首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2938篇
  免费   169篇
  国内免费   170篇
  2024年   11篇
  2023年   45篇
  2022年   83篇
  2021年   111篇
  2020年   96篇
  2019年   149篇
  2018年   101篇
  2017年   88篇
  2016年   76篇
  2015年   146篇
  2014年   212篇
  2013年   190篇
  2012年   137篇
  2011年   130篇
  2010年   95篇
  2009年   126篇
  2008年   150篇
  2007年   136篇
  2006年   142篇
  2005年   109篇
  2004年   115篇
  2003年   92篇
  2002年   66篇
  2001年   66篇
  2000年   46篇
  1999年   81篇
  1998年   64篇
  1997年   35篇
  1996年   42篇
  1995年   38篇
  1994年   44篇
  1993年   33篇
  1992年   34篇
  1991年   43篇
  1990年   28篇
  1989年   12篇
  1988年   14篇
  1987年   25篇
  1986年   12篇
  1985年   13篇
  1984年   6篇
  1983年   3篇
  1982年   9篇
  1981年   7篇
  1980年   2篇
  1979年   2篇
  1978年   6篇
  1977年   2篇
  1976年   2篇
  1974年   2篇
排序方式: 共有3277条查询结果,搜索用时 15 毫秒
51.
Fungal spores are an important component of library air   总被引:4,自引:0,他引:4  
The airborne fungal spore types were studied in different libraries in Delhi, using an Andersen sampler and a Burkard personal sampler, for culturable and non-culturable fungi respectively. The concentration inside the libraries, before and after the agitation of books, were compared with outside air. The major contributors to the library air areCladosporium, aspergilli/penicillia, smuts andAlternaria, varying from 50 to 14%. Some fungi (Cladosporium, Alternaria, smut,Penicillium chrysogenum andnigricans) showed seasonal occurrence, corresponding to their occurrence in the extramural environment. Aspergilli/penicillia,Drechslera, Curvularia andAspergillus flavus had a significantly higher concentration (P<0.01) inside the library, and recorded a significant increase in concentration after agitation of books. Air-conditioned libraries have low fungal spore concentrations, as compared to naturally ventilated libraries.  相似文献   
52.
Conserving biological diversity requires a major effort in conducting survey and inventories, establishing priorities, selecting protected areas, managing resources and monitoring the effects of management. Systematics has an important contribution to make to each of these five major activities. Further, the new Convention on Biological Diversity requires systematics information to support action under virtually all of its substantive conservation and sustainable use articles. It seems apparent that large reference collections contribute directly to development, and development assistance agencies should recognize that investing in maintaining these collections is a legitimate form of development assistance.  相似文献   
53.
兰属、兜兰属、石斛属植物叶片的扫描电镜观察   总被引:10,自引:0,他引:10  
对兰科植物的兰属、兜兰属及石斛属16个种折叶片及其横断面进行了扫描电镜的观察。兰属各种叶片上表皮细胞均为矩形,上表皮细胞表面具小乳突或不明显突起。石斛属及兜兰属的各个种上下表皮细胞均为多边形,但石斛属表皮细胞表面无坦无纹饰,而兜兰属花叶类上表皮细胞表面明显呈乳突状,绿叶类呈龟背状隆起。兰属及石斛属叶片叶肉组织没有栅栏组织及海绵组织的分化,而兜兰属的绿叶类叶肉不分化;花叶类叶肉有分化。  相似文献   
54.
Eighty-six species of fungi belonging to sixty-four genera were examined for their ability to metabolize naphthalene. Analysis by thin-layer and high pressure liquid chromatography revealed that naphthalene metabolism occurred in forty-seven species belonging to thirty-four genera from the major fungal taxa. All organisms tested from the order Mucorales oxidized naphthalene with species of Cunninghamella, Syncephalastrum and Mucor showing the greatest activity. Significant metabolism was also observed with Neurospora crassa, Claviceps paspali and four species of Psilocybe. The predominant metabolite formed by most organisms was 1-naphthol. Other products identified were, 4-hydroxy-1-tetralone, trans-1,2-dihydroxy-1,2-dihydronaphthalene, 2-naphthol, 1,2-and 1,4-naphthoquinone.  相似文献   
55.
