全文获取类型
收费全文 | 966篇 |
免费 | 40篇 |
国内免费 | 69篇 |
专业分类
林业 | 112篇 |
农学 | 68篇 |
基础科学 | 5篇 |
257篇 | |
综合类 | 376篇 |
农作物 | 64篇 |
水产渔业 | 30篇 |
畜牧兽医 | 100篇 |
园艺 | 54篇 |
植物保护 | 9篇 |
出版年
2024年 | 4篇 |
2023年 | 13篇 |
2022年 | 24篇 |
2021年 | 22篇 |
2020年 | 24篇 |
2019年 | 33篇 |
2018年 | 13篇 |
2017年 | 21篇 |
2016年 | 54篇 |
2015年 | 49篇 |
2014年 | 58篇 |
2013年 | 29篇 |
2012年 | 67篇 |
2011年 | 95篇 |
2010年 | 77篇 |
2009年 | 90篇 |
2008年 | 91篇 |
2007年 | 75篇 |
2006年 | 66篇 |
2005年 | 44篇 |
2004年 | 32篇 |
2003年 | 16篇 |
2002年 | 19篇 |
2001年 | 14篇 |
2000年 | 12篇 |
1999年 | 8篇 |
1998年 | 5篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 1篇 |
1990年 | 2篇 |
1989年 | 1篇 |
排序方式: 共有1075条查询结果,搜索用时 15 毫秒
51.
通过对绵阳市区各类绿地植物的调查,分析了城市绿地植物的种类构成和植物多样性特征。结果表明,绵阳市区植物种类丰富,共有植物72科,167个属,480种。其中人工栽培植物占49.6%,野生植物占50.4%;乡土植物占64.5%,外来植物占35.5%。乡土植物中的乡土栽培植物仅占14.1%,比例过低,缺乏地方风貌特色。绵阳城市各功能区的植物丰富度以草本植物最高,其次是灌木植物,乔木植物最低。植物多样性指数在乔木层、灌木层和草本层的多样性指数都是其他绿地最高。绵阳植物开花集中在春夏季,秋冬季节开花植物少,花色比例是红色∶白色∶黄色∶蓝紫色=8∶7∶3∶1。 相似文献
52.
Laccases of fungal origin have been intensively studied due to their importance in various biotechnological applications. There is a constant demand for new laccases with improved properties such as stability at higher temperatures or at an alkaline pH. Growing molecular evidence suggests that laccases may also be widespread in bacteria. While only a handful of bacterial laccases have been purified and characterized, several novel traits have already been discovered (e.g. pH-stability and 2-domain organization of the enzyme as opposed to the usual 3-domain structure of fungal laccases). The aim of this study was to examine the diversity of bacterial laccase-like genes in two types of high-organic peat soil using a cloning and sequencing approach. Gene libraries prepared of small fragments (150 base pairs) revealed an amazing diversity of bacterial laccases. The fragments clustered in 11 major lineages, and one third of the 241 sequences resembled laccase-like genes of Acidobacteria. Additionally, a new primer was used to retrieve several larger fragments of the putative bacterial laccase genes that spanned all four copper-binding sites. Both “conventional” 3-domain laccases and the recently described 2-domain small laccases have been obtained using this approach, demonstrating the potential of the primer. The present study thus contributes to the understanding of the diversity of bacterial laccases and provides a new tool for finding laccase-like sequences in bacterial strains and soil samples. 相似文献
53.
载畜率对荒漠草原植物群落影响的研究 总被引:1,自引:0,他引:1
对不同载畜率下短花针茅荒漠草原植物群落特征进行比较,结果表明:不同植物种群的盖度对载畜率的响应不同,无芒隐子草盖度在轻度放牧区最大,栉叶蒿盖度在7月份、8月份分别以中度放牧区和轻度放牧区最大;短花针茅、冷蒿、阿氏旋花、细叶葱、木地肤随载畜率增大高度下降明显,猪毛菜高度在四个载畜率下差异均不显著;随着载畜率的增大,植物群落生物量呈下降趋势,且重度放牧与不放牧、轻度放牧有显著差异;随着载畜率的增大,建群种、优势种地位改变不明显,但一年生的猪毛菜重要值增大,在重度放牧区达到最大并占据优势地位;Shannon-wiener指数、Simpson指数、Pielou均匀度指数均以轻度放牧区最大,Shannon-wiener指数、Simpson指数均为轻度放牧>中度放牧>重度放牧>不放牧,且不放牧与轻度放牧差异显著。 相似文献
54.
