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   25845篇
  免费   59篇
  国内免费   72篇
系统科学   120篇
丛书文集   295篇
教育与普及   66篇
理论与方法论   121篇
现状及发展   10555篇
研究方法   1146篇
综合类   13240篇
自然研究   433篇
  2013年   242篇
  2012年   384篇
  2011年   827篇
  2010年   174篇
  2008年   428篇
  2007年   520篇
  2006年   515篇
  2005年   519篇
  2004年   470篇
  2003年   455篇
  2002年   472篇
  2001年   902篇
  2000年   878篇
  1999年   513篇
  1992年   486篇
  1991年   380篇
  1990年   409篇
  1989年   440篇
  1988年   407篇
  1987年   393篇
  1986年   433篇
  1985年   509篇
  1984年   382篇
  1983年   353篇
  1982年   284篇
  1981年   283篇
  1980年   342篇
  1979年   828篇
  1978年   638篇
  1977年   623篇
  1976年   489篇
  1975年   532篇
  1974年   781篇
  1973年   624篇
  1972年   659篇
  1971年   778篇
  1970年   972篇
  1969年   740篇
  1968年   772篇
  1967年   678篇
  1966年   653篇
  1965年   453篇
  1964年   114篇
  1959年   246篇
  1958年   413篇
  1957年   313篇
  1956年   263篇
  1955年   235篇
  1954年   230篇
  1948年   188篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
72.
73.
74.
Blinking     
CLARK RE  LAWSON RW 《Nature》1948,162(4129):970
  相似文献   
75.
76.
77.
78.
Q Gu  F A Popp 《Experientia》1992,48(11-12):1069-1082
By considering an exciplex system consisting of collective molecules in interaction with both the 'pumping' fields and the biophoton fields, the two-level exciplex model and the three-level exciplex model are presented. They are useful for the investigation of the quasi-stationary behaviour of biophoton emission, and biophoton emission as a dynamic process in the presence of external perturbations. Our theoretical results predict a series of nonlinear effects, such as chaos, fractal behaviour, and non-equilibrium phase transition. These effects characterize the coherence nature of living systems. In our approaches, there are two important quantities f and x, which can be used to mark the working points of the two-level and three-level exciplex systems. All the influences of external perturbations on the exciplex systems, e.g. change of temperature, the addition of agents, exposure to light, etc., can be interpreted as shifts of the working points of the systems, leading to a diversity of nonlinear response of biophoton emission. In addition, the agreements of the theoretical results and the corresponding experimental observations on biophoton emission from biological systems in the presence of external perturbations are demonstrated.  相似文献   
79.
Transgenesis in fish   总被引:1,自引:0,他引:1  
Gene transfer into fish embryo is being performed in several species (trout, salmon, carps, tilapia, medaka, goldfish, zebrafish, loach, catfish, etc.). In most cases, pronuclei are not visible and microinjection must be done into the cytoplasm of early embryos. Several million copies of the gene are generally injected. In medaka, transgenesis was attempted by injection of the foreign gene into the nucleus of oocyte. Several reports indicate that the injected DNA was rapidly replicated in the early phase of embryo development, regardless of the origin and the sequence of the foreign DNA. The survival of the injected embryos was reasonably good and a large number reached maturity. The proportion of transgenic animals ranged from 1 to 50% or more, according to species and to experimentators. The reasons for this discrepancy have not been elucidated. In all species, the transgenic animals were mosaic. The copy number of the foreign DNA was different in the various tissues of an animal and a proportion lower than 50% of F1 offsprings received the gene from their parents. This suggests that the foreign DNA was integrated into the fish genome at the two cells stage or later. An examination of the integrated DNA in different cell types of an animal revealed that integration occurred mainly during early development. The transgene was found essentially unrearranged in the fish genome of the founders and offsprings. The transgenes were therefore stably transmitted to progeny in a Mendelian fashion. Southern blot analysis revealed the presence of possible junction fragments and also of minor bands which may result from a rearrangement of the injected DNA. In all species, the integrated DNA appeared mainly as random end-to-end concatemers. In adult trout blood cells, a small proportion of the foreign DNA was maintained in the form of non-integrated concatemers, as judged by the existence of end fragments. The transgenes were generally only poorly expressed. The majority of the injected gene constructs contained essentially mammalian or higher vertebrates sequences. The comparison of the expression efficiency of these constructs in transfected fish and mammalian cells indicates that some of the mammalian DNA sequences are most efficiently understood by the fish cell machinery. Chloramphenicol acetyl transferase gene under the control of promoters from Rous sarcoma virus, and human cytomegalovirus, was expressed in several tissues of transgenic fish. Chicken delta-crystallin gene was expressed in several tissues of transgenic fish.(ABSTRACT TRUNCATED AT 400 WORDS)  相似文献   
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