo, Shags Phalacrocorax aristotelis and Great-Northern Divers (but not auks, Alcidae) differed significantly between areas of mussel aquaculture and control
sites in this study. Mussel suspension buoys were heavily utilized by gulls, cormorants and shags as safe perching platforms
for preening activities. Gulls also fed extensively on epifauna attached to the buoys. Great-Northern Divers spent more time
snorkelling (head submerged in search of food) in mussel sites than in the control sites, however dive duration and visible
foraging success rates did not differ significantly between sites. Overall, the environmental impact of mussel suspension
culture appears to be positive or neutral on marine bird species occurring at this study site. 相似文献
102.
103.
This study was carried out to investigate the influences of feeding frequency of extruded pellet and moist pellet on growth
and body composition of juvenile flounder (initial mean weight 6.3 g) in sub optimal water temperatures. A 2 (diets: extruded
pellet and moist pellet)×2 (feeding frequencies: two and three times daily)×2 (water temperatures: 12 and 17°C) factorial
design with three replications was used. After 60 days of feeding, the feeding frequency did not significantly affect growth
performance of fish. Weight gain, daily feed intake, feed efficiency and protein efficiency ratio were significantly (P<0.05) higher for fish reared at the higher water temperature. At the same water temperature, weight gain, feed efficiency,
and protein efficiency ratio were significantly (P<0.05) higher for fish fed the extruded pellet than moist pellet. The results of this study indicate that feeding frequency
of two times daily is sufficient for optimal growth of juvenile flounder when reared in suboptimal water temperatures, and
an increase in water temperature from 12 to 17°C improved growth and feed efficiency. 相似文献
104.
Recording cerebellar neuron activities in swimming goldfish 总被引:1,自引:0,他引:1
ABSTRACT: Neuronal activities were investigated in the cerebellum of immobilized and swimming goldfish Carassius auratus . Extracellularly recorded neural activities of the cerebellum in immobilized goldfish were characterized and classified into five types. Based on the waveforms and recording depths, these five neural activity types were estimated to originate from three identified classes of cerebellar neurons: Purkinje cells, eurydendroid cells, and granule cells. Chronic recording of cerebellar neuron activities in unrestrained goldfish was performed for more than 100 h. During the chronic recordings, a submersible amplifier attached to the goldfish head, and a multielectrode array developed for the present study were used. Neuronal activities in the cerebellum of free-swimming fish could also be classified into five types as in the immobilized condition. Firing patterns of two neurons identified as Purkinje cells and eurydendroid cells were analyzed during turning movements of the goldfish. The firing patterns of these neurons changed in relation to turning movements. Although some improvements are required, the chronic recording method developed in the present study can be applied to further investigations concerning the direct relationship between brain neural activities and certain behavior. 相似文献
105.
106.
ABSTRACT: The thermal stability of carp G-actin was investigated by monitoring loss of actin polymerization ability. To determine the amount of native actin remaining after heat treatment, actin was labeled with a fluorescence reagent, N-(1-pyrene)iodoacetamide. The loss of polymerization ability of carp actin during heat treatment, at between 45 and 55°C, occurred faster than that of chicken actin. The inactivation rate was influenced by concentrations of ATP and Ca2+ in solution. With the increase of Ca2+ concentration, the inactivation of carp actin was markedly suppressed. Furthermore, the activation energy of the inactivation of carp actin obtained from an Arrhenius plot was similar to that of chicken actin. These results indicated that the thermal instability of carp G-actin was due to the low affinites of ATP and Ca2+ for carp actin described in a previous report. 相似文献
107.
