全文获取类型
收费全文 | 6168篇 |
免费 | 364篇 |
国内免费 | 1篇 |
专业分类
6533篇 |
出版年
2023年 | 62篇 |
2022年 | 90篇 |
2021年 | 166篇 |
2020年 | 113篇 |
2019年 | 147篇 |
2018年 | 189篇 |
2017年 | 147篇 |
2016年 | 236篇 |
2015年 | 331篇 |
2014年 | 380篇 |
2013年 | 419篇 |
2012年 | 522篇 |
2011年 | 528篇 |
2010年 | 324篇 |
2009年 | 236篇 |
2008年 | 328篇 |
2007年 | 359篇 |
2006年 | 346篇 |
2005年 | 268篇 |
2004年 | 257篇 |
2003年 | 217篇 |
2002年 | 213篇 |
2001年 | 54篇 |
2000年 | 37篇 |
1999年 | 37篇 |
1998年 | 52篇 |
1997年 | 40篇 |
1996年 | 46篇 |
1995年 | 32篇 |
1994年 | 30篇 |
1993年 | 25篇 |
1992年 | 32篇 |
1991年 | 25篇 |
1990年 | 21篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 10篇 |
1986年 | 16篇 |
1985年 | 17篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 12篇 |
1981年 | 7篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 9篇 |
1976年 | 6篇 |
1975年 | 9篇 |
1974年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有6533条查询结果,搜索用时 15 毫秒
41.
Bernd Nidetzky Marianne Hayn Ricardo Macarron Walter Steiner 《Biotechnology letters》1993,15(1):71-76
Summary The synergistic action of purified cellulases from Trichoderma reesei in hydrolysis of cellulose decreased with increasing substrate concentration, depended strongly on the the type of cellulose used, and was maximal on crystalline cellulose. Contrarily, the activity of the individual cellulases was highest on amorphous cellulose. The binary combinations CBH I/EG III and CBH I/CBH II exhibited the greatest degree of synergism on crystalline cellulose. 相似文献
42.
Physiological ecology of a metalimnetic Cryptomonas population: relationships to light, sulfide and nutrients 总被引:1,自引:0,他引:1
Gasol Josep M.; Garcia-Cantizano Josefina; Massana Ramon; Guerrero Ricardo; Pedros-Alio Carlos 《Journal of plankton research》1993,15(3):255-275
The possible mechanisms by which Cryptomonas phaseolus maintainsa large metalimnetic population during stratification were studiedin Lake Cisó. The population was shown to always remainat the oxygen-sulfide interface despite the fact that sometimesthe amount of light reaching was <1% of incident light. Thismeant that the amount of light reaching the peak was sometimesmuch lower than the optimum light intensity for photosynthesisof the population, which was 相似文献
43.
A scanning electron microscope study of normal and vitrified leaves from Datura insignis plantlets cultured in vitro 总被引:5,自引:0,他引:5
Flavio Costa Miguens Ricardo Pereira Louro Raul Dodsworth Machado 《Plant Cell, Tissue and Organ Culture》1993,32(1):109-113
The surface anatomy of normal and vitreous leaves of plantlets obtained from Datura insignis Barb Rodr nodal segment cultures was compared using scanning electron microscopy. Normal and vitrified leaves are similar in several ways. They are both amphistomatic, and have similar distributions of glandular and non-glandular trichomes. Stomata have similar length, diameter and distribution in normal and vitreous plants. Immature stomata, which have closed pores, and plugged stomata, which contain an amorphous material between their guard cells, occur in both normal and vitrified leaves. Normal and vitreous leaves differ in the frequency of normal and abnormal stomata. Normal stomata have kidney-shaped guard cells and resemble closely those found in field-grown plants, whereas abnormal stomata have deformed guard cells. Normal stomata represent approximately 80% of the total number of stomata in normal leaves, but only 7% of the total number of stomata in vitreous leaves. Abnormal stomata represent 90% of the total number in vitreous leaves. The deformation of guard cells could possibly be a mechanical impediment to stomatal function. 相似文献
44.
45.
Hypnea musciformis is the only species so far exploited in Brazil as raw material for the production of k-carrageenan. Due to the erratic production in space and time, increasing harvest and transportation costs, experiments have been performed in order to assess the viability of H. musciformis mariculture.In nature the species occurs as an epiphyte, and so mariculture using artificial substrates that simulated the natural host of the species was tried. These substrates were attached, at regular intervals, to linear ropes. In the sea, these ropes were stretched between cement blocks.Seeding occurs naturally, by means of spores, or detached pieces of H. musciformis scattered in the water column that get entangled on the ropes. The best yields (0.54 wet kg m–1 month–1) were obtained with unthreaded rope substrates maintained in a vertical position by small rafts. Production is highest in the first 18 m off the rocky shore (0–2.1 m deep), at the highest substrate density utilized (10 m–1), 2–3 months after installing the ropes in seawater. The main factor controlling seasonal production is water movement. 相似文献
46.
