全文获取类型
收费全文 | 6254篇 |
免费 | 539篇 |
国内免费 | 5篇 |
出版年
2023年 | 34篇 |
2022年 | 85篇 |
2021年 | 175篇 |
2020年 | 90篇 |
2019年 | 96篇 |
2018年 | 138篇 |
2017年 | 129篇 |
2016年 | 220篇 |
2015年 | 400篇 |
2014年 | 439篇 |
2013年 | 428篇 |
2012年 | 622篇 |
2011年 | 567篇 |
2010年 | 334篇 |
2009年 | 290篇 |
2008年 | 400篇 |
2007年 | 398篇 |
2006年 | 385篇 |
2005年 | 291篇 |
2004年 | 313篇 |
2003年 | 211篇 |
2002年 | 218篇 |
2001年 | 43篇 |
2000年 | 27篇 |
1999年 | 47篇 |
1998年 | 42篇 |
1997年 | 46篇 |
1996年 | 30篇 |
1995年 | 20篇 |
1994年 | 38篇 |
1993年 | 20篇 |
1992年 | 23篇 |
1991年 | 19篇 |
1990年 | 13篇 |
1989年 | 16篇 |
1988年 | 5篇 |
1987年 | 7篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 12篇 |
1982年 | 9篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1978年 | 6篇 |
1977年 | 13篇 |
1975年 | 5篇 |
1973年 | 6篇 |
1968年 | 5篇 |
1962年 | 4篇 |
排序方式: 共有6798条查询结果,搜索用时 15 毫秒
51.
Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F0, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (Fv)/maximum fluorescence (Fm)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F0, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and Fv/Fm, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.Abbreviations Elips
early light-induced proteins
- EPS
epoxidation state
- F0
instantaneous fluorescence
- Fm
maximum fluorescence
- Fv
variable fluorescence
- LHC II
light-harvesting complex of PSII
- LiDS
lithium dodecyl sulfate
This research was supported by the Swedish Natural Science Research Council. We wish to thank Dr. Adrian Clarke1 (Department of Plant Physiology, University of Umeå, Sweden) for advice on electrophoresis, valuable discussion and providing antibodies. Dr. Stefan Jansson1 and Dr. Torill Hundal (Department for Biochemistry, University of Stockholm, Sweden) provided antibodies. Jan Karlsson1 helped with the HPLC, Dr. Marianna Krol gave advice on green gels and Dr. Vaughan Hurry (Cooperative Research Centre for Plant Sciences, Australian National University, Canberra, Australia) provided valuable discussion. 相似文献
52.
The Allodon River, a tributary of the Rhône, has suffered considerably from the recent expansion of human activities in the Geneva region. This study documents changes in its benthic fauna by comparing species richness before and after 1986 and by considering the possibilities of recolonization by drift.
Résumé L'Allondon, affluent du Rhône, est une rivière qui a considérablemen t souffert d'une expansion récente des activités humaines dans la région genevoise. Cette étude met en lumière l'évolution de certains éléments de la faune benthique en comparant les richesses specifiques avant et après 1986, année critique pour la macrofaune benthique du bassin genevois. Elle met en évidence les possibilités de recolonisation par dérive de certains recours de l'Allondon à partir d'affluents moins perturbés.相似文献
53.
Attempts to induce enantioselectivity in the catalytic hydrogenation of unsymmetrically substituted aromatics using covalently bound, well known chiral auxiliaries are described. Marked differences in stereoselectivity and rate of hydrogenolysis are noted as a function of the auxiliary used. Enantioselectivities obtained in the resulting cyclohexyl derivatives are rather poor. © 1995 Wiley-Liss, Inc. 相似文献
54.
Three endocyclic sulfoximides of the 1-aryl- and 1-alkyl-3-oxo-benzo[d]-isothia (IV)-azole 1-oxide type (1-substituent = 2′-carboxyphenyl, 2′-carbethoxyphenyl, and octyl, respectively) were found to be well resolved on a chiral phase derived from bovine serum albumin (BSA). Selectivities (α) of 1.74, 1.12, and 1.44, respectively, were obtained. The retention behaviour of 1-octyl-3-oxo-benzo[d]isothia(IV)-azole 1-oxide was further investigated in some detail as a function of the mobile phase composition and the elution order was established from optically active material obtained from the enantiopure sulfoxide precursor. An enantiomeric excess of 85.4% was obtained in the cyclocondensation reaction of the octyl-substituted sulfoxide precursor with hydrazoic acid to the corresponding endocyclic sulfoximide. © 1995 Wiley-Liss, Inc. 相似文献
55.