Maciej Zalewski Barbara Bis Mazgorzata apiska Piotr Frankiewicz Wojciech Puchalski 《水产资源保护:海洋与淡水生态系统》1998,8(2):287-307
1. The effect of riparian ecotone functional complexity and stream hydraulics on an upland river ecosystem has been analysed. 2. The amount of nutrients retained by bottom sediment was lowest on a sandy substrate (range: 26–104 mg m−2 P-PO4) and highest in wetland bays (range: 558–5368 mg m−2 P-PO4). A stream bed covered by Berula erecta had about three times higher retentive nutrient capacity ( x̄ = 584 mg m−2 day−1) than did a sandy substrate( x̄ = 205 mg m−2 day−1). 3. The amount of allochthonous organic matter (CPOM) deposited on the stream bed decreased with current velocity. The trophic potential of CPOM, measured as total protein, was significantly correlated with the amount of deposited CPOM (r = 0.863; p<0.00001) and depended on stream order. 4. Both invertebrate and fish biomass in the upland river were significantly correlated with calcium/bicarbonate (benthos: r = 0.858; p<0.006; fish: r = 0.918; p<0.001). 5. Fish biomass, diversity and species richness were highest in pools, lower in riffles and lowest in the run/transition zone. 6. Macroinvertebrate biomass was highest at an intermediate riparian ecotone complexity with an adequate supply of organic matter and incident light. Fish biomass followed the same trend, being lowest in heavily shaded areas and in open channels without riparian vegetation (range: 1–4.5 g m−2), but highest in ecotones of intermediate complexity (range: 1.6–92.8 g m−2). The ‘cascading effect’ of invertebrate density depletion, which was inversely related to fish biomass, was observed seasonally. 7. The above results indicate that riparian ecotone structure and the heterogeneity of the stream channel may regulate biodiversity, productivity and nutrient retention in the fluvial corridor. These quantitative data help to create alternative scenarios for sustainable river basin management. © 1998 John Wiley & Sons, Ltd. 相似文献
108.
The present study examines the pattern of accumulation of cadmium (Cd) and its biochemical effects on selected tissues of
a variety of African catfish (Clarias gariepinus), after exposure to various doses of Cd. The results obtained indicate that at the end of 21 days of exposure, the total
tissue organ cadmium concentration followed the pattern kidney > gill > liver > muscle for each of the exposure concentrations.
The levels of Cd in these organs were higher than those in ambient water. Moreover, while the rate of uptake of Cd increased
with time in the kidney, liver and muscle, it decreased in the gill. Superoxide dismutase (SOD) was significantly elevated
only in the kidney of catfish treated with 0.2 and 0.4 ppm of Cd for 7 days compared with the control. Conversely, gill SOD
was significantly decreased in the same concentrations of Cd-treated catfish relative to the control. Statistically similar
levels of SOD were observed in the liver, brain and muscle with all the treatments after the same duration of treatment. In
the fish exposed for 21 days, SOD activity was significantly decreased in the kidney with a corresponding increase in lipid
peroxidation (LPO), but it manifested only with the 0.2- and 0.4-ppm Cd treatment relative to the control. In the liver, however,
Cd exposure significantly increased SOD in the 0.2- and 0.4-ppm treatments of the same duration. In conclusion, the present
study indicates that the accumulation of Cd and its effect on SOD and LPO in C. gariepinus is dependent on concentration, tissue and time 相似文献
109.
We have shown previously that riboflavin is stored in chloragosomes of chloragocyte-derived earthworm eleocytes what may have an adaptive value for worms vulnerable to soil-derived pathogen invasion. Thus, it is important to determine whether riboflavin stores are also present in earthworm species with low eleocyte counts, especially in the chloragocytes of their chloragogen tissue that surrounds the coelomic site of the intestine, and find out its putative roles in earthworm immunity. Experiments were performed on adult worms from 6 species. Freely floating coelomocytes (consisting of amoebocytes plus species-specific numbers of eleocytes) were extruded by mild electric shock and then chloragocytes from the same anaesthetized worm were mechanically detached into buffer. Both cell suspensions were analyzed by spectrofluorometry for riboflavin content. It turned out that riboflavin is stored in species-specific quantities in all investigated species. Riboflavin storage predominates in free coelomocytes of eleocyte-rich species (Eisenia andrei, Dendrobaena veneta, Allolobophora chlorotica) while in chloragocytes of species with few eleocytes (Lumbricus terrestris, L. rubellus, and Aporrectodea caliginosa). Upon a massive microbial impact, the coelomocytes (both amoebocytes and eleocytes) are involved in the formation of multicellular bodies encapsulating soil-derived pathogens, what is connected with enhanced coelomocyte mobility. Thus the second aim of the present investigation was to check if riboflavin can participate in coelomocyte accumulations. Tests performed in a 48-well chemotaxis chamber revealed that riboflavin behave as chemoattractant for coelomocytes of all investigated earthworm species, which may be one of mechanisms underpinning its immunomodulatory functions. 相似文献