全文获取类型
收费全文 | 25625篇 |
免费 | 1429篇 |
国内免费 | 3111篇 |
专业分类
林业 | 1574篇 |
农学 | 2758篇 |
基础科学 | 1148篇 |
7473篇 | |
综合类 | 10221篇 |
农作物 | 2256篇 |
水产渔业 | 935篇 |
畜牧兽医 | 2068篇 |
园艺 | 652篇 |
植物保护 | 1080篇 |
出版年
2024年 | 155篇 |
2023年 | 563篇 |
2022年 | 952篇 |
2021年 | 1050篇 |
2020年 | 1107篇 |
2019年 | 1196篇 |
2018年 | 945篇 |
2017年 | 1429篇 |
2016年 | 1612篇 |
2015年 | 1186篇 |
2014年 | 1410篇 |
2013年 | 1968篇 |
2012年 | 2159篇 |
2011年 | 1938篇 |
2010年 | 1487篇 |
2009年 | 1471篇 |
2008年 | 1207篇 |
2007年 | 1350篇 |
2006年 | 1215篇 |
2005年 | 946篇 |
2004年 | 690篇 |
2003年 | 535篇 |
2002年 | 427篇 |
2001年 | 379篇 |
2000年 | 317篇 |
1999年 | 319篇 |
1998年 | 270篇 |
1997年 | 237篇 |
1996年 | 224篇 |
1995年 | 253篇 |
1994年 | 164篇 |
1993年 | 180篇 |
1992年 | 152篇 |
1991年 | 134篇 |
1990年 | 122篇 |
1989年 | 115篇 |
1988年 | 86篇 |
1987年 | 61篇 |
1986年 | 47篇 |
1985年 | 18篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1981年 | 11篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1962年 | 7篇 |
1955年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 468 毫秒
991.
Summary The hybrid orchid clones of Aranda Christine show differential meiotic behaviour with respect to meiotic irregularities in the form of non-congressing chromosomes, laggards and complement fractionation. These irregularities occur in significantly variable but high frequencies, which results in the observed sterility of these hybrids. The observed differential meiotic behaviour is probably due to the transmission of a variable set of chromosomes from the third generation hybrid parent, Vanda Hilo Blue. Deviant spores, mainly polysporous and polyploid, are the end products of the meiotic aberrations. Such spores appear to be produced regularly in the intergeneric orchid hybrids.Meiosis in one of the Aranda Christine clones, C80, was found to be highly aberrant and could be unique among orchid hybrids. 相似文献
992.
Summary Variation in pigment content of the flour of bread wheats (Triticum aestivum L.) was studied in the progenies of F1 and F2 of three crosses and their reciprocals. Reciprocal differences in pigment content were observed in the F1 and F2 means. Low pigment content was found to be partially dominant or over dominant in the crosses studied. There was evidence of substantial mid-parent F1 heterosis in all crosses and betterparent F1 heterosis in three crosses. In the F2, heritability estimates were moderate to high. The F2 frequency distributions were not normal. Estimation of effective factor pairs indicated the presence of one or two major gene pairs involved in the expression of pigment content in the flour. Action of modifiers was also assumed in one cross and its reciprocal. A factorial approach to metrical character suggested that the F2 segregation ratios of low pigment content to high pigment content were 3:1, 15:1, 13:3 and 9:7 for the different crosses. Utilization of the findings in a wheat breeding program is briefly discussed. 相似文献
993.
S. K. Yau M. Bounejmate J. Ryan R. Baalbaki A. Nassar R. Maacaroun 《European Journal of Agronomy》2003,19(4):599-610
In arid and semi-arid areas of West Asia and North Africa, including the northern Bekaa Valley of Lebanon, farmers have been increasingly practicing continuous barley cultivation. The objectives of the study were to (1) determine whether barley monoculture is unsustainable1, (2) ascertain if barley and total dry matter yields can be increased and sustained by including a legume crop in the rotation, and (3) determine which barley–legume rotations are more productive. The trial was set up in a randomised complete block design with two replicates under rain-fed conditions in 1994–1995 at the Agricultural Research and Educational Center (33°56′ N, 36°5′ E, 995 m above sea level). Eight two-phase barley-based rotations were compared: barley in rotation with barley, lentil, common vetch, bitter vetch, common vetch for grazing, medics for grazing, common vetch for hay, and common vetch with barley for hay. Seed and straw were harvested from barley and legumes in the first four rotations. Relative to the trial mean, seed and straw yield under barley monoculture slumped in 1997–1998 and did not recover since then. Infestation of wild barley was a cause of this yield decline. Barley–legume rotations yielded 44–80% more barley grain and 27–53% more barley straw than the barley monoculture over the 6 years (1995–1996 to 2000–2001). Furthermore, in the legume phase, common and bitter vetch gave higher seed yield than barley monoculture. Thus, all barley–legume rotations, except barley–medics, yielded more total dry matter than barley monoculture on the basis of per rotation cycle. Among the barley–legume rotations, the barley-common vetch for seed rotation gave the highest and most stable dry matter yield. In conclusion, barley monoculture was unsustainable, but barley yields could be increased and sustained by including legumes in the rotation. Farmers in semi-arid areas of Lebanon should discontinue practicing barley monoculture and adopt a barley–legume, such as common vetch, rotation. 相似文献
994.
