全文获取类型
收费全文 | 26351篇 |
免费 | 753篇 |
国内免费 | 45篇 |
专业分类
林业 | 4329篇 |
农学 | 1663篇 |
基础科学 | 240篇 |
5332篇 | |
综合类 | 1126篇 |
农作物 | 2634篇 |
水产渔业 | 2814篇 |
畜牧兽医 | 5085篇 |
园艺 | 1301篇 |
植物保护 | 2625篇 |
出版年
2023年 | 80篇 |
2022年 | 142篇 |
2021年 | 254篇 |
2020年 | 257篇 |
2019年 | 291篇 |
2018年 | 3012篇 |
2017年 | 2996篇 |
2016年 | 1575篇 |
2015年 | 397篇 |
2014年 | 432篇 |
2013年 | 709篇 |
2012年 | 1519篇 |
2011年 | 2850篇 |
2010年 | 2488篇 |
2009年 | 1651篇 |
2008年 | 1918篇 |
2007年 | 2144篇 |
2006年 | 527篇 |
2005年 | 550篇 |
2004年 | 573篇 |
2003年 | 566篇 |
2002年 | 381篇 |
2001年 | 258篇 |
2000年 | 296篇 |
1999年 | 227篇 |
1998年 | 71篇 |
1997年 | 62篇 |
1996年 | 45篇 |
1995年 | 54篇 |
1994年 | 33篇 |
1993年 | 43篇 |
1992年 | 57篇 |
1991年 | 42篇 |
1990年 | 45篇 |
1989年 | 67篇 |
1988年 | 46篇 |
1987年 | 47篇 |
1986年 | 32篇 |
1985年 | 37篇 |
1984年 | 24篇 |
1983年 | 28篇 |
1980年 | 16篇 |
1979年 | 25篇 |
1978年 | 19篇 |
1977年 | 17篇 |
1976年 | 20篇 |
1973年 | 19篇 |
1968年 | 18篇 |
1967年 | 16篇 |
1966年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
951.
MA-SEBS as compatibilizer and impact modifier was incorporated into Polypropylene/Wood Fiber (PP/WF) to enhance interface adhesion and impact strength of the composite. The effect of MA-SEBS content on the impact fracture behavior of PP/WF composites was studied. The impact properties of composites with 8% MA-SEBS reached the maximum value. And further increasing of MA-SEBS content to 10% did not improve the fracture toughness, but improved the stiffness of composites by DMA analysis. This was attributed to the improved PP/WF adhesion. As the MA-SEBS content is more than 8%, the molecule interaction of PP and WF was expected to much stronger than lower MA-SEBS. Scanning electron microscopy (SEM) was performed to analyze the impact fracture surface and showed a stronger affinity for the wood surfaces. 相似文献
952.
This paper describes the use by family forest landowners of educational programs provided by Washington State University Cooperative
Extension (WSUCE), and the associated use of technical assistance programs provided by state and federal agencies and the
private sector. Approximately 100,000 family forest owners controlled 19% or over 1.2 M ha of Washington’s forestland and
accounted for 29% of the timber harvested in the state on a volume basis in 1998. A variety of public and private assistance
and education programs are available to encourage and help family forest owners manage their forests. In 1999 a mail survey
was conducted to evaluate use and effectiveness of Washington’s family forest assistance and education programs. Over half
of the 872 responding family forest landowners had contact with an extension educator, program or educational material, and
about three quarters of these respondents gave an overall rating of the usefulness of extension programs and materials as
good or excellent. Respondents attending WSUCE forestry educational programs have larger median land ownership size, are older,
have owned their forests longer, have a higher rate of absentee ownership, and are better educated than non-users. They are
more likely to actively manage their forests for timber production and exhibit a clearer understanding of the multiple-use
capabilities of their forests. 相似文献
953.
