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   2500篇
  免费   81篇
  国内免费   13篇
  2594篇
  2015年   16篇
  2014年   14篇
  2013年   57篇
  2012年   60篇
  2011年   53篇
  2010年   64篇
  2009年   111篇
  2008年   81篇
  2007年   63篇
  2006年   60篇
  2005年   53篇
  2004年   50篇
  2003年   27篇
  2002年   19篇
  2001年   17篇
  2000年   19篇
  1999年   31篇
  1998年   36篇
  1997年   43篇
  1996年   36篇
  1995年   27篇
  1994年   23篇
  1993年   20篇
  1992年   34篇
  1991年   15篇
  1990年   22篇
  1989年   22篇
  1988年   19篇
  1987年   21篇
  1986年   20篇
  1985年   21篇
  1984年   14篇
  1983年   20篇
  1981年   21篇
  1980年   14篇
  1979年   20篇
  1977年   15篇
  1971年   18篇
  1959年   49篇
  1958年   123篇
  1957年   137篇
  1956年   134篇
  1955年   149篇
  1954年   150篇
  1953年   119篇
  1952年   104篇
  1951年   98篇
  1950年   59篇
  1949年   14篇
  1948年   23篇
排序方式: 共有2594条查询结果,搜索用时 15 毫秒
71.
72.
ABSTRACT Depredation of livestock by large carnivores is an important but poorly understood source of human-carnivore conflict. We examined patterns of livestock depredation by jaguars (Panthera onca) and pumas (Puma concolor) on a ranch-wildlife reserve in western Brazil to assess factors contributing to prey mortality. We predicted jaguars would kill a greater proportion of calves than yearling and adult cattle and that proximity to suitable habitat would increase mortality risk. We further speculated that exposure to predation risk would promote livestock grouping and increased movement distance. We recorded 169 cattle mortality incidents during 2003–2004, of which 19% were due to predation by jaguars and pumas. This level of mortality represented 0.2–0.3% of the total livestock holdings on the ranch. Jaguars caused most (69%) cattle predation events, and survival in allotments was lower for calves than for other age classes. Forest proximity was the only variable we found to explain patterns of livestock mortality, with predation risk increasing as distance to forest cover declined. Due to low predation risk, cattle movement patterns and grouping behavior did not vary relative to level of spatial overlap with radiocollared jaguars. The overall effect of predation on cattle was low and livestock likely constituted an alternative prey for large cats in our study area. However, selection of calves over other age cohorts and higher predation risk among cattle in proximity to forest cover is suggestive of selection of substandard individuals. Cattle ranchers in the Pantanal region may reduce cattle mortality rates by concentrating on losses due to nonpredation causes that could be more easily controlled.  相似文献   
73.
Evaluation of PAPNET-assisted cervical rescreening
We have compared the results of targeted manual rescreening of 1211 randomly selected smears with the results of PAPNET-assisted rescreening of 1613 cervical smears, containing at least 6.3% low-grade squamous intraepithelial lesion (SIL). PAPNET diagnosis and the targeted rescreening diagnosis were compared with the initial report, issued on the corresponding smear. Reproducibility scores for inadequacy, presence of endocervical and endometrial cells, specific infections and squamous cell abnormalities were determined. The reproducibility scores for the diagnosis of inadequate smears and specific infections were lower with the PAPNET-assisted rescreening. The detection of squamous cell abnormalities was excellent for both methods (>0.95), with a higher detection rate for false-negative smears with the PAPNET testing system.  相似文献   
74.
clonality V.0.4 is a program for testing heterozygosity-genet size relationships in clonal organisms using a randomization procedure. The software has been developed under the Borland Delphi developing environment and a Windows-executable version is freely downloadable from http://gemi.mpl.ird.fr/SiteSGASS/Prugnolle/ClonalityPage.html. The program compares the observed F(IS) of the population with the F(IS) expected if genets (multilocus genotypes present in multiple copies within the population) were chosen randomly from the set of different multilocus genotypes. The randomization procedure is performed with the same number of genets and the same number of repetitions per genet as what is observed in the original data set.  相似文献   
75.
