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   1708篇
  免费   98篇
  2023年   8篇
  2022年   10篇
  2021年   14篇
  2020年   20篇
  2019年   20篇
  2018年   18篇
  2017年   22篇
  2016年   30篇
  2015年   54篇
  2014年   65篇
  2013年   90篇
  2012年   102篇
  2011年   109篇
  2010年   61篇
  2009年   67篇
  2008年   97篇
  2007年   91篇
  2006年   85篇
  2005年   95篇
  2004年   100篇
  2003年   98篇
  2002年   79篇
  2001年   31篇
  2000年   20篇
  1999年   36篇
  1998年   35篇
  1997年   18篇
  1996年   23篇
  1995年   24篇
  1994年   23篇
  1993年   36篇
  1992年   27篇
  1991年   28篇
  1990年   16篇
  1989年   12篇
  1988年   10篇
  1987年   21篇
  1986年   16篇
  1985年   11篇
  1983年   10篇
  1982年   12篇
  1981年   9篇
  1980年   5篇
  1979年   16篇
  1978年   3篇
  1977年   8篇
  1975年   2篇
  1974年   3篇
  1973年   2篇
  1972年   6篇
排序方式: 共有1806条查询结果,搜索用时 15 毫秒
31.
Acacia ochracea , a new species in the A. Senegal complex, is described and illustrated. It is a common and conspicuous tree over large areas in the western Bay Region and in the Gedo Region in SW Somalia.  相似文献   
32.
The following new species are described: Chamaecrista dunensis (Somalia), Acacia densispina (Somalia), A. flagellaris (Somalia), Tephrosia karkarensis (Somalia), T. scopulata (Somalia), Indigofera ancistrocarpa (Kenya), I. curvirostrata (Ethiopia), I. hiranensis (Somalia), I. karinensis (Somalia), I. kucharii (Somalia), I. nugalensis (Somalia), I. pellucida (Kenya, Somalia), I. sabulosa (Ethiopia, Somalia), I. sessilis (Somalia), I. ienuirostris (Somalia), Rhynchosia megalocalyx (Somalia), Crotalaria allophylla (Somalia), C. intricata (Somalia), C. warfae (Somalia), and Coronilla somalensis (Somalia). 2n = 22 is reported in Tephrosia karkarensis. Cassia somalensis, Acacia puccioniana, Sophora somalensis , and Crotalaria sennii are reduced to synonyms of respectively Senna hookeriana, Dichrostachys kirkii, Millettia usaramensis and Crotalaria boranica subsp. trichocarpa . Thirty-seven taxa are reported from Somalia for the first time, including Ptycholobium biflorum , known previously only from Southern Africa. Crotalaria pieropoda , known previously only from Socotra, is reported from Somalia and Oman. Neptunia oleracea and Vatovaea pseudolablab are new generic records for respectively Ethiopia and Oman. The fruits of Indigofera boranica are described. Vigna somaliensis and an allied species are discussed. Rhynchosia gansole is lectotypified and its flowers and fruits are described.  相似文献   
33.
34.
The Greater Bilby has shown a rapid decline in range during this century and now occupies only a small isolated area in south-western Queensland (QLD) and a larger, but mostly low-density area in the north-western deserts of the Northern Territory (NT) and Western Australia (WA). We have examined variation in the control region of mitochondrial DNA (mtDNA) and at nine microsatellite loci in order to investigate the extent of current and historical subdivision across the species range, and to provide a preliminary assessment of genetic structuring and mating system on a finer scale within the QLD population. Both mtDNA and microsatellite loci had substantial variation within and among populations, with mtDNA divergence being greater between QLD and NT than between NT and WA. The QLD population had two unique and divergent mtDNA lineages, but there was no evidence for strong phylogeographical structure across the range. The available evidence suggests that the bilby should be considered as a single Evolutionarily Significant Unit consisting of multiple Management Units. Augmentation of the remnant QLD population from the NT does not appear necessary at this stage, at least not on genetic grounds. Finer-scale analysis of microsatellite variation for two QLD colonies revealed a deficiency of heterozygotes and significantly greater relatedness within than between colonies. However, structuring was observed only for males; relatedness values for females did not depart from those expected under panmixia. Parentage exclusion analysis for one colony allowed the construction of a partial pedigree which indicated strong polygyny, with one male fathering all but one of the eight offspring assigned. The extent to which fine-scale genetic structuring and differences between sexes is due to sex-biased dispersal vs. effects of mating system remain to be determined.  相似文献   
35.
