全文获取类型
收费全文 | 6201篇 |
免费 | 422篇 |
国内免费 | 473篇 |
出版年
2024年 | 14篇 |
2023年 | 124篇 |
2022年 | 127篇 |
2021年 | 154篇 |
2020年 | 169篇 |
2019年 | 202篇 |
2018年 | 150篇 |
2017年 | 178篇 |
2016年 | 180篇 |
2015年 | 181篇 |
2014年 | 205篇 |
2013年 | 327篇 |
2012年 | 189篇 |
2011年 | 231篇 |
2010年 | 195篇 |
2009年 | 280篇 |
2008年 | 318篇 |
2007年 | 332篇 |
2006年 | 360篇 |
2005年 | 307篇 |
2004年 | 308篇 |
2003年 | 281篇 |
2002年 | 224篇 |
2001年 | 215篇 |
2000年 | 205篇 |
1999年 | 176篇 |
1998年 | 138篇 |
1997年 | 155篇 |
1996年 | 144篇 |
1995年 | 101篇 |
1994年 | 135篇 |
1993年 | 108篇 |
1992年 | 106篇 |
1991年 | 92篇 |
1990年 | 71篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 45篇 |
1986年 | 42篇 |
1985年 | 38篇 |
1984年 | 36篇 |
1983年 | 29篇 |
1982年 | 19篇 |
1981年 | 19篇 |
1980年 | 10篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1975年 | 7篇 |
排序方式: 共有7096条查询结果,搜索用时 15 毫秒
171.
Christopher A. Zoladeski 《植被学杂志》1991,2(2):255-258
Vegetation zones in dune slacks on the Leba Bar on the Polish Baltic Sea coast are described. Plant colonization starts on moist sand in eastern ends of slacks, in the wake of receding mobile dunes. Communities on deflation ribs, dominated by psammophytic grasses, sedges and lichens, are structurally simple. Communities in troughs, dominated by species of fresh habitats, become structurally more complex in a western direction and they culminate with the Pinus silvestris forest. The main lines of vegetation variation correlate with groundwa-ter depth, micro topography, soil profile build-up and substrate acidification. 相似文献
172.
Koukichi Nagasaka 《Population Ecology》1991,33(1):115-128
Three Athalia sawflies, A. japonica, A. rosae and A. infumata, feeding on cruciferous plants, coexist in Japan. However, it is not known what ecological strategies they use and what environmental factors are crucial to such strategies. I attempted to explain these questions by examining the relationship between the spatio-temporal distribution patterns of three Athalia sawflies and their habitats in three districts (Lowland, Intermediate and Mountain). The three sawflies have different spatio-temporal distribution patterns, though they usually used common cruciferous plants. A. japonica was abundant in spring and autumn but disappeared during summer in all the districts. In the Lowland, populations of A. rosae and A. infumata, like that of A. japonica, crashed in summer. However A. rosae occurred mainly in summer in the Intermediate and Mountain. Although A. infumata occurred in the same seasons as A. rosae in all districts, population levels of A. infumata were always lower than those of A. rosae. The crucial factors controlling their population patterns were the availability of host plants and temperature. Population crashes of A. rosae and A. infumata were due to food depletion, and those of A. japonica were due to heat stress. On the other hand, their population patterns may be interpreted as phenological synchronization with their original host plants, though they all existed on common cruciferous plants. The three sawflies may have evolved different strategies to escape from unfavorable habitat conditions. Such strategies are speculated to be summer diapause in A. japonica, long distance migration in A. rosae, and local dispersal in A. infumata. 相似文献
173.
