全文获取类型
收费全文 | 43616篇 |
免费 | 1551篇 |
国内免费 | 2649篇 |
专业分类
林业 | 6491篇 |
农学 | 4452篇 |
基础科学 | 2011篇 |
6938篇 | |
综合类 | 10494篇 |
农作物 | 3795篇 |
水产渔业 | 2865篇 |
畜牧兽医 | 5135篇 |
园艺 | 2265篇 |
植物保护 | 3370篇 |
出版年
2024年 | 108篇 |
2023年 | 451篇 |
2022年 | 977篇 |
2021年 | 1269篇 |
2020年 | 1175篇 |
2019年 | 1203篇 |
2018年 | 3470篇 |
2017年 | 3856篇 |
2016年 | 2131篇 |
2015年 | 1268篇 |
2014年 | 1344篇 |
2013年 | 1614篇 |
2012年 | 2711篇 |
2011年 | 4055篇 |
2010年 | 3955篇 |
2009年 | 3001篇 |
2008年 | 2827篇 |
2007年 | 3090篇 |
2006年 | 1302篇 |
2005年 | 1160篇 |
2004年 | 674篇 |
2003年 | 726篇 |
2002年 | 793篇 |
2001年 | 689篇 |
2000年 | 629篇 |
1999年 | 525篇 |
1998年 | 375篇 |
1997年 | 365篇 |
1996年 | 301篇 |
1995年 | 275篇 |
1994年 | 280篇 |
1993年 | 201篇 |
1992年 | 208篇 |
1991年 | 188篇 |
1990年 | 129篇 |
1989年 | 133篇 |
1988年 | 111篇 |
1987年 | 55篇 |
1986年 | 25篇 |
1985年 | 13篇 |
1984年 | 26篇 |
1983年 | 16篇 |
1982年 | 15篇 |
1981年 | 14篇 |
1980年 | 10篇 |
1979年 | 13篇 |
1977年 | 6篇 |
1964年 | 5篇 |
1956年 | 8篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 140 毫秒
991.
An experiment to examine the effect of streptobacillary arthritis on mouse populations was conducted in maturing sorghum crops
in mouse-proofed pens. There were three treatment pens and three control pens in which mice with and without clinical signs
of streptobacillary arthritis were introduced, respectively. A constant mouse density (533 mice ha−1) was introduced to the pens by releasing 12 mice in each pen. The sex ratio of introduced mice was also constant by releasing
in each pen nine males and three females. Mean track index was lower in treatment pens than in control pens, indicating that
streptobacillary arthritis reduced the activity of mice. However, this was not translated into a reduction in other population
responses. Mean mouse density at harvest, percentage of female pregnant at harvest, yield loss caused by mice and a feeding
index (oil card index) did not differ significantly between treatment and control groups. A plausible explanation for these
findings was that mice with the clinical signs might be less active but were able to consume as much grains as mice without
the clinical signs because food was in abundant supply. 相似文献
992.
Martin Häglund 《Wood Science and Technology》2007,41(6):477-490
Knowledge of effects of moisture exposure on timber structures is important for serviceability and load bearing capacity of
timber elements and systems. Varying climate affects the moisture content profiles in timber as the relative humidity changes
in the ambient air. This paper presents methods for calculation of diffusion based moisture transport and corresponding results
on how timber responds to varying moisture with regard to moisture penetration and distribution. The calculations are based
on measured outdoor relative humidity and temperature at different climatic locations in Sweden. No remarkably large deviations
between the different climatic locations were found in general, which is positive in a design code perspective. 相似文献
993.
Chang-Hua Fang Bruno Clair Joseph Gril Tancrède Alméras 《Wood Science and Technology》2007,41(8):659-671
Transverse drying shrinkage was measured at microscopic and mesoscopic levels in poplar wood characterised by an increasing
growth strain (GS), from normal to tension wood. Results show that: (a) the drying shrinkage, measured as a relative thickness
decrease, was significantly higher for G-layer (GL) than for the other layers (OL), GL shrinkage was not significantly correlated
with GS, and OL shrinkage was negatively correlated with GS. (b) In gelatinous fibre (G-fibre), lumen size increased during
drying and this increase was positively related with GS, but in normal wood fibre, lumen size decreased during drying. These
findings suggest that GL shrank outwards (i.e., its internal perimeter increases), so that its shrinkage weakly affected the
total cell shrinkage and the mesoscopic shrinkage was controlled by the OL shrinkage which shrank inwards (i.e., its external
perimeter decreases). (c) Measurements done on 7 × 7 mm2 thin sections evidenced a negative correlation between transverse
shrinkage and GS, significant in T direction but weak in R direction. These observations at both levels allow to discuss the
contribution of GL to the mesoscopic shrinkage of tension wood. 相似文献
994.
995.
We caught the adults of a secondary wood-boring insect, Callidiellum rufipenne (Motschulsky), on Japanese cedar, Cryptomeria japonica D. Don., trees and logs with manipulated bark water contents to clarify the mechanism of host selection by C. rufipenne. More C. rufipenne were trapped when the bark water content range was ca. 120–200%. Fewer C. rufipenne were trapped at higher and lower bark water contents. These findings indicate that C. rufipenne adults do not simply select weakened or dying trees. Bark water content is one of important factors for C. rufipenne choosing their host for oviposition. 相似文献
996.
997.
This article discusses the determinants of profitability expressed as return on capital among Japanese pulp and paper companies. Return on capital is measured as income before tax related to stockholders’ equity. Return on capital has been divided into profit margin and asset utilization rate. The hypothesis is that return on capital can be explained by mill size, productivity, product line, financial situation, and use of wood resources. The dataset consists of 13 larger Japanese pulp and paper companies listed in Pulp &; Paper Statistics. Data on financial performance, production, and use of wood fibers were collected for the period 1991–2001. The average return on equity for the studied companies is 4.2%. Labor productivity has increased quite significantly, from about 600 to 1,000 kg/employee and year. An econometric analysis indicates that the best model fit is found when the asset utilization rate is used as the dependent variable. Significant variables are, among others, asset utilization rate lagged one time period (?), labor productivity (+), capital productivity (?), paper production as a share of total paper and board production (?), total value of assets on the books (?) and solidity (+). This may indicate that the total value of assets on the books is large in relation to the total production of paper, paperboard and pulp. 相似文献
998.
999.
1000.
Peroxidase plays important roles in many stress-related interactions and catalyzes important reactions in various physiological processes. Since peroxidase played critical roles in the evolution of almond(Prunus dulcis Miller(D.A Webb) syn P. amygdalus Batsch), peroxidase-gene-based analyses may increase the understanding of evolution of this species. Peroxidase gene polymorphism(POGP) markers were used to determine genetic diversity and relationships among 69 Turkish genotypes/cultivars and 27 foreign almond cultivars by using unweighted pair group method with arithmetic mean(UPGMA) analysis. This study is the first evaluation of the use of POGP markers for diversity analysis in almond.Totally, 83 fragments were obtained from eight peroxidase primer pairs, and polymorphism was identified as 94 %.Similarity level among the genotypes ranged between 0.63 and 0.93, and all materials were distinguished. In general,Turkish and foreign genotypes were mixed in clusters since they share a common genetic background and gene migration among the sites. Clusters were not based on geographic regions except for some minor groupings. This study indicated that peroxidase gene markers can be reliably used to determine genetic relationships in almonds. 相似文献