全文获取类型
收费全文 | 2345篇 |
免费 | 60篇 |
国内免费 | 141篇 |
出版年
2023年 | 19篇 |
2022年 | 38篇 |
2021年 | 57篇 |
2020年 | 40篇 |
2019年 | 60篇 |
2018年 | 60篇 |
2017年 | 60篇 |
2016年 | 56篇 |
2015年 | 45篇 |
2014年 | 85篇 |
2013年 | 114篇 |
2012年 | 68篇 |
2011年 | 122篇 |
2010年 | 78篇 |
2009年 | 144篇 |
2008年 | 154篇 |
2007年 | 150篇 |
2006年 | 106篇 |
2005年 | 101篇 |
2004年 | 107篇 |
2003年 | 84篇 |
2002年 | 62篇 |
2001年 | 40篇 |
2000年 | 42篇 |
1999年 | 48篇 |
1998年 | 50篇 |
1997年 | 51篇 |
1996年 | 45篇 |
1995年 | 46篇 |
1994年 | 31篇 |
1993年 | 46篇 |
1992年 | 51篇 |
1991年 | 24篇 |
1990年 | 34篇 |
1989年 | 30篇 |
1988年 | 34篇 |
1987年 | 33篇 |
1986年 | 25篇 |
1985年 | 17篇 |
1984年 | 18篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 4篇 |
1974年 | 2篇 |
1965年 | 2篇 |
排序方式: 共有2546条查询结果,搜索用时 15 毫秒
61.
A guild of leeches and triclads coexist and are the most numerous invertebrate predators on the stony shores of productive British lakes. Populations of all species are food-limited. Mortality of recruited young is considerably higher in leech than in triclad populations, and this paper investigates reasons for this. In particular, the feeding success of young leeches and triclads in relation to prey species, prey size, prey condition (alive or crushed), spatial heterogeneity (with or without the presence of stones or gravel), and the presence or absence of other young or adults predators (leeches or triclads) of the same or different species are investigated in the laboratory. Feeding success by young leeches and triclads on crushed prey without the presence of stones was high, but declined dramatically in leeches but not triclads when stones were present. Young leeches and triclads were inept at capturing live prey, of a small or large size, with the exception of soft-bodied prey such as oligochaetes. Feeding success by young predators on live prey was not increased by the presence of other young predators of the same or different species. With only a few exceptions, the presence of adult leeches, and to a much lesser extent adult triclads, increased the feeding success, growth and survival of young leeches and triclads. It is concluded that the high mortality of young leeches, compared to triclads, in field populations is due to their inability to locate damaged food in an environment with spatial heterogeneity due to a poorly developed chemosensory system. High and low levels of juvenile morality are accompanied by high and low reproductive rates in leech and triclad populations, respectively. It is unusual for a food limited population to have a high level of recruitment, but it is speculated that the characteristically high reproductive output in parasitic leeches, from which predaceous leeches are derived or have affinities, has been retained to counterbalance high juvenile mortality rates. 相似文献
62.
Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production? 总被引:26,自引:2,他引:24
A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N2 via biological N2 fixation (BNF).Although BNF has long been a component of many farming systems throughout the world, its importance as a primary source of N for agriculture has diminished in recent decades as increasing amounts of fertilizer-N are used for the production of food and cash crops. However, international emphasis on environmentally sustainable development with the use of renewable resources is likely to focus attention on the potential role of BNF in supplying N for agriculture. This paper documents inputs of N via symbiotic N2 fixation measured in experimental plots and in farmers' fields in tropical and temperate regions. It considers contributions of fixed N from legumes (crop, pasture, green manures and trees), Casuarina, and Azolla, and compares the relative utilization of N derived from these sources with fertilizer N. 相似文献
63.
Somatic embryogenesis and plant regeneration from callus culture of Acacia catechu-a multipurpose leguminous tree 总被引:1,自引:0,他引:1
Plant regeneration via somatic embryogenesis was achieved from callus derived from immature cotyledons of Acacia catechu Willd. on Woody Plant Medium (WPM) supplemented with 13.9 M kinetin and 2.7 M 1-naphthaleneacetic acid. The addition of 0.9–3.5 mM L-proline to the medium influenced development of somatic embryos and also promoted secondary somatic embryogenesis. The light-green somatic embryos germinated on half-strength MS medium supplemented with 2% (w/v) sucrose. Somatic embryos germinated into plantlets that were acclimatized in the greenhouse and subsequently transferred to the field. 相似文献
64.
