全文获取类型
收费全文 | 5644篇 |
免费 | 995篇 |
国内免费 | 1961篇 |
出版年
2024年 | 35篇 |
2023年 | 207篇 |
2022年 | 186篇 |
2021年 | 295篇 |
2020年 | 421篇 |
2019年 | 425篇 |
2018年 | 370篇 |
2017年 | 433篇 |
2016年 | 455篇 |
2015年 | 391篇 |
2014年 | 368篇 |
2013年 | 438篇 |
2012年 | 304篇 |
2011年 | 320篇 |
2010年 | 279篇 |
2009年 | 386篇 |
2008年 | 376篇 |
2007年 | 389篇 |
2006年 | 318篇 |
2005年 | 334篇 |
2004年 | 240篇 |
2003年 | 248篇 |
2002年 | 192篇 |
2001年 | 168篇 |
2000年 | 154篇 |
1999年 | 132篇 |
1998年 | 112篇 |
1997年 | 95篇 |
1996年 | 64篇 |
1995年 | 54篇 |
1994年 | 58篇 |
1993年 | 38篇 |
1992年 | 37篇 |
1991年 | 37篇 |
1990年 | 44篇 |
1989年 | 22篇 |
1988年 | 33篇 |
1987年 | 26篇 |
1986年 | 16篇 |
1985年 | 25篇 |
1984年 | 12篇 |
1983年 | 7篇 |
1982年 | 22篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1972年 | 2篇 |
1958年 | 4篇 |
排序方式: 共有8600条查询结果,搜索用时 546 毫秒
51.
Primary production and nitrogen allocation of field grown sugar maples in relation to nitrogen availability 总被引:2,自引:1,他引:1
Above ground net primary production (NPP), nitrogen (N) allocation, and retranslocation from senescing leaves were measured in 7 sugar-maple dominated sites having annual net N mineralization rates ranging from 26 to 94 kg · ha–1 · yr–1. The following responses were observed: (1) Green sun leaves on richer sites had higher N mass per unit leaf area than sun leaves on poorer sites; (2) Total canopy N varied much less than annual net mineralization, ranging from 81 to 111 kg · ha–1; (3) This was due to the existence of a large and relatively constant pool of N which was retranslocated from senescing leaves for use the following year (54 to 80 kg · ha–1); (4) The percentage of canopy N retranslocated by sugar maple was also relatively constant, but was slightly higher on the richer sites. Percent N in leaf litter did not change across the gradient; (5) Above ground NPP increased linearly in relation to N allocated above ground. Therefore, N use efficiency, expressed as above ground NPP divided by N allocated above ground was constant; (6) N use efficiency expressed as (NPP above ground/total N availability) was a curvilinear function of N availability; and (7) This pattern reflected a decreasing apparent allocation of N below ground with decreasing N availability. 相似文献
52.
The effect of (2RS, 3RS)-1-(4-Chlorophenyl)-4, 4-dimethyl-2-(1H-1,2,4 triazol-1-yl) pentan-3-ol (PP333) on the growth and transpiration of normal and root pruned colt rootstocks was measured. PP333 reduced plant height, stem diameter increment, leaf number, area and weight and stem weight. Root pruning reduced root, leaf and stem weight, and plant height in control plants. PP333 reduced both total water use and transpiration per unit leaf area and increased stomatal resistance. In control plants root pruning also reduced total water use and increased stomatal resistance. 15 days after the beginning of the experiment half the plants in all treatments were allowed to dry out. The effects of drought, i.e. reduced transpiration, growth and leaf water potentials, were smaller in PP333 treated than in control plants. 相似文献
53.
Cynthia A. Paszkowski 《Environmental Biology of Fishes》1986,17(3):227-233
Synopsis Laboratory experiments examined the foraging performances of a dietary generalist, bluegill,Lepomis macrochirus, and a dietary specialist, golden shiner,Notemigonus crysoleucas, as they fed from devices simulating four foraging sites (bottom substrate, water column, submerged macrophytes, and water surface). Fishes foraged in monospecific and mixed-species groups of two and four individuals. For monospecific groups, foraging rates of bluegills did not differ among the four sites, but golden shiners had significantly higher rates on bottom and midwater sites than on plant and surface sites. The size of monospecific groups did not affect foraging rates of either species. In mixed-species trials, bluegills removed more food items than golden shiners from plant and surface sites in two- and four-fish groups and from bottom sites in two-fish groups. Bluegills' foraging performances improved with experience, golden shiners' performances did not. Experimental results are discussed with respect to interactions between bluegills and golden shiners in natural assemblages. 相似文献
54.
A late Wenlock flora from Co. Tipperary, Ireland 总被引:1,自引:0,他引:1
D. EDWARDS F.L.S. J. FEEHAN D. G. SMITH 《Botanical journal of the Linnean Society. Linnean Society of London》1983,86(1-2):19-36
An assemblage of macroplants preserved as highly coalified compressions which lack anatomy is described from a Wenlock locality in County Tipperary, Ireland. Most of the fertile specimens are assigned to Cooksonia Lang. The taxonomic status of this genus is discussed. Some poorly preserved palynomorphs, including miospores, acritarchs, chitinozoans and a variety of tubes, have been isolated from associated sediments, but the age of the flora is based on graptolites. Sedimentological and palaeontological studies of the region are summarized. They provide little direct evidence for the habitats of the plants which are considered to have been terrestrial. The relevance of this flora to the current debate on the colonization of the land is evaluated and it is concluded that these plants provide the earliest record of erect fertile land plants of possible pteridophyte affinity. 相似文献
55.
56.