Mobile gene cassettes and integrons: capture and spread of genes by site-specific recombination 总被引:38,自引:0,他引:38
An integron is a genetic unit that includes the determinants of the components of a site-specific recombination system capable of capturing and mobilizing genes that are contained in mobile elements called gene cassettes. An integron also provides a promoter for expression of the cassette genes, and integrons thus act both as natural cloning systems and as expression vectors. The essential components of an integron are an int gene encoding a site-specific recombinase belonging to the integrase family, an adjacent site, attl, that is recognized by the integrase and is the receptor site for the cassettes, and a promoter suitably oriented for expression of the cassette-encoded genes. The cassettes are mobile elements that include a gene (most commonly an antibiotic-resistance gene) and an integrase-specific recombination site that is a member of a family of sites known as 59-base elements. Cassettes can exist either free in a circularized form or integrated at the attl site, and only when integrated is a cassette formally part of an integron. A single site-specific recombination event involving the integron-associated attl site and a cassette-associated 59-base element leads to insertion of a free circular cassette into a recipient integron. Multiple cassette insertions can occur, and integrons containing several cassettes have been found in the wild. The integrase also catalyses excisive recombination events that can lead to loss of cassettes from an integron and generate free circular cassettes. Due to their ability to acquire new genes, integrons have a clear role in the evolution of the genomes of the plasmids and transposons that contain them. However, a more general role in evolution is also likely. Events involving recombination between a specific 59-base-element site and a nonspecific secondary site have recently been shown to occur. Such events should lead either to the insertion of cassettes at non-specific sites or to the formation of stable cointegrates between different plasmid molecules, and a cassette situated outside the integron context has recently been identified. 相似文献
56.
57.
Albert de la Chapelle Christina Kari Markku Nurminen Sven Hernberg 《Human genetics》1977,37(2):183-194
Summary An epidemic of agranulocytosis and granulocytopenia occurred in 1975 in conjunction with clozapine treatment of mental patients in Finland. An attempt was made to assess the epidemiologic and genetic factors contributing to the adverse drug effect. The estimated incidence rate in Finland was 2.1/1000 patient-months. This figure could not be compared with rates from other countries because of the inexact nature of the figures reported so far. All 16 cases occurred in seven hospitals in southwestern Finland, whereas the overall hospital net use of the drug was geographically evenly distributed. The difference between the observed and the proportionally expected incidence of cases amongst the hospitals where clozapine was used was statistically significant. The average consumption of the drug did not differ between the hospitals where cases occurred and those where no definite cases could be diagnosed. Six-generation pedigree analyses failed to reveal significant parental consanguinity or genetic kinship between probands. Neither did the birth places of the ancestors of the probands disclose a typical isolate pattern. In conclusion, the cases appeared to be confined to a few hospitals in southwestern Finland. Although a genetic factor is not excluded, we found no evidence in support of a genetic mechanism. 相似文献
58.
Christina M. Hansen Bay 《Journal of insect physiology》1978,24(2):141-149
The salivary glands of adult Calliphora contain enzymes which hydrolyze starch, sucrose and trehalose. Amylase and sucrase are shown to be secretory enzymes, while trehalase remains in the gland. Results of electrophoretic and ultrastructural studies suggest that protein secretion is confined to the abdominal region of the gland. Secretion of both fluid and protein occurs from a single type of cell. While a fly is feeding on solid sugar, amylase and sucrase are lost from the gland and appear in saliva, while the level of trehalase in the gland increases slightly. The mixture of food and saliva passes mainly to the crop where carbohydrate is digested by the salivary enzymes. 相似文献
59.
60.
Christina Skarpe 《植被学杂志》1992,3(3):293-300
Abstract. Theories concerning the factors involved in the dynamics of savannas, particularly the tree-grass interface, are reviewed. Emphasis is put on factors related to soil moisture, soil nutrients, fire and large herbivores. The distinction between external (independent) and internal (dependent, interactive) environment is discussed and it is explained how this distinction is affected by the scale of observation. 相似文献