Barbara Moser Christoph Bachofen Jonathan D. Müller Marek Metslaid Thomas Wohlgemuth 《Annals of Forest Science》2016,73(4):959-970
Key message
Pinus sylvestris seedlings quickly expand their roots to deeper soil layers while Pseudotsuga menziesii concentrates its root system in the topsoil, thereby running the risk of desiccation during long dry spells, as indicated by lower survival after simulated summer drought.Context
Pseudotsuga menziesii (Douglas-fir) is regarded as a promising species to maintain the productivity of Central European lowland forests given the projected increase of long dry spells.Aims
Will the species be able to regenerate from seed and spread outside plantations in a drier temperate Europe?Methods
We measured the relative growth rate, biomass allocation, root architecture, and phenotypic plasticity of Pseudotsuga menziesii seedlings sown in a common garden and grown under current precipitation and prolonged drought, respectively. The species’ competitive ability with respect to Pinus sylvestris L., the most drought-tolerant native conifer in Central Europe, was assessed during three growing seasons.Results
Pinus sylvestris seedlings had higher relative growth rates than did Pseudotsuga menziesii seedlings, first in terms of aboveground biomass and later in terms of shoot height. This resulted in heavier and taller seedlings after three growing seasons under both moist and dry conditions. Shorter vertical roots corresponded with lower survival of Pseudotsuga menziesii seedlings under dry conditions.Conclusion
Fast root proliferation allows Pinus sylvestris seedlings to reach deeper water pools that are less rapidly depleted during transient drought. By contrast, the shallow root system might put Pseudotsuga menziesii seedlings at the risk of desiccation during prolonged dry spells.954.
What is new about new forest owners? A typology of private forest ownership in Austria 总被引:1,自引:5,他引:1
With structural changes in agriculture, new types of forest owners have become increasingly important. This article develops
an empirically-based typology of forest owners in Austria. Based on a representative survey and by means of cluster analysis,
seven types of forest owners are identified. These types form a sequence, ranging from forest owners with a strong agricultural
background to forest owners with no agricultural background at all. The latter exhibit markedly different behaviour in various
respects, e.g. in their interest in forest-related information. The increasing number of ‘new’ forest owners raises important
questions for forest policy, especially how policy instruments can reach these owners and how extension services can address
them. 相似文献
955.
It has been postulated that osmotic effects on plant tissue are mediated by abscisic acid (ABA). Hybrid larch (Larix kaempferi (Lambert) Carr. x L. decidua Mill.) plantlets, transformed with the ABA-inducible wheat Em promoter associated with the Gus reporter gene, were axenically inoculated with two ectomycorrhizal fungi: Cenococcum geophilum Fr., considered tolerant to water stress, and Laccaria bicolor (Marie) Orton, considered less tolerant to drought. The mycorrhizal and non-mycorrhizal transgenic plantlets were subjected to osmotic stress by adding polyethylene glycol (PEG) to the culture medium. In the presence of PEG, L. bicolor and C. geophilum reduced shoot water potential and turgor potential, but increased host osmotic potential. Treatment of plantlets with PEG induced a significant increase in endogenous ABA concentrations. Laccaria bicolor and C. geophilum behaved similarly and significantly decreased the ABA response of plantlets to PEG treatment. Moreover, inoculation with either fungus regulated the ABA response of the plantlets even when the fungus was separated from the host by a cellophane sheet that prevented mycorrhiza formation. Although the wheat Em promoter was inducible in larch plantlets, it was not regulated by endogenous ABA. Induction of the wheat Em promoter in larch plantlets depended on organ type, with maximum induction in the root apex. Induction of the Em promoter was significantly decreased by mycorrhizal inoculation. 相似文献
956.
L. Szalay-Marzsó 《Journal of pest science》1963,36(12):189-193
Summary
Lepyrus palustris Scop., a pest of the basket-willow has in Hungary one generation in a year. The overwintered adults appear in the early spring and cause occasionally severe damages on the young shoots, especially in the southern parts of Hungary. The females lay 100–140 eggs during the months May–June on the earth surface near to the willow-stocks. The larvae enter the soil and damage outside on the main roots, which can cause severe losses mainly in the young plantations. The 4th instar larva pupate in the soil. The young adults appear in July, the damage caused by them in the second part of the summer is not more important. The best control has been achieved applying Wofatox dust (3 per cent) against the overwintered adults at the beginning of the vegetation period. 相似文献
957.