Using data for 27 Texas counties from 1978-1987, it is shown that the incidence rates of suicide, homicide, and rape are significantly higher in counties whose drinking water supplies contain little or no lithium than in counties with water lithium levels ranging from 70-170 micrograms/L; the differences remain statistically significant (p less than 0.01) after corrections for population density. The corresponding associations with the incidence rates of robbery, burglary, and theft were statistically significant with p less than 0.05. These results suggest that lithium has moderating effects on suicidal and violent criminal behavior at levels that may be encountered in municipal water supplies. Comparisons of drinking water lithium levels, in the respective Texas counties, with the incidences of arrests for possession of opium, cocaine, and their derivatives (morphine, heroin, and codeine) from 1981-1986 also produced statistically significant inverse associations, whereas no significant or consistent associations were observed with the reported arrest rates for possession of marijuana, driving under the influence of alcohol, and drunkenness. These results suggest that lithium at low dosage levels has a generally beneficial effect on human behavior, which may be associated with the functions of lithium as a nutritionally-essential trace element. Subject to confirmation by controlled experiments with high-risk populations, increasing the human lithium intakes by supplementation, or the lithiation of drinking water is suggested as a possible means of crime, suicide, and drug-dependency reduction at the individual and community level. 相似文献
57.
试论云阳县人口激增与土地锐减的矛盾及对策 总被引:2,自引:0,他引:2
土地是人们赖以生存的物质基础,是其它任何东西不可代替的生产资料。离开了土地,人就不能生存下去,这是众所周知的真理。但是,目前人口的激增与土地锐减的矛盾越来越尖锐。据有关统计资料表明,1978—1988年全国耕地面积减少3.66×10~6ha,与此同时全国人口又猛增1.33亿,给农业粮食生产、 相似文献
58.
水土保持耕作法是治理黄土高原地区坡耕地的根本措施 总被引:3,自引:0,他引:3
坡耕地所造成的土壤侵蚀,已成为全世界共同关注的问题。黄土高原地区,沟壑纵横,地形破碎,沟深坡陡,是世界上水土流失最强烈的地区。坡耕地的水土流失不仅是造成河流水库淤积的一个重要原因,更重要的是每年要丧失大量的肥沃表土,导致农业减产和土壤退化。一遇暴雨,人民的生命财产将遭受到巨大的威胁。黄土高原地区每年水力侵蚀土层深度达0.2—2cm,每1ha 损失表土120t 左右,损失 相似文献
59.
Stable carbon isotopes as indicators of increased water use efficiency and productivity in white spruce (Picea glauca (Moench) Voss) seedlings 总被引:6,自引:1,他引:5
The relationship among water use efficiency (WUE), productivity and carbon isotopic composition (δ13C) in white spruce (Picea glauca (Moench) Voss) seedlings was investigated. Sixteen hundred seedlings representing 10 controlled crosses were planted in the field in individual buried sand-filled cylinders. The soil water content in the cylinders was measured using time domain reflectometry over two growing seasons and seedling water use determined by water balance. Two watering treatments were imposed: irrigation and dry land. There was significant (1.6–2.0%c) genetic variation in needle δ13C. Ranking of crosses in terms of δ13C was generally maintained over watering treatments and there was not a significant genetic versus environmental interaction. There was a positive correlation between δ13C and both intrinsic and long-term WUE (more positive δ13C with increased WUE) and between δ13C and productivity, suggesting a correlation due to variation in photosynthetic capacity. Root to shoot ratios did not increase in water-stressed plants, indicating that responses to drought were primarily at the level of gas exchange, rather than through morphological changes. Our results indicate that it should be possible to use δ13C as a surrogate for WUE and to select white spruce genotypes for high WUE without compromising yield. 相似文献
60.
R. S. SENOCK J. M. HAM T. M. LOUGHIN B. A. KIMBALL D. J. HUNSAKER P. J. PINTER G. W. WALL R. L. GARCIA R. L. LaMORTE 《Plant, cell & environment》1996,19(2):147-158
The effects of elevated carbon dioxide (CO2) concentration on plant water use are best evaluated on plants grown under field conditions and with measurement techniques that do not disturb the natural function of the plant. Heat balance sap flow gauges were used on individual main stems of wheat (Triticum aestivum L. cv Yecora rojo) grown under normal ambient conditions (control) and in a free-air CO2 enrichment (FACE) system in Arizona with either high (control + high H2O = CW; FACE + high H2O = FW) or low (control + low H2O = CD; FACE + low H2O = FD) irrigation regimens. Over a 30d period (stem elongation to anthesis), combinations of treatments were monitored with,10–40 gauges per treatment. The effects of increased CO2 on tiller water use were inconsistent in both the diurnal patterns of sap flow and the statistical analyses of daily sap flow (Ftot). Initial results suggested that the reductions in Ftot, from CO2 enrichment were small (,0–10%) in relation to the H2O treatment effect (,20–30%). For a 3d period, Ftot of FW was,19–26% less than that of CW (P = 0.10). Examination of the different sources of variation in the study revealed that the location of gauges within the experimental plots influenced the variance of the sap flow measurements. This variation was probably related to positional variation in subsurface drip lines used to irrigate plots. A sampling design was proposed for use of sap flow gauges in FACE systems with subsurface irrigation that takes into account the main treatment effects of CO2 enrichment and the other sources of variation identified in this study. Despite the small and often statistically non-significant differences in Ftot between the CW and FW treatments, cumulative water use of the FW treatment at the end of the first three test periods ranged from 7 to 23% lower than that of the CW treatment. Differences in sap flow between FW and CW compared well with treatment differences in evapotranspiration. The results of the study, based on the first reported sap flow measurements of wheat, suggest that irrigation requirements for wheat production, in the present climatic regimen of the south-western US, may be predicted to decrease slightly because of increasing atmospheric CO2. 相似文献