Nutritional indices, development rates, percent dry weights and total lipids were determined in gypsy moth larvae (Lymantria dispar L.) reared on a high wheat germ (HWG) diet or diets prepared from lyophilized, ball-milled oak or pine foliage as the only source of dietary nitrogen (N). With regard to both total and proteinaceous N content, HWG diet>oak diet>pine diet. All nutritional indices measured were significantly lower in second instars fed pine diet vs. oak diet. Protein supplementation of pine diet with either casein or ovalbumin to bring total N up to the level present in oak diet resulted in small increased in approximate digestibility (AD) and effciency of conversion of ingested food (ECI), but relative growth rate (RGR) remained unaffected. The low RGR of larvae fed pine diet (unsupplemented or protein supplemented), as compared to those fed HWG or oak diet, was accompanied by significantly lower larval percent dry weight and percent total lipid. In contrast, RGR, larval percent dry weight and total lipid values were comparable in second instars fed HWG or oak diet. Insects reared from the first through the final instar on oak diet exhibited lower pupal weights compared to those reared on HWG. Casein addition to oak diet generally resulted in even more extended larval development times and further reduced pupal weights, but wheat germ addition to oak diet did not alter development rates and caused an increase in pupal weights. 相似文献
174.
175.
Variation in resource abundance affects diet and feeding morphology in the pumpkinseed sunfish (Lepomis gibbosus) 总被引:3,自引:0,他引:3
Summary Growth responses and accumulation of N and P were studied in two pygmy south-west Australian species of Drosera following supplementary feeding of arthropods (collembolans, Hypogastrura vernalis and fruit flies, Drosophila melanogaster) and/or a balanced mineral nutrient supplement (N as nitrate) via the roots. One feeding experiment used glasshouse-raised germlings from vegetative propagules (gemmae) of the perennial Drosera closterostigma, the other three (two on D. closterostigma and one on the annual D. glanduligera) involved natural populations engaging in natural captures of indigenous prey. All experiments recorded highly significant increases in plant dry matter, N and P (all plant age groups) and in reproductive performance (adult plants only) from artificial feeding of arthropods, but no apparent benefits from minerals alone or additive effects of minerals above that due to insects. Unresponsiveness to mineral nutrients was suggested to relate to inability of the species to use nitrate, while up to three-fold growth and nutrient uptake response to insects indicated that growth of natural populations might be severely limited by inadequate catches of prey. It is concluded that the highly nutrient-poor conditions typical of the habitat of pygmy species of Drosera may have promoted marked specialization towards carnivory and an attendant decline in ability to utilize soil-derived sources of nutrients. 相似文献
176.
177.
Although members of the Oxalidaceae family have been described as host plants of vesicular-arbuscular mycorrhizal fungi, Oxalis pes-caprae did not become colonized by Glomus mosseae. Extracts of Ox. pes-caprae root inhibited the germination of G. mosseae spores. However, the presence of G. mosseae in the rhizosphere of Ox. pes-caprae produced browning of the roots, which was interpreted as a hypersensitivity response of the plant to the presence of VA fungus. 相似文献
178.
Advances in cereal protoplast research 总被引:5,自引:0,他引:5
Beginning in 1986, plants have been regenerated from protoplasts of all of the important cereal species, including wheat, rice, maize, and barley, and grasses such as sugarcane. In addition, somatic hybrids/cybrids as well as transgenic plants with introduced useful agronomic traits have been obtained in several instances. This rapid and impressive progress in the genetic manipulation of cereals has been made possible by two critical technical advances during the past decade: the establishment of embryogenic suspension cultures as a source of totipotent protoplasts and the direct delivery of DNA into protoplasts for genetic transformation. 相似文献
179.
Carbon: terrestrial C4 plants 总被引:1,自引:1,他引:0
The carbon isotope composition of terrestrial C4 plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO2 to the carboxylation sites, but is also influenced by the final carboxylation of ribulose-1,5-bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C4 photosynthesis. As a new feature, the model considers four types of limitation of the overall CO2 assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO2, irradiance and water stress on short-term discrimination by leaves of several C4 species measured simultaneously with CO2 gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C4 plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C4 plants, evolutionary and genetical aspects, and some ecological implications are reviewed. 相似文献
180.
The plastid ribisomal RNA (rRNA) operon of the achlorophyllous root parasite Conopholis americana was completely sequenced. Full-length rRNA genes are retained in the gene cluster, but significant divergence has occurred in the 16S, 23S and 5S genes. Both the 16S–23S intergenic spacer and the 4.5S–5S intergenic spacer have suffered substantial deletions, including the two tRNA genes typically found in prokaryotic and plastid 16S–23S spacers. 相似文献