全文获取类型
收费全文 | 8329篇 |
免费 | 554篇 |
国内免费 | 1篇 |
出版年
2023年 | 80篇 |
2022年 | 77篇 |
2021年 | 159篇 |
2020年 | 124篇 |
2019年 | 154篇 |
2018年 | 385篇 |
2017年 | 308篇 |
2016年 | 376篇 |
2015年 | 329篇 |
2014年 | 389篇 |
2013年 | 634篇 |
2012年 | 693篇 |
2011年 | 695篇 |
2010年 | 446篇 |
2009年 | 309篇 |
2008年 | 317篇 |
2007年 | 290篇 |
2006年 | 314篇 |
2005年 | 178篇 |
2004年 | 153篇 |
2003年 | 154篇 |
2002年 | 122篇 |
2001年 | 108篇 |
2000年 | 89篇 |
1999年 | 104篇 |
1998年 | 81篇 |
1997年 | 70篇 |
1996年 | 76篇 |
1995年 | 88篇 |
1994年 | 73篇 |
1993年 | 55篇 |
1992年 | 61篇 |
1991年 | 59篇 |
1990年 | 44篇 |
1989年 | 61篇 |
1988年 | 48篇 |
1987年 | 54篇 |
1986年 | 45篇 |
1985年 | 58篇 |
1984年 | 51篇 |
1983年 | 59篇 |
1982年 | 53篇 |
1981年 | 61篇 |
1980年 | 45篇 |
1979年 | 61篇 |
1978年 | 46篇 |
1977年 | 55篇 |
1974年 | 41篇 |
1973年 | 44篇 |
1969年 | 46篇 |
排序方式: 共有8884条查询结果,搜索用时 406 毫秒
991.
Exocytotic fusion pore stability and topological defects in the membrane with orientational degree of ordering 总被引:1,自引:0,他引:1
Regulated exocytosis is a process that strongly depends on the formation and stability of the fusion pore. It was indicated experimentally and theoretically that narrow and highly curved fusion pore may be stabilized by accumulation of anisotropic membrane components possessing orientational ordering. On the other hand, narrow fusion pore may also undergo repetitive opening and closing, disruption in the so called kiss and run process or become completely opened in the process of full fusion of the vesicle with the membrane. In this paper we attempt to elucidate the subtle interplay between the stabilizing and destabilizing processes in the fusion neck. A possible physical mechanism which may lead to disruption of the stable fusion pore or complete fusion of the vesicle with the membrane is discussed. It is indicated that topologically driven defects of the in-plane orientational membrane ordering in the region of the fusion pore may disrupt the fusion. Alternatively, it may facilitate repetitive opening and closing of the fusion pore or induce full fusion of the vesicle with the target membrane. 相似文献
992.
A Popelka I Novák M Lehocký I Junkar M Mozetič A Kleinová I Janigová M Slouf F Bílek I Chodák 《Carbohydrate polymers》2012,90(4):1501-1508
Low-density polyethylene (LDPE) belongs to commodity polymer materials applied in biomedical applications due to its favorable mechanical and chemical properties. The main disadvantage of LDPE in biomedical applications is low resistance to bacterial infections. An antibacterial modification of LDPE appears to be a solution to this problem. In this paper, the chitosan and chitosan/pectin multilayer was immobilized via polyacrylic acid (PAA) brushes grafted on the LDPE surface. The grafting was initiated by a low-temperature plasma treatment of the LDPE surface. Surface and adhesive properties of the samples prepared were investigated by surface analysis techniques. An antibacterial effect was confirmed by inhibition zone measurements of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The chitosan treatment of LDPE led to the highest and most clear inhibition zones (35mm(2) for E. coli and 275mm(2) for S. aureus). 相似文献
993.
Eliška Zapomělová Olga Skácelová Petr Pumann Radovan Kopp Emil Janeček 《Hydrobiologia》2012,698(1):353-365
Questions of biogeography of freshwater cyanobacteria and their ability to colonize new areas have been recently discussed in connection with increasing occurrence of some formerly rare morphospecies in temperate zones. Nevertheless, the general knowledge about the distribution of cyanobacterial species is still fragmentary, and any new findings on cyanobacterial biogeography and spread are valuable. In this study, we provide updated information on the occurrence of Anabaena bergii, Raphidiopsis mediterranea, and Sphaerospermopsis aphanizomenoides in the Czech Republic. In addition, more nostocacean morphospecies are newly reported from the Czech Republic (A. fusca, A. tenericaulis, Dolichospermum curvum, D. mucosum, and S. reniformis). All of these morphospecies were characterized from a morphological point of view, and their phylogenetic affiliations were assessed on the basis of their 16S rRNA gene sequences. Based on these results, Anabaena bergii was reclassified into Chrysosporum gen. nov., and D. tenericaule comb. nova was established. 相似文献
994.
995.
996.
