全文获取类型
收费全文 | 1221篇 |
免费 | 85篇 |
专业分类
林业 | 54篇 |
农学 | 13篇 |
基础科学 | 12篇 |
128篇 | |
综合类 | 187篇 |
农作物 | 48篇 |
水产渔业 | 46篇 |
畜牧兽医 | 689篇 |
园艺 | 14篇 |
植物保护 | 115篇 |
出版年
2019年 | 21篇 |
2018年 | 10篇 |
2017年 | 19篇 |
2016年 | 15篇 |
2014年 | 11篇 |
2013年 | 73篇 |
2012年 | 32篇 |
2011年 | 40篇 |
2010年 | 30篇 |
2009年 | 24篇 |
2008年 | 38篇 |
2007年 | 46篇 |
2006年 | 30篇 |
2005年 | 42篇 |
2004年 | 37篇 |
2003年 | 30篇 |
2002年 | 43篇 |
2001年 | 30篇 |
2000年 | 47篇 |
1999年 | 30篇 |
1998年 | 22篇 |
1997年 | 18篇 |
1996年 | 14篇 |
1995年 | 19篇 |
1994年 | 12篇 |
1993年 | 19篇 |
1992年 | 29篇 |
1991年 | 26篇 |
1990年 | 27篇 |
1989年 | 29篇 |
1988年 | 22篇 |
1987年 | 19篇 |
1986年 | 13篇 |
1985年 | 19篇 |
1984年 | 13篇 |
1983年 | 18篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 10篇 |
1979年 | 22篇 |
1978年 | 13篇 |
1976年 | 12篇 |
1975年 | 11篇 |
1974年 | 11篇 |
1973年 | 14篇 |
1972年 | 16篇 |
1971年 | 15篇 |
1970年 | 10篇 |
1967年 | 15篇 |
1938年 | 10篇 |
排序方式: 共有1306条查询结果,搜索用时 31 毫秒
81.
David J. Walker M. Pilar Bernal Enrique Correal 《Water, air, and soil pollution》2007,184(1-4):335-345
With a view to the selection of plants for the re-vegetation of contaminated, semi-arid land, two populations of the perennial species Bituminaria bituminosa (Fabaceae) from the south of Spain were studied: one (“LA”) from a non-contaminated soil and the other (“C2”) from a similar soil having elevated total levels of Pb and Zn (1,112 and 4,249 μg g?1, respectively). For sand-cultured plants receiving nutrient solution, flow cytometry showed that heavy metals, at the concentrations measured in aqueous extracts from contaminated soils, had only slight genotoxic effects on root tip cell nuclei. Both populations were also grown in both soils, in two pot experiments. In the first, shoot biomass of LA and C2 in the contaminated soil was decreased to similar extents, with respect to the “clean” soil. Tissue heavy metal concentrations were unlikely to have been phytotoxic, except in the case of shoot Zn for population LA, but there were tissue deficiencies of P and K for populations LA and C2, respectively. In the second pot assay, the stimulation of growth by NPK fertiliser confirmed that even though this soil had high total heavy metal levels, nutrient availability was the principal factor limiting growth. The lesser transport of heavy metals (Cd, Mn and Zn) to the shoot by the population from the contaminated site is a factor that should be considered when selecting B. bituminosa lines for the phytostabilisation of such sites. 相似文献
82.
W. W. Miller D. W. Johnson C. Denton P. S. J. Verburg G. L. Dana R. F. Walker 《Water, air, and soil pollution》2005,163(1-4):3-17
It is generally held that surface runoff in heavily forested ecosystems is minimal and therefore nutrient fluxes via runoff are unimportant. This is based in large measure on the absence of direct observation or remnant physical evidence. It is further held that protected forests with heavy understory and litter serve as a nutrient sink due to maximum uptake and interception. Our Sierran studies have detected the presence of surface runoff at several sites in the form of both overland and litter interflow with concentrations of NH4-N as high as 87.2 mg L?1, NO3-N as high as 95.4 mg L?1, and PO4-P as high as 24.4 mg L?1. Data suggest that nutrients are derived from the mature O-horizons, and that there has been little contact with the mineral soil or root zone where strong retention and/or uptake of these ions would be expected. Such contributions from overland/interflow could be particularly important in areas where ultra-oligotrophic lakes (e.g., Lake Tahoe) are now trending towards meso-oligotrophic status. We believe that fire exclusion in these systems may have exacerbated N and P inputs to Lake Tahoe and elsewhere by allowing unnatural buildups of O-horizons that are apparently a source of nutrients to surface runoff. 相似文献
83.