Life history and reproductive potential of the agarophyte Gelidium robustum in California 总被引:1,自引:1,他引:0
The reproductive potential of the tetrasporangial phase of Gelidium robustum was studied for 16 months at two sites off Santa Barbara, California. In all samples tetrasporangial thalli were always more abundant than gametangial ones. Tetratrasporangial sori were present throughout the duration of the study but relative fecundity was highest [300–400 sori g–1 (w. wt)] in spring/summer samples of consecutive years, as a result of increasing numbers both of tetrasporangial branchlets per plant and of sori per branchlet. On the other hand, laboratory experiments showed that tetraspore release per sorus was highest (150–250 spores sorus–1 d–1) in winter. Inferring from these field and laboratory data plants released up to ± 34 000 tetraspores g–1 (w. wt) d–1 in the spring/summer of the second study year. Tetraspore germination, under defined culture conditions, also showed a marked seasonality increasing sharply from less than 10% in winter up to almost 60% in spring/summer, thus coinciding with the period of maximal spore output per plant. These results suggest that although relatively high numbers of tetraspores may be released by G. robustum plants all year round these might not always have the potential to germinate and recruit. 相似文献
47.
Gerardo H. Vázquez Nin Ernestina Flores Olga M. Echeverría Hilde Merkert Rodolfo Wettstein Ricardo Benavente 《Chromosoma》1993,102(7):457-463
The distribution of DNA in synaptonemal complexes of rat and mouse spermatocytes, and of chick oocytes was investigated by immunogold electron microscopy. Except for a few specific sites, DNA was not immunolocalized in the space between lateral elements of the complex. Some labeled fibrils connecting the lateral elements with the central element were observed associated with recombination nodules or near them. However, other labeled fibrils in the space between lateral elements did not appear to present any relationship to recombination nodules. The immunocytochemical approaches used here confirmed the presence of significant amounts of DNA in the lateral elements as previously indicated by preferential DNA staining methods. Furthermore, our findings support the view that recombination nodules are the site of chiasma formation. 相似文献
48.
Camila Tamburrini Silvia Lucrecia Dahinten Rubén Ricardo Romero Saihueque María C. Ávila-Arcos María Laura Parolin 《American journal of physical anthropology》2023,182(2):161-176
Ethical discussions around ancient DNA (aDNA) research predate the technological breakthroughs that led to the accelerated generation of ancient genomic data, revealing a long-due need to address these aspects in the field. Given the diverse conflicts that genomics has raised towards the communities associated with the Non-living Human Ancestors under study, it has been suggested that the ethical and legal implications of genetically studying present-day and ancient human populations should be considered case-by-case. Nevertheless, the discussions have focused on US and European perspectives. To contribute from a local and Latin American position to the problem, we present the history of consensus and disagreement of the relationships between scientists and Indigenous communities of the Atlantic coast of the central Argentinian Patagonia. We describe how these relationships resulted in the approval of a groundbreaking provincial law that acknowledges the Indigenous community's right to be involved in decision-making concerning their Ancestors. In addition, we emphasize how these established relationships allowed the development of aDNA studies. With this background, we address the main ethical concerns of genomic studies of Ancestors identified in the reference literature and commit to applying some of the recommendations suggested in those ethical guidelines. Then, we reflect on possible negative consequences of ongoing research and propose some suggestions based on personal experiences that will contribute to moving the ethical field towards a more contextualized science with a local perspective. 相似文献
49.
As urban areas expand due to increasing human populations, natural habitats are diminishing in quantity and quality. We conducted a study to examine the response of diurnal acoustic communities along a gradient of environments, ranging from native to dense urban areas. We hypothesized that acoustic indices would decline with urbanization, transitioning from natural and rural areas to urban environments. We conducted the research in and around Brasília, Central Brazil. We deployed 24 digital recorders in native, rural, low-density and high-density urban areas. We employed five commonly used acoustic indices (ADI, ACI, NDSI, H and BI) to characterize the acoustic communities and represent the existing biodiversity numerically. We initially compared the index values across different land use types using a non-parametric rank sum test. Subsequently, we selected eight landscape metrics and performed a principal component analysis to summarize the surrounding matrix at each sampling point. We then employed generalized linear models to determine if the acoustic indices exhibited the anticipated variations. The results indicated significant variations in all indices among the different land use types, indicating their responsiveness to distinct acoustic communities and environmental characteristics. ADI, H and NDSI exhibited declining values from native to urban areas, while ACI and BI displayed the opposite trend. These findings underscore the influence of landscape structure on acoustic indices. Consequently, we concluded that adopting appropriate landscape planning, mainly through integrating natural and urban areas, could help preserve biodiversity in tropical urban regions. 相似文献