全文获取类型
收费全文 | 2824篇 |
免费 | 259篇 |
国内免费 | 253篇 |
专业分类
林业 | 714篇 |
农学 | 384篇 |
基础科学 | 100篇 |
378篇 | |
综合类 | 849篇 |
农作物 | 256篇 |
水产渔业 | 217篇 |
畜牧兽医 | 284篇 |
园艺 | 58篇 |
植物保护 | 96篇 |
出版年
2024年 | 6篇 |
2023年 | 28篇 |
2022年 | 45篇 |
2021年 | 86篇 |
2020年 | 66篇 |
2019年 | 95篇 |
2018年 | 77篇 |
2017年 | 119篇 |
2016年 | 84篇 |
2015年 | 88篇 |
2014年 | 132篇 |
2013年 | 170篇 |
2012年 | 196篇 |
2011年 | 183篇 |
2010年 | 202篇 |
2009年 | 193篇 |
2008年 | 195篇 |
2007年 | 193篇 |
2006年 | 199篇 |
2005年 | 203篇 |
2004年 | 138篇 |
2003年 | 95篇 |
2002年 | 69篇 |
2001年 | 59篇 |
2000年 | 47篇 |
1999年 | 46篇 |
1998年 | 44篇 |
1997年 | 27篇 |
1996年 | 24篇 |
1995年 | 27篇 |
1994年 | 35篇 |
1993年 | 31篇 |
1992年 | 32篇 |
1991年 | 27篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 9篇 |
1987年 | 13篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1963年 | 1篇 |
排序方式: 共有3336条查询结果,搜索用时 78 毫秒
121.
播期对麦(油)后直播棉产量、品质及氮磷钾利用的影响 总被引:3,自引:0,他引:3
为明确长江流域下游棉区麦(油)后直播棉生产的适宜播期,本研究以?中棉所50‘为材料,于2017—2018年进行大田试验,研究不同播期(5月15日、5月25日、6月4日、6月14日和6月24日)对直播棉产量、品质及养分利用的影响。结果表明,播期推迟,棉花生育进程推迟;花铃期日均温和日照时数降低、有效积温先升高后降低且以6月4日播期花铃期有效积温最高。可见,前3个播期棉花花铃期温光条件较好。棉株与生殖器官的生物量及氮磷钾累积量随播期推迟降低,且前3个播期的棉株生物量和养分累积进入较高速率累积期早于后2个播期的棉株。随着播期的推迟,直播棉成铃数与皮棉产量显著降低。与5月15日播期相比,其余4个播期直播棉的皮棉产量分别降低15.9%~16.9%、13.1%~16.9%、26.9%~33.5%、58.2~62.0%。100kg皮棉氮、磷和钾吸收量随播期推迟而增加,但养分利用效率降低。不同播期间,以6月4日和6月14日播期棉纤维综合品质较优。综上,本试验条件下,5月15日—6月4日是长江流域棉区麦(油)后直播棉获得较高产量的适宜播期,该播期内适当推迟播种有利于棉花优质纤维的形成。 相似文献
122.
采用热磨法研磨纤维,研究不同纤维目数(40~100目,20~120目,10~130目)对纤维板力学性能和成本的影响。在保证制作质量合格的纤维板试件基础上,对其进行力学性能试验,验证确定纤维目数区间的合理性,并利用确定的纤维目数区间进行密度和施胶量的单因素对比分析;分析其对力学性能和成本的影响。研究结果表明:纤维目数在40~100目时,可通过降低试件密度或减小施胶量,在保证制品较好力学性能的前提下降低制备成本,可实现优质优价经济效益最大化;从纤维板制品制备成本分析得出:采用上述三种纤维目数区间,在保证制品质量要求的前提下,可实现纤维目数区间与制品质量等级的合理匹配,达到合理利用原材料降低成本的效果。 相似文献
123.
124.
Tiger flathead, Neoplatycephalus richardsoni (Castelnau), and sand flathead, Platycephalus bassensis Cuvier, are undifferentiated and managed with a common legal minimum length (LML). The Commonwealth Trawl Sector (CTS) and the Tasmanian Danish‐seine fishery (TDSF) use a minimum codend mesh sizes of 90 and 70 mm, respectively. The codend mesh size should be tailored to the LML, which is based on the length of first maturity of females (M50). This study found the length–girth relationship of N. richardsoni and P. bassensis was not significantly different. Using the covered codend method, these two species had 50% retention lengths (L50) of 294 ± 2 and 307 ± 3 mm, in 70‐mm and 90‐mm codends, respectively. L50 estimates for mesh sizes from this study and others produced a curvilinear relationship: y = 120 ln(x) ? 214, with an r2 of 0.8504. The size at maturity (M50) for female N. richardsoni was 337 mm, which is larger than the estimate for female P. bassensis (247 mm). There is a mismatch between the estimates of L50, the estimates of M50 and the LMLs in each fishery, leading to suboptimal exploitation of female Platycephalus. The model produced in this paper recommends a codend mesh size of 98 mm for both fisheries to exploit Platycephalus species sustainably. 相似文献
125.