Lack of evidence for integration of Trypanosoma cruzi minicircle DNA in South American human genomes
Horizontal gene transfer involving kinetoplast DNA minicircles between Trypanosoma cruzi and its mammalian hosts has recently been proposed as a usual consequence of infection (Hecht et al., 2010). However, we have found no sequences longer than 29 bp perfectly matching minicircles of T. cruzi in the unassembled reads from Colombian and Peruvian human populations provided by the 1,000 Genome project (129 individuals in total, coverage from 1.4× to 36.3×, read length from 42 to 101 bp). The weak sequence matches that were identified are shared with a Finnish population used as a control from a non-endemic area. 相似文献
997.
Apodemus sylvaticus stankovici, described from the topographically rough landscape of the western Balkan glacial refugium, was recently proposed as being
either a junior synonym of Apodemus flavicollis or a species on its own right. To untangle this taxonomic vagueness, we sequenced complete cytochrome b gene in 28 field mice collected at 12 locations in the mountains of Bosnia and Herzegovina, Montenegro, western Macedonia
and northern Greece. Samples yielded 27 new haplotypes which clustered into two distinct groups. One of these clades also
included the reference haplotype of A. flavicollis, while another cluster emerged as being identical with the reference sample for A. sylvaticus. As is common in Apodemus, both species retrieved in our analysis were characterized by low levels of intraspecific variation (0.4–0.9%) as opposed
to a high level of differentiation between them (8.0–10.0%); therefore, the taxonomic classification of our material was without
doubt. We found no evidence regarding the presence of an additional cryptic species in the mountains of the western Balkans.
The very similar values of genetic variability in the two species imply their common evolutionary history of a long-term coexistence
in the western Balkan refugium. 相似文献
998.
Abstract Many classic models of speciation incorporate assortative mating based on mating groups, such as plants with different flowering times, and they investigate whether an ecological trait under disruptive natural selection becomes genetically associated with the selectively neutral mating trait. It is well known that this genetic association is potently destroyed by recombination. In this note, we point out a more fundamental difficulty: if a "knife-edge" symmetry assumption of previous models is violated, then the mating trait is no longer neutral and sexual selection eliminates the polymorphism in the mating locus. This result strengthens the growing consensus that magic traits are the more likely route to nonallopatric speciation. We expand the model assuming also ecological selection on the mating trait and investigate the conditions for natural selection to overcome sexual selection and maintain mating polymorphism; we find that the combination of natural and sexual selection can cause also bistability of allele frequencies. 相似文献
999.
Crown age estimation of a monocotyledonous tree species Dracaena cinnabari using logistic regression
Unique woodlands of Dracaena cinnabari (DC) are at risk throughout most of their range (Socotra Island, Yemen) as a result of missing regeneration and overmaturity. Effective conservation measures depend on reliable predictions of future population dynamics, which depend on accurate data on current age structure. However, age determination of Dracaena sp. has long been a scientific challenge, because the common method of tree ring counts cannot be applied to this or to most other monocotyledonous trees. In the present study, the indirect method for crown age estimation proposed by Adolt and Pavlis (Trees 18:43–53, 2004) was further developed using a more appropriate statistical technique and an intuitive model formulation. This new technique is based on the relationship between the number of branching orders and the number of flowering events that result from a specific growth pattern. We used logistic regression to directly model annual flowering probability, the reciprocal value of which corresponds to the length of the interval between flowering events. Our methodology was applied to data sets collected at two ecologically distinct sites. In Firmihin, the time between flowering events decreases from 28?years between the first and second event to 10?years between the 25th and 26th event. The length of time between flower events in Skant, however, was estimated to be a constant value of 6.5?years. We propose the application of generalised mixed-effects models and methods of survey sampling to improve the accuracy of crown age estimation in DC. Our methodology may also be useful for age estimations of other tree species with similar growth patterns, such as Dracaena draco and Aloe dichotoma. 相似文献
1000.
Effect of prey feeding (ants Formica fusca) on the quantitative changes in the accumulation of free amino acids, soluble proteins, phenolic metabolites and mineral
nutrients in the leaves of carnivorous plant Drosera capensis was studied. Arginine was the most abundant compound in Drosera leaves, while proline was abundant in ants. The amount of the majority of amino acids and their sum were elevated in the
fed leaves after 3 and 21 days, and the same, but with further enhancement after 21 days, was observed in ants. Accumulation
of amino acids also increased in young non-fed leaves of fed plants. Soluble proteins decreased in ants, but were not enhanced
in fed leaves. This confirms the effectiveness of sundew’s enzymatic machinery in digestion of prey and suggests that amino
acids are not in situ deposited, but rather are allocated within the plant. The content of total soluble phenols, flavonoids
and two selected flavonols (quercetin and kaempferol) was not affected by feeding in Drosera leaves, indicating that their high basal level was sufficient for the plant’s metabolism and prey-induced changes were mainly
N based. The prey also showed to be an important source of other nutrients besides N, and a stimulation of root uptake of
some mineral nutrients is assumed (Mg, Cu, Zn). Accumulation of Ca and Na was not affected by feeding. 相似文献