55.
Development of Core Subset of Finger Millet Germplasm Using Geographical Origin and Data on 14 Quantitative Traits 总被引:6,自引:0,他引:6
Hari D. Upadhyaya C. L. L. Gowda R. P. S. Pundir V. Gopal Reddy Sube Singh 《Genetic Resources and Crop Evolution》2006,53(4):679-685
Finger millet [Eleusine coracana (L.) Gaertn.] is an important cereal food crop in Africa and South Asia. It is a hardy crop that can be grown in very diverse
environments from almost at sea level to about 2400 m.a.s.l. Finger millet has an excellent food value as its seeds contain
protein ranging from 7 to 14% and are particularly rich in methionine amino acid, iron, and calcium. Despite all these merits,
this crop has been neglected from the main stream of crop improvement research. One of the means to boost its production and
productivity is to enhance utilization of finger millet germplasm to breed superior varieties. Keeping this objective in view,
a core subset of finger millet germplasm (622 accessions) based on origin and data on 14 quantitative traits was developed
from the entire global collection of 5940 accessions held in the genebank at ICRISAT, Patancheru, India. The comparison of
means, variances, frequency distribution, Shannon-Weaver diversity index (H′) and phenotypic correlations indicated that the
core subset represents the entire collection. These tests indicated that sampling was optimal and the diversity has been captured
very well in the core subset. The correlation analysis indicated that panicle exsertion and longest finger length could be
given lower priority in the future germplasm evaluation work of finger millet. 相似文献
56.
M.?Vetel?inenEmail author E.?Gammelg?rd J.?P.?T.?Valkonen 《Genetic Resources and Crop Evolution》2005,52(8):999-1010
The management of genebank collections of cultivated potato is costly due to the need for in vitro maintenance and virus eradication. Therefore, it is important to set up conservation strategies, which prevent duplicates
entering the collections. In this study, 32 Nordic potato landraces were studied for 57 morphological traits and analysed
for amplified fragment length polymorphism (AFLP). Most landraces could be distinguished based on the morphological characters,
except five accessions. Using five primer combinations to generate 114 reproducible AFLPs, of which 63 (55%) were polymorphic,
the five morphologically indistinguishable accessions were placed into two groups with identical AFLP patterns, suggesting
that some of the accessions were redundant for long-term preservation. The AFLP data showed that the Nordic collection of
potato landraces is composed of genetically different clones, and morphological analysis revealed a wide range of variability.
This variability seems to be distributed randomly over the Nordic region since the cluster analysis based on AFLPs and morphological
traits revealed no grouping based on the country of origin. Principal component analysis suggests that fewer morphological
traits than used in this study will be sufficient to discriminate between different genotypes of cultivated potato (Solanum tuberosum L.). Future possibilities for rationalising potato collections are discussed. 相似文献
57.