Food and water resources used by the Madagascan hissing-cockroach mite,Gromphadorholaelaps schaeferi
We determined the food source and water balance properties of the hissing-cockroach mite, Gromphadorholaelaps schaeferi. The food source for mites was identified using Evans blue dye by direct injection into a fasting host cockroach, Gromphadorhina portentosa, or by incorporation into cockroach food. No coloration was observed in mites on dye-injected cockroaches, but coloration was present in mites when only the food for the cockroaches had been stained. Thus, the mites are scavengers of cockroach food, and are not parasitic as previously thought. Our results demonstrate that the mites can absorb water from the air anywhere between 0.84 and 0.93 a
v
(%RH/100), and wax-block experiments revealed that the mouth is the site of uptake. The mites are normally clumped together on the host, typically in between the cockroach's legs and around the spiracles. Water loss rates for mites in groups (0.16% h-1) were far lower than for isolated mites (0.30% h-1), suggesting a group effect with regard to water balance. Above the transition temperature of 30°C rate of water loss was rapid. The sites occupied by mites on the cockroach's body seem to be highly specific for feeding and absorption of water vapour. 相似文献
65.
K. T. Castle B. A. Wunder 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1995,164(8):609-617
We fed prairie voles (Microtus ochrogaster) rat chow diluted with variable amounts of -cellulose to determine 1) how much fiber the voles could tolerate in their diet; 2) changes in food intake and digestibility of dry matter and of fiber; 3) the extent to which voles utilized fiber as an energy source; and 4) whether any of these variables differed between groups of animals maintained at 5 or 22°C. Fiber content of the diets ranged from 20 to 84%. Animals held at 5°C maintained body mass through a diet containing 69% fiber, while animals held at 22°C maintained body mass through the 84% fiber diet. Dry matter intake increased with fiber level from 9.3 to 15.0 g·day-1 for animals at 5°C and from 5.6 to 14.0 g·day-1 for animals at 22°C; intake on the highest fiber diet eaten by either group was not different. Dry matter digestibility decreased significantly as the fiber in the diets increased, but was not affected by temperature treatments. Digestible dry matter intake for each group remained constant regardless of diet quality, but on each diet digestible dry matter intake for animals at 5°C was significantly higher than that of the animals held at 22°C. Digestibility of the fiber portion of the experimental diets remained constant as food quality decreased, so the percent of daily energy need met by fiber utilization increased with higher food intake. On the lowest quality diet each group tolerated, fiber digestion provided approximately 42 and 68% of the energy needs of voles at 5 and 22°C, respectively.Abbreviations BM
body mass
- BMR
basal metabolic rate
- DE
digestible energy
- DM
dry matter
- DMD
dry matter digestibility
- DDMI
digestible dry matter intake
- MR
metabolic rate
- NDF
neutral detergent fiber (=cell walls)
- NDS
neutral detergent solubles (=cell solubles)
- SEM
standard error of mean
-
T
a
ambient temperature 相似文献
66.
67.
The Coccinellidae is an economically important family within the Coleoptera. Some members are phytophagous pests, but many are beneficial predators and valuable biocontrol agents. This study investigates the morphology of the mandibles of adult Coccinellidae in relation to diet. Using scanning electron microscopy on 86 species of Coccinellidae, it was found that the morphology of the mandibles was dictated by the general feeding method, and could only be used to indicate a phytophagous, mycophagous or carnivorous diet. Phytophagous Coccinellidae of the subfamily Epilachninae had mandibles with denticulate apical teeth and setae for feeding on leaf material. The mandibles of the mycophagous Psylloborini had secondary teeth on the ventral apical tooth for collecting fungal spores. The mandibles of carnivorous Coccinellidae and Scymninae had either a bifid or unidentate apex. The unidentate mandible seemed to be restricted to coccidophagous species. Many species also had a mandibular groove along which prey body juices were conducted. Although mandible morphology could be related to the overall feeding method, there was no relationship between specific diet or food taxon and mandible shape. Mandible shape does not appear to be especially restricting for changes in diet either in the ecological sense or over evolutionary time. Mandible morphology is of limited use in determining diet and host specificity in Coccinellidae that are being selected as potential biocontrol agents. 相似文献
68.
69.
70.
V. A. A. Jansen 《Journal of mathematical biology》1995,34(2):195-224
The aim of this paper is to understand how dispersal in a patchy environment influences the stability properties of tri-trophic metapopulations. Differential equation models for tri-trophic metapopulations are formulated and analysed. The patchy nature of the metapopulations is incorporated through dispersal phases. Two variants are studied: one with a dispersal phase for the top and one with a dispersal phase for the middle level. A complete characterisation of stable and unstable equilibria is given and the possibility of invasion in these food chains is studied. A dispersal phase for the middle level can destabilize the bottom level-middle level interaction, because of the delay that dispersal causes in the reaction to the resource. When the middle level is not efficiently controlled by the top level, the unstable bottom level-middle level pair can destabilize the entire food chain. Dispersal for the top level can destabilize in the same way. A characterisation of the long term behaviour of the models is given. Bistability with a stable three species equilibrium and a stable limit cycle is one of the possibilities. 相似文献