Koichi Sone Keita Oishi Tatsuya Shibato Kunihiko Hata 《Journal of Forest Research》2016,21(4):185-192
We conducted field studies to evaluate the impact that curculio weevil attacks on Pasania edulis (Fagacea) acorns have on the regeneration of this tree. The germination ratio of weevil-attacked acorns was significantly lower than that of sound acorns. The number and position of the attacks on acorns affected the germination ratio. The seedlings from weevil-attacked acorns were shorter and had a smaller leaf area than those from sound acorns. The negative effects on seedling height lasted for at least three years after germination. We also studied the handling of the two types of acorns by granivorous Apodemus mice in a broad-leaved forest dominated by P. edulis. The mice picked up, transported, hoarded, and recovered sound and weevil-attacked acorns similarly. In the study site, the seedlings from weevil-attacked acorns were estimated to account for 1.5–20.4 % of the total seedlings for cohorts of mast years and 0–3.7 % for those of poor or medium acorn production. From these results, we conclude that acorns with the lower part of their cotyledons slightly damaged by weevil larvae might still be able to contribute to the regeneration of P. edulis in the field but that their contribution would be negligible or small even in mast years in this study site. 相似文献
958.
Picea mongolica is an endemic but endangered species in China. The spruce forest is only found in sandy forest-steppe ecotones. In this study, we examined the initial response of the quantity and refilling process of fine roots in an artificial canopy gap with a diameter of 36 m in a P. mongolica forest. Under the canopy, the fine root length densities of trees, shrubs and herbs were 2,622, 864 and 3,086 m·m–2, respectively. The fine root biomass of trees, shrubs and herbs were 148, 62 and 65 g·m–2, respect... 相似文献
959.
The bending properties of split bamboo culm were compared with those of spruce and beech wood specimens. The bamboo allowed
large flexural deformation since its outer layer retains the tensile stress while the softer inner layer undergoes large compressive
deformation. The results suggested that the combination of the fiber-rich outer part and the compressible inner part was responsible
for the flexural ductility of split bamboo. To clarify the compressible nature of the inner part of bamboo, the longitudinal
surfaces of the bamboo and wood specimens were microscopically observed before and after a large longitudinal compression.
Although the wood specimens showed serious and localized buckling, the inner part of the bamboo specimens showed no such visible
buckling. In the latter case, the foam-like parenchyma cells absorbed the large compressive deformation by their microscopic
buckling and simultaneously, the alignment of sclerenchyma fibers was maintained by the surrounding parenchyma matrix. The
flexural elasticity of the bamboo was compared to that of the wood in respect of remaining strain during cyclic bending tests.
No clear difference was recognized between their remaining strains. This fact indicated that the bamboo was not so flexible
elastically, although its fiber–foam combination and intelligent fiber distribution improve flexural ductility. 相似文献
960.
A new approach is proposed for the evaluation of the brittleness of heat-treated Styrax tonkinensis wood. Heat treatment made wood more brittle when wood was heated at a higher temperature or for a longer time. The brittleness
increased to four times that of the control when wood was heated at 200°C for 12 h. For treatment at 160°C, the increase in
brittleness without any change in weight is thought to be possibly caused by the relocation of lignin molecules. At higher
temperatures, loss of amorphous polysaccharides due to degradation is thought to become the main factor affecting brittleness.
The crystallites that were newly formed after 2 h of treatment showed brittleness that was different from that of the inherent
crystallites remaining after 12 h of heat treatment. This inherent crystalline cellulose possibly plays a role in brittleness.
There is also the possibility of using color to predict the brittleness of heat-treated wood. 相似文献