Maas, F. M., De Kok, L. J., Peters, J. L. and Kuiper, P. J.C. 1987. A comparative study on the effects of H2S and SO2 fumigationon the growth and accumulation of sulphate and sulphydryl compoundsin Trifolium pratense L., Glycine max Merr. and Phaseolus vulgarisL.—J. exp. Bot. 38: 1459-1469. The effects of 0—25 mm3 dm3 H2S and SO2 on growth andsulphur content of shoots of Trifolium pratense, Glycine maxand Phaseolus vulgaris were studied. After 2 weeks of fumigationthe yield of T. pralense was reduced by 32% by H2S, but notaffected by SO2. Yield of G. max was not affected by H2S, butreduced by 20% by SO2, whereas that of P. vulgaris was increasedby 11% by H2S and not affected by SO2. Increases in sulphydrylcontent were already observed after 24 h of exposure to H2Sand SO2 in all plants. The increase was greatest in T. pratenseand smallest in P. vulgaris and, except for T. pratense, alwaysgreater in the H2S-exposed than the SO2-exposed plants. Oneday of exposure resulted in an increase in sulphate contentonly in the SO2-fumigated plants, with the highest accumulationin T. pratense and the lowest in P. vulgaris. After 2 weeksan increase in sulphate content was also observed in the H2S-exposedplants. This increase was also highest in T. pratense and lowestin P. vulgaris. Transpiration rate was not affected by a 24 h exposure to H2Sor SO2 and was highest in T. pratense, intermediate in G. maxand lowest in P. vulgaris. The order of theoretical rates of deposition of H2S and SO2correlated with the observed increases in sulphydryl contentduring the first 24 h of exposure in both H2S and SO2-fumigatedplants and with the increase in sulphate content in the SO2-exposedplants. The increases in sulphydryl content were only 8% ofthe theoretical H2S and SO2-deposition fluxes, whereas sulphateaccumulation accounted for at least 57% of the theoretical SO2-depositionflux. Key words: Air pollution, clover, French bean, Glutathione, Soybean, sulphur metabolism.  相似文献   
76.
In Argentina, 58.2% out of the 8126 Cutaneous Leishmaniasis (CL) incident cases accumulated from 1954 to 2006 were reported in the provinces of Salta and Jujuy. The aim of this study was to develop an exploratory risk map and a potential distribution map of the vector, in order to offer recommendations for CL prevention. A total of 12 079 Phlebotominae (Diptera: Psychodidae) belonging to the species Lutzomyia neivai (Pinto), Lu. migonei (França), Lu. cortelezzii (Brèthes), Lu. shannoni (Dyar), Lu. quinquefer (Dyar) and Brumptomyia spp. (França & Parrot) were captured. Potential distribution models were created for two species, Lu. neivai (incriminated vector of Leishmania braziliensis) and Lu. migonei, associated with domestic animals in Argentina and that in turn could be involved as a link between zoonotic transmission cycles and anthropozoonotic. The Maximum Entropy Modeling System (MaxEnt) was used. The Jackknife test was performed, and the ‘rainfall of the driest month’ was the variable that best generalized the models. Accuracy was evaluated by the area under the curve (AUC) and validated by the Cohen's kappa index. This approximation provides a new analytical resource of high potential for the prevention of the disease, in order to allocate resources properly and to develop the most suitable strategies for action.  相似文献   
77.
Caesalpinia echinata and C. ferrea var. ferrea have different seed behaviours and seed and fruit types. Comparison of the seed ontogeny and anatomy partly explained the differences in seed behaviour between these two species of Brazilian legumes; some differences were also related to fruit development. The seed coat in C. ferrea consisted of two layers of osteosclereids, as well as macrosclereids and fibres, to form a typical legume seed coat, whereas C. echinata had only macrosclereids and fibres. In C. echinata , the developing seed coat had paracytic stomata, a feature rarely found in legume seeds. These seed coat features may account for the low longevity of C. echinata seeds. The embryogeny was similar in both species, with no differences in the relationship between embryo growth and seed growth. The seeds of both species behaved as typical endospermic seeds, despite their different morphological classification (exendospermic orthodox seeds were described for C. echinata and endospermic orthodox seeds for C. ferrea ). Embryo growth in C. ferrea accelerated when the sclerenchyma of the pericarp was developing, whereas embryonic growth in C. echinata was associated with the conclusion of spine and secretory reservoir development in the pericarp. Other features observed included an endothelial layer that secreted mucilage in both species, a nucellar summit, which grew up into the micropyle, and a placental obturator that connected the ovarian tissue to the ovule in C. ferrea . © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society , 2004, 146 , 57–70.  相似文献   
78.
79.
80.
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