Summary The Irish approach to the computer processing of grassland relevés is introduced. A suite of programs in Fortran IV has been developed for the purpose. The preparation and analysis of the relevés using these programs is explained. The cost and ecological effectiveness is discussed. It is concluded that the approach is not radically different to the familiar Braun-Blanquet technique of tabular comparison of relevés.Contribution from the Working Group for Data-Processing in Phytosociology, International Society for Vegetation Science.  相似文献   
56.
Summary Diallel crosses among Triticum boeoticum (4 lines from different geographical areas), T.urartu, Aegilops squarrosa and Ae. speltoides exhibited reciprocal differences in hybrid seed morphology, endosperm development, and embryo viability. T. urartu and Ae. squarrosa as females with T. boeotiaum and Ae. speltoides lead to shrivelled inviable seed. T.boeoticum accessions as female with Ae.speltoides also lead to shrivelled seeds. The reciprocal crosses produced plump seeds which either resembled the maternal parent or showed size differences. By altering the endospermic genome ratios, hybrid seeds with 1 (PF)/1 (PM) showed extreme shrivelling whereas those with 4 (PF)/1 (PM) were medium shrivelled to plump. Genetic experiments involving hybrids of T. boeoticum, T. urartu and T. monococcum showed that a factor is present in pollen or male gametes, which shows dosage effect and which, by interacting with the maternal genome, leads to endosperm abortion.  相似文献   
57.
Summary Endosperm protein components from common bread wheats (Triticum aestivum L.) and related species were extracted with aluminum lactate, pH 3.2, and examined by electrophoresis in the same buffer. Electrophoretic patterns of the albumins and globulins were compared to evaluate the possibility that a particular species might have contributed its genome to tetraploid or hexaploid wheat. Together with protein component mobilities, differential band staining with Coomassie Brilliant Blue R250 was employed to test the identity or non-identity of bands. Eight species and 63 accessions, representative of Triticum and Aegilops were tested. Considerable intraspecific variation was observed for patterns of diploid but not for tetraploid or hexaploid species. Patterns of some accessions of Triticum urartu agreed closely with major parts of the patterns of Triticum dicoccoides and T. aestivum. A fast-moving, green band was found in all accessions of T. urartu and of Triticum boeoticum, however, that was not found in those of T. dicoccoides or T. aestivum. This band was present in all accessions of Triticum araraticum and Triticum zhukovskyi. Patterns of Aegilops longissima, which has been suggested as the donor of the B genome, differed substantially from those of T. dicoccoides and T. aestivum. Finally, two marker proteins of intermediate mobility were also observed and may be used to discriminate between accessions of T. araraticum/T. zhukovskyi and those of T. dicoccoides/T. aestivum.  相似文献   
58.
作者对新疆维吾尔自治区的9个市、县进行了药用延胡索的资源调查,采集植物标本70余份,弄清药用种类主要是3种,即Corydalis glaucescens Regel,C.ledebouriana Kir.et Kar.和C.schanginii(Pall.)Fedtsch.;初步了解了3种药用延胡索植物的分布状况,并对块茎中总生物碱的含量作了分析。  相似文献   
59.
Summary We report a collection of 53 prototypic sequences representing known families of repetitive elements from the human genome. The prototypic sequences are either consensus sequences or selected examples of repetitive sequences. The collection includes: prototypes for high and medium reiteration frequency interspersed repeats, long terminal repeats of endogenous retroviruses, alphoid repeats, telomere-associated repeats, and some miscellaneous repeats. The collection is annotated and available electronically.[/ap ]Offprint requests to: J. Jurka  相似文献   
60.
Summary The construction and characterization of two genome-specific recombinant DNA clones from B. nigra are described. Southern analysis showed that the two clones belong to a dispersed repeat family. They differ from each other in their length, distribution and sequence, though the average GC content is nearly the same (45%). These B genome-specific repeats have been used to analyse the phylogenetic relationships between cultivated and wild species of the family Brassicaceae.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号