The mainly neotropical genus Calliandra Benth. (Leguminosae–Mimosoideae–Ingeae) is reported for the first time as native in continental Africa. There are two species known, C. gilbertii Thulin & Hunde sp. nov. in E Kenya and Somalia and C. redacta (J. H. Ross) Thulin & Hunde comb. nov. in South Africa near the Namibia border. On pollen morphological grounds they are supposed to be most related to certain neotropical species but the pollen does not agree entirely with any previously known group of the genus. The distribution of C gilbertii and C. redacta stresses the phytogeographical link between the arid zones of northern and southern Africa.  相似文献   
36.
37.
Anoura geoffroyi (Chiroptera, Phyllostomidae, Glossophaginae), Geoffroy's hairy-legged long-tongued bat, were collected from September 1984 to August 1985, and these bats were found to breed seasonally in the wild on Trinidad, West Indies, at 10 degrees N latitude. Histological examination of these samples indicated that females became pregnant in July or August, and young were born in late November or early December. The testes and epididymides were small from September to mid-April, increased threefold in weight between mid-April and late May, reached a peak weight in July, and decreased in weight in August. Spermatogenesis occurred throughout the testes of males captured from May to August. In 1990, the timing of parturition in females that gave birth in the laboratory to young conceived in the wild was similar to the timing in the field in 1984-1985. Groups of 10-13 males were subjected in the laboratory to (i) a gradually changing, civil twilight photoperiod that mimicked the natural cycle of annual change at 10 degrees N latitude, (ii) the same gradually changing cycle of photoperiod accelerated to a six-month period, or (iii) a constant photoperiod (light 12:54 h: dark 11:06 h). These treatments began in mid-December, four months before the initiation of testicular recrudescence in the wild. In all three groups, testicular volume remained low until April, and then increased two- to threefold between late April and late June, rising to a peak in July, as occurred in the wild.(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
38.
Summary A certain proportion of protein S7 exists in an altered form in E. coli rpsD (S4) mutants. Depending on the type of S4 mutation involved, two different forms of the altered S7 can be distinguished. The unusual form is longer than normal S7 by about 500 daltons due to extra material at the carboxyl end of the protein. It is suggested that a mutationally altered S4 might lower the efficiency of termination during translation of the messenger for S7. This results in an increased frequency of translational read-through, which gives the observed longer forms of S7. Data are interpreted to mean that one class of S4 mutants might suppress UGA and UAG whereas another class only suppresses UGA.  相似文献   
39.
Physical and chemical criteria of lipoproteins containing apolipoprotein B, extracted from human aortic intima, were compared with those of plasma low density lipoproteins (LDL). Homogenates of grossly normal intima and advanced atherosclerotic lesions were subjected to differential ultracentrifugation to isolate a d = 1.006--1.063 g/ml density fraction which was extensively characterized. By electroimmunoassay, over 90% of the recovered apolipoprotein B immunological reactivity was found in isolates from both plaques and normal intima. In isolates of plaque and normal intima, particles of the same size as LDL were found, although a small population of very large structures was also present in plaque fractions. Apolipoprotein composition was similar to that of plasma LDL except for the presence of human serum albumin in aortic isolates. Fractions from aorta demonstrated greater electrophoretic mobilities than LDL. The lipid composition of isolates from normal intima was similar to that of LDL. The lipid composition of plaque fractions showed a significant decrease in the cholesteryl ester to free cholesterol ratio and in the triglyceride content in comparison to LDL and to fractions from normal intima. The fatty acid pattern of the cholesteryl ester fraction from isolates of both normal and plaque aortic homogenates demonstrated a significant decrease in the linoleate to oleate ratio as compared to LDL. Our initial studies suggest that althought aortic fractions are similar to LDL by certain criteria, some differences observed are more pronounced in fractions from lesions than from normal intima.  相似文献   
40.
Pavonia friisii , sp. nov., from south-eastern Ethiopia and south-central and southern Somalia, P. nigrescens , sp. nov., from south-central and southern Somalia, P. matteiana , sp. nov., from south-central Somalia, P. longipilosa , sp. nov., from eastern Ethiopia, P. rotundifolia , sp. nov., from eastern Ethiopia and northern and central Somalia, P. marginata , sp. nov., from central Somalia, and P. paucibracteata , sp. nov., from central Somalia, are described and illustrated.  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