James R Simpson Leo J Fritschen Richard B Walker 《Agricultural and Forest Meteorology》1985,33(4):299-313
Stomatal diffusion (leaf) resistance (rs) of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), lodgepole pine (Pinus contorta Dougl. ex Loud.) and white oak (Quercus alba L.) in a variety of natural environments is described during conditions of light saturation as a function of the product of the predawn (base) xylem pressure potential of twigs (ψxb) or the soil water potential (ψs), and the saturation deficit of the atmosphere (δe). This general functional form was derived from an analysis based on an Ohm's law analog for water flow in the soil—plant—atmosphere system. The specific form used in each case was determined using regression analysis, where dependent variables considered were xylem pressure potential of the twigs or leaves (ψx) and air temperature (T) as well as δe and ψxb (or ψs). For three independent sets of Douglas-fir data, it is shown qualitatively that a single function describes the combined data as well as the functions derived for each of three data sets individually. The results imply that it may be possible to describe stomatal response of Douglas-fir, and perhaps other species, over a range of environmental conditions using the same model. 相似文献
84.
Fleischer RL Price PB Walker RM Filz RC Fukui K Friedlander MW Holeman E Rajan RS Tamhane AS 《Science (New York, N.Y.)》1967,155(3759):187-189
Cosmic ray nuclei have been observed with the use of plastic trackdetecting solids in satellites and high-altitude balloon flights. Nuclear emulsions in the stacks of plastic sheets allowed the positive identification of cosmic raynuclei as light as nitrogen. The most striking new information was the failure to observe relativistic iron nuclei, a result which has led to an advance in the understanding of track registration criteria. 相似文献
85.
Large numbers of two species of mole crickets flew to loudspeakers playing the appropriate calling song outdoors. Mated females were more frequently captured than unmated ones, and males were 12 percent of the catch. Crickets of three other subfamilies were trapped as they flew to mole cricket songs resembling their own. 相似文献
86.
The discovery of a new type of fossil nuclear track in mica is reported. This track is produced by the recoil nucleus accompanying the alpha-particle decay of uranium and thorium impurities. The tracks are very short and can be seen with phase contrast microscopy. Measurement of fossil alpha-recoil track densities, coupled with determinations of the thorium and uranium contents, provides a new dating technique analogous to the previously discovered "fission-track method." The primary advantage of the alpha-recoil method is a several-thousand-fold increase in sensitivity over the fission-track technique. The alpha-recoil method should also prove useful in studying the problem of extinct isotopes in meteorites. 相似文献
87.
88.
I G Walker 《Science (New York, N.Y.)》1966,151(706):99-101
After exposure to 5-fluorodeoxyuridine, L-cells are considerably more sensitive to the lethal effect of sulfur mustard than after they have been released from this block by addition of thymidine and allowed to proceed into the G2 phase of the division cycle. Nevertheless, for both populations, the amounts of mustard bound per cell and per nucleus (expressed as the amount of mustard per unit of protein) were the same. Likewise, the amounts of mustard bound per unit of DNA were the same for both populations. 相似文献
89.
90.
Mechanosensory transduction underlies a wide range of senses, including proprioception, touch, balance, and hearing. The pivotal element of these senses is a mechanically gated ion channel that transduces sound, pressure, or movement into changes in excitability of specialized sensory cells. Despite the prevalence of mechanosensory systems, little is known about the molecular nature of the transduction channels. To identify such a channel, we analyzed Drosophila melanogaster mechanoreceptive mutants for defects in mechanosensory physiology. Loss-of-function mutations in the no mechanoreceptor potential C (nompC) gene virtually abolished mechanosensory signaling. nompC encodes a new ion channel that is essential for mechanosensory transduction. As expected for a transduction channel, D. melanogaster NOMPC and a Caenorhabditis elegans homolog were selectively expressed in mechanosensory organs. 相似文献