Yong Li Katsutaro Yamamoto Tomonori Hiraishi Katsuaki Nashimoto Hiroyuki Yoshino 《Fisheries Science》2006,72(6):1147-1152
ABSTRACT: A field experiment was conducted in the Matsumae area of Hokkaido, Japan, during June and July 2002, to investigate the effects of different entrance designs on the catch efficiency of fish traps by fishing with commercial traps (entrance inclination angle [α] = 37°; funnel length of entrance [ L f ] = 22 cm) and experimental traps. The experimental traps were of the same size and similar design as commercial traps, with different entrance inclination angles (trap E1: α = 46°; E2: α = 27°; E3: α = 0°; all L f = 22 cm) or funnel lengths (E4: α = 37°, L f = 8 cm). In total, 2200 fish during 200 trap hauls were captured. The catch was significantly higher using both traps E2 and the commercial trap than with trap E3 ( P < 0.05), and the catch of trap E2 was higher than that of the commercial trap. There were no significant differences in mean fish body length or the frequency distributions of body length among trap types (E1, E2, E3 and commercial). The funnel length of the entrance also affected the catch of traps. Trap E4 had significantly higher catches than the commercial trap ( P = 0.04) when traps were deployed for a 1-day soak time. Fish body length frequency distributions did not differ between trap E4 and the commercial trap. The results showed that catch can be greatly affected by trap entrance designs. 相似文献
126.
从Nd3+:YAG脉冲激光器调Q技术出发,研究了不同腔长条件下电光调Q激光器输出脉冲的宽度、能量和峰值功率特征,得到了不同腔长的调Q激光器输出脉冲特性曲线。研究结果表明,激光器腔长的改变能较好地调节脉冲的输出特性,对脉冲激光器的灵活应用具有一定的指导意义。 相似文献
127.
128.
《Communications in Soil Science and Plant Analysis》2012,43(12):1391-1398
Abstract A short hand notation for depicting inped soil macropores is presented. By utilizing symbols for macropore diameter, facial shape, pore length, inped shape and pore orientation; it is possible to quickly portray inped field macropores. Circles, ellipses, and triangles of differing dimensions are used to symbolize round, elliptical, and irregular facial shaped pores of different diameters. The pore length and orientation are symbolized by line length and line direction. The inped shapes (spherical, tubular, and elongated) are respectively represented by one line, two parallel lines, or three parallel lines within or intersecting the pore shape symbols. Field application of this methodology saves time and is valuable when numerous inped pores need to be depicted. 相似文献
129.
The morphological basis of resistance to the whiteflyBemisia tabaci Genn. (Aleyrodidae: Hemiptera) was studied. The plant characters examined were leaf area, thickness of leaf lamina, hair
density, hair length, angle of insertion of leaf hair, and density of gossypol glands. Hair density and leaf thickness were
positively correlated with the population ofB. tabaci, and a positive correlation was obtained between the adult whitefly population and gossypol glands on stem internodes. Cotton
genotype USA-22 (sparsely hairy) was found to be more tolerant toB. tabaci than was genotype USA-13 (velvety hairy). The use of thinner and glabrous leaved cotton varieties is suggested to minimize
the whitefly menace in cotton. 相似文献
130.
Association of aster yellows subgroup 16SrI-B phytoplasmas with a disease of Rehmannia glutinosa var. purpurea 总被引:1,自引:0,他引:1
A new phytoplasma disease of Rehmannia glutinosa var. purpurea was observed in the Czech Republic in 1998. Infected plants showing severely proliferating shoots, leaves reduced in size with vein clearing and chlorosis, shortened internodes and virescent petals died in advanced stages of the disease. Electron microscopy examination of the ultra-thin sections revealed the presence of numerous polymorphic bodies in phloem tissue of leaf midribs and petioles. The disease was successfully transmitted from infected plant via a dodder bridge into periwinkle ( Catharanthus roseus ). The phytoplasma aetiology of this disease was further confirmed by polymerase chain reaction (PCR) using universal primers R16F2/R16R2. Restriction fragment length polymorphism (RFLP) analysis of amplification products indicated the presence of aster yellows related phytoplasmas (16SrI-B) in naturally infected samples of R. glutinosa var . purpurea and in symptomatic periwinkle after dodder transmission of the agent. A comparison of the amplified sequence with 17 sequences available in the GenBank confirmed the classification of the phytoplasma in the subgroup 16SrI-B. This is the first report of natural occurrence of phytoplasma-associated disease in R. glutinosa var. purpurea. 相似文献