Using a Chlorophyll Meter to Optimize Nitrogen Fertilizer Application for Intensively-Managed Small-Grain Cereals 总被引:1,自引:0,他引:1
Most methods used to estimate N status require collection, processing and analysis of soil and/or plant tissue samples. In this study, leaf chlorophyll content of cereals, determined in the field with a portable chlorophyll meter (Minolta SPAD-502), was used to predict N availability of wheat ( Triticum aestivum L.), barley ( Hordeum vulgare L.), oats ( Avena sativa L.) and rye ( Secale cereale L.). The results indicated that accurate nitrogen (N) fertilizer recommendations based on chlorophyll content have both environmental and economic advantages. Determination of leaf chlorophyll content with a chlorophyll meter accurately indicated plant N status allowing N fertilizer requirement to be accurately determined and resulting in increased N uptake efficiency. Applying N fertilizer on this basis improved production economics and improved physical input-output ratio during grain yield formation. Using a chlorophyll meter, responsive and non-responsive categories can be determined at the stage of maximum number of florets per ear primordium (Zadoks' GS 37-41) and at pollination (i.e. pollen grains on well-developed stigmatic hairs, GS 52-58). 相似文献
995.
New aspects of soybean somatic embryogenesis 总被引:4,自引:0,他引:4
Somatic embryo formation from immature cotyledons was improved in the following ways: by cutting into sections, supplementing culture media with spermine and using solid/liquid/solid type of culture. Cut cotyledons of the eight genotypes examined expressed a higher ability for somatic embryogenesis than whole cotyledons. Of the three polyamines tested, spermine considerably stimulated and putrescine slightly inhibited induction of somatic embryos. The ability of embryoid formation on medium with spermidine depended on the genotype. The solid/liquid/solid type of culture was better than the continuous solid culture. The best nitrogen ion content for the subculture of somatic embryos was 10 mM NH4NO3 and 30 mM KNO3. The possibility of using these modifications in Agrobacterium transformation is discussed. 相似文献
996.
997.
Overall effects of HRT ( 11 h, 9 h, 8 h, 6 h ) on the Spiral up-flow reactor (SUFR) system are analyzed. The results of the operation under different HRT condition shows that the removal efficiencies of COD, TN and TP decreased not obviously with decreasing HRT when HRT is longer than 8h. Anaerobic retention time, anoxic retention time and aerobic retention time were 1.51 h,2.25 h and 4.24 h,respectively. The removal efficiencies of COD, TN and TP is kept over 93%,87% and 90.2%, respectively. Although the removal efficiencies changed obviously when the HRT was 6 h. Anaerobic retention time, anoxic retention time and aerobic retention time are 1.13 h,1.69 h and 3.18 h,respectively. COD-removal efficiency is always up to 85%, the removal efficiencies of TN and TP is kept over 76% and 72%,also has upper removal efficiencies. It is indicated that the SUFR system, which is similar to plug-flow-reactor, has strong stability and can compensate negative effects of increasing hydraulic loading. 相似文献
998.
通过大量试验研制出与国际法(GB10463-89)配套的仪器-玉米粉皮胚含量测定器,并对国际法进行了适当改进,提高了方法的准确度。 相似文献
999.
Response of Rapeseed (Brassica napus L.) Growth, Yield, Oil Content and Its Fatty Acids to Nitrogen Rates and Application Times 总被引:1,自引:0,他引:1
A. F. Ibrahim E. O. Abusteit El-M. A. El-Metwally 《Journal of Agronomy and Crop Science》1989,162(2):107-112
A steady and progressive increase in rapeseed yield was observed with each increment in applied nitrogen rates up to 213 kg/ha in both seasons. As for nitrogen application times, the analyzed data showed that adding a split dose (either 1/2 or 1/3) before the third irrigation was a common part between high yielding treatments in 1985/86 season. Nitrogen rates X application times interaction affected rapeseed yield significantly during the first winter season. The highest seed yield of 2.5 t/ha was obtained by adding 213 kg N/ha in two split doses at sowing and just before the third irrigation. The second yield value of 2.47 t/ha was produced under the same N rate when applied in two split doses before second and third irrigation. However, in the second season (1986/87), rapeseed plants did not exhibit significant responses to nitrogen rates X application times interaction.
Chemical analyses showed that rapeseed oil content and its fatty acids (Palmetic, Stearic, Oleic, Linoleic, Linolenic, Arachidic and Erucic) percentages were not significantly affected by either nitrogen rates or application times in both seasons. A very low content of Erucic acid (0.1–0.9 %) in all tested sample was noticed. 相似文献
Chemical analyses showed that rapeseed oil content and its fatty acids (Palmetic, Stearic, Oleic, Linoleic, Linolenic, Arachidic and Erucic) percentages were not significantly affected by either nitrogen rates or application times in both seasons. A very low content of Erucic acid (0.1–0.9 %) in all tested sample was noticed. 相似文献
1000.