全文获取类型
收费全文 | 3279篇 |
免费 | 181篇 |
国内免费 | 13篇 |
专业分类
3473篇 |
出版年
2021年 | 24篇 |
2017年 | 23篇 |
2016年 | 25篇 |
2015年 | 51篇 |
2014年 | 42篇 |
2013年 | 87篇 |
2012年 | 99篇 |
2011年 | 95篇 |
2010年 | 92篇 |
2009年 | 123篇 |
2008年 | 115篇 |
2007年 | 99篇 |
2006年 | 95篇 |
2005年 | 80篇 |
2004年 | 77篇 |
2003年 | 59篇 |
2002年 | 32篇 |
2001年 | 41篇 |
2000年 | 41篇 |
1999年 | 63篇 |
1998年 | 44篇 |
1997年 | 50篇 |
1996年 | 48篇 |
1995年 | 39篇 |
1994年 | 37篇 |
1992年 | 66篇 |
1991年 | 33篇 |
1990年 | 38篇 |
1989年 | 41篇 |
1988年 | 32篇 |
1987年 | 34篇 |
1986年 | 36篇 |
1985年 | 42篇 |
1984年 | 27篇 |
1983年 | 23篇 |
1982年 | 23篇 |
1981年 | 28篇 |
1979年 | 28篇 |
1977年 | 22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 |
1948年 | 23篇 |
排序方式: 共有3473条查询结果,搜索用时 0 毫秒
1.
The revision of the antarctic–subantarctic species Orchomenopsis reducta Schellenberg, 1931, has led to its attribution to a new, highly apomorphic genus: Falklandia gen.n. A new definition of the uristid group is given and Falklandia with 36 other lysianassoid genera are attributed to this supposedly monophyletic group. 相似文献
2.
D H Doherty 《Journal of virology》1982,43(2):641-654
The T4 mutation ptg19-80 affects the mechanism of capsid-length determination. It is located in gene 23, which encodes the major structural protein of the capsid. The mutation results in the production of abnormal-length capsids in high frequencies. This paper describes the isolation and partial characterization of second-site revertants of ptg19-80. In the course of their analysis, it was discovered that ptg19-80 is itself a double mutation consisting of a gene 23 mutation (ptg19-80c), which causes the morphogenetic defect, and a suppressor mutation located near the lysozyme gene. Phenotypic characterization of nine pseudo-wild-type revertants of this double-mutation revealed that these revertants all produced lower frequencies of abnormal capsids than did ptg19-80. Seven of these revertants were shown to contain two suppressor mutations, one mapping in or near gene 22 and done mapping in or near gene 24. Both mutations were required for suppression. These suppressors displayed no discernible phenotype in the absence of ptg19-80c. 相似文献
3.
Monitoring the integrity of self: biology of MHC-restriction of virus-immune T cells 总被引:2,自引:0,他引:2
All available evidence indicates that the cytotoxic thymus-derived lymphocyte (T cell), which is lytic for virus-infected target cells in vitro, is also the effector in cell-mediated immunity in vivo. Such T cell show two orders of specificity: for the virus in question, and for a particular self major histocompatibility complex (MHC) glycoprotein. Recirculating T cells amy thus be considered to survey the integrity of self, the self components involved being the cell-surface structures that are recognized as foreign during graft rejection. Virus-infected liver cells are apparently eliminated in much the same way as a transplanted organ. The necessary balance between self-tolerance (absence of autoreactivity) and self-monitoring effector T cell function seems to be established during the process of differentiation in thymus. The molecular nature of the underlying recognition events is, as yet, obscure. 相似文献
4.
5.
Cyanide-Resistant Root Respiration and Tap Root Formation in Two Subspecies of Hypochaeris radicata 总被引:1,自引:0,他引:1
Root respiration of the tap root forming species Hypochaeris radicata L. was measured during tap root formation. A comparison was made of two subspecies: H. radicata L. ssp. radicata L., a subspecies from relatively rich soils, and H. radicata L. ssp. ericetorum Van Soest, a subspecies from poor acidic soils. Root respiration was high and to a large extent inhibited by hydroxamic acid (SHAM) before the start of the tap root formation, indicating a high activity of an alternative non-phosphorylative electron transport chain. The rate of root respiration was much lower and less sensitive to SHAM when a considerable tap root was present. However, root respiration was also cyanide-resistant when a tap root was present, indicating that the alternative pathway was still present. A decreased rate of root respiration coincided with an increase of the content of storage carbohydrates, mainly in the tap root. The level of reducing sugars was constant throughout the experimental period, and it was concluded that the activity of the alternative oxidative pathway was significant in oxidation of sugars that could not be utilized for purposes like energy production, the formation of intermediates for growth or for storage. Root respiration decreased after the formation of a tap root. This decrease could neither be attributed to a gradual disappearance of the alternative chain, nor to a decreased level of reducing sugars. No differences in respiratory metabolism between the two subspecies have been observed, suggesting that a high activity of the alternative oxidative pathway is not significant in adaptation of the present two subspecies to relatively nutrient-rich or poor soils. 相似文献
6.
7.
8.
9.
Several leafhopper variants of the Circulifer tenellus complex were collected in “citrus stubborn” affected areas in Israel. Two of these variants transmitted the Spiroplasma citri to Matthiola incana after being injected with the disease agent. The variant from Atriplex halimus was designated Circulifer tenellus-A (CTA) and the variant from Portulaca oleracea was designated Circulifer tenellus-? (CTP). Transmission characteristics were determined for both leafhoppers. A high rate of transmission (43.3%) was obtained by single CTA leafhoppers that were injected with the Amiad S. citri isolate from the Upper Galilee, compared with 7% transmission obtained with the CTP leafhoppers. The Gilgal S. citri isolate from the Jordan Valley, was not transmitted by either. Injection was more effective than acquisition access feeding to render the leafhopper infective for both CTA and CTP. The minimum acquisition access period needed for the CTA variant to transmit the Amiad isolate was 1 h. Longer AAPs did not necessarily result in a higher rate of transmission. The minimum incubation period was 6 days and the maximum was 32 days. The LP50 calculated from the logarithmic curve y = 45.74Ln(x)–53.68 was 9.64 days. The minimum inoculation access period (IAP) was lh. The same transmission parameters for the CTP variant could not be determined, as no transmission was obtained even when groups of five-six insects were placed on a single plant. 相似文献
10.
Evaluation of xylazine hydrochloride as the sole immobilizing agent in moose and caribou--and its subsequent reversal with idazoxan 总被引:1,自引:0,他引:1
Xylazine hydrochloride was used as the sole immobilizing agent in moose and caribou. The animals were free-ranging and immobilization was accomplished from a helicopter using powered darts. Following a period of immobilization during which radiotelemetry collars were fitted, the animals were revived using idazoxan (RX 781094) or its methoxy analogue RX 821002. Xylazine was administered at dose rates of approximately 3.0 mg/kg and 5.0 mg/kg to the moose and caribou, respectively. Moose received 430 +/- 27 mg of xylazine and a mean dose of 10 mg idazoxan (RX 781094). Caribou received 485 +/- 30 mg xylazine and a mean dose of 4 mg idazoxan (RX 821002). This technique gave adequate immobilization with rapid recovery of consciousness in both species. 相似文献