N.R. Maslen 《Soil biology & biochemistry》2006,38(10):3141-3151
Nematode worms are one of the most important soil faunal groups in Antarctica. However, relatively little is known about their wider distribution, biogeography and history in the region, and taxonomic information remains confused or incomplete. Here, we hypothesise that the Alexander Island (southern maritime Antarctic) fauna includes elements that have survived (at least) the period of Pleistocene glaciation in situ, forming a regional centre of endemism and biodiversity hotspot. We describe nematological surveys carried out across a latitudinal gradient between 68 and 77°S along the southern Antarctic Peninsula, comparing the data obtained with the maritime Antarctic fauna described in the few previous studies between northern Marguerite Bay and the South Orkney Islands (60-68°S). In general, our survey supports previous findings of a lack of overlap at species level between the maritime and continental Antarctic biogeographical zones, with the large majority of specimens obtained from all survey sites being attributable to known maritime or new and currently endemic taxa. However, collections from Alexander Island, Alamode Island and the most westerly site sampled, Charcot Island, include specimens morphologically very close to two known continental Antarctic species, which may indicate a link between the two regions. The fauna obtained at the northern study sites (ca. 68°S, Adelaide Island, Marguerite Bay) closely matches that described previously. However, in contrast with widely described patterns of decreasing diversity in other Antarctic biota, species richness increased markedly at locations on Alexander Island (ca. 72°S), including a substantial element of undescribed species (50% of taxa across all locations, 40% of taxa found on Alexander Island). Finally, the most southerly samples obtained, from inland nunataks in Ellsworth Land (75-77°S), indicate a fauna that does not include nematodes, which is exceptional not only in an Antarctic context but also for soils worldwide. 相似文献
58.
M. B. Barry J. L. Pham B. Courtois C. Billot N. Ahmadi 《Genetic Resources and Crop Evolution》2007,54(8):1675-1690
Rice genetic diversity partitioning between farms, varieties and, within-variety diversity, were analysed in two villages
of Maritime Guinea with contrasted agroecological conditions. One thousand and two hundred individual plants belonging to
45 accessions collected in eight farms were genotyped using 10 SSR markers. The molecular variance was evenly shared between
and within accessions, while the farm effect was almost nil. Local varieties had a multi-line genetic structure. The number
of multilocus genotypes was proportional to the utilisation rate of the variety in the village. The F
ST values between different accessions of each variety were significant which indicated low genetic consistency in the variety
names. This varietal structure could mainly be explained by the migration phenomenon and the high varietal turnover. Compared
to allelic diversity, multilocus genotypic diversity seemed to be the most suitable indicator of the quantitative distribution
of diversity at different management scales (accession, farm and village). The within- and between-farm F
ST values were in the same order of magnitude. The within-farm diversity was not farm-specific but quantitatively high, i.e.
up to 50% of the total genotypic diversity of a given village. Given the relative importance of the within-variety diversity,
the in situ approach stands out as the most effective solution. As farms do not host specific diversity the in situ approach
could be implemented by working with a small number of farms. 相似文献
59.
Plant conservation: old problems, new perspectives 总被引:1,自引:0,他引:1
Vernon H Heywood 《Biological conservation》2003,113(3):321-335
Nature conservation has changed from an idealistic philosophy to a serious technology(J. Harper, 1992)A review is given of the major conceptual changes that have taken place during the last 50 years in our understanding of the nature of plant conservation and of the principal methodological advances in undertaking conservation assessments and actions, largely through the incorporation of tools and techniques from other disciplines. The interrelationships between conservation and sustainable use are considered as well as the impact of the development of the discipline of conservation biology, the effects of the general acceptance of the concept of biodiversity and the practical implications of the implementation of the Convention on Biological diversity. The effect on conservation policy and management of the accelerating loss or conversion of habitats throughout the world and approaches for combating this are discussed. 相似文献
60.
Werner Ulrich 《Pedobiologia》2004,48(1):59-69
Community structures and local diversity patterns of parasitic Hymenoptera with soil and leaf litter hosts were studied in a German beech forest and a meadow. Hymenoptera appeared to be one of the most species-rich taxa associated with the soil. Eighty-eight species were found in the meadow (total density of 128 ind. m−2 yr−1) and 188 species (149 ind. m−2 yr−1) in the forest. The mean parasitism rates were above 60% for parasitoids of mycetophagous Diptera and between 7% and 26% for parasitoids of saprophagous Diptera. Species overlap between both habitats was higher than expected from a random sample model. Species common to both habitats were primarily parasitoids of predatory Coleoptera. High mean densities of these species support the hypothesis of a positive correlation between local abundance and range size. 相似文献