全文获取类型
收费全文 | 2146篇 |
免费 | 182篇 |
国内免费 | 128篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 63篇 |
2021年 | 120篇 |
2020年 | 65篇 |
2019年 | 72篇 |
2018年 | 85篇 |
2017年 | 71篇 |
2016年 | 95篇 |
2015年 | 152篇 |
2014年 | 149篇 |
2013年 | 172篇 |
2012年 | 201篇 |
2011年 | 195篇 |
2010年 | 110篇 |
2009年 | 91篇 |
2008年 | 100篇 |
2007年 | 76篇 |
2006年 | 71篇 |
2005年 | 57篇 |
2004年 | 41篇 |
2003年 | 42篇 |
2002年 | 49篇 |
2001年 | 33篇 |
2000年 | 32篇 |
1999年 | 42篇 |
1998年 | 15篇 |
1997年 | 21篇 |
1996年 | 15篇 |
1995年 | 12篇 |
1994年 | 8篇 |
1993年 | 9篇 |
1992年 | 12篇 |
1991年 | 15篇 |
1990年 | 19篇 |
1989年 | 9篇 |
1987年 | 9篇 |
1986年 | 5篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 6篇 |
1973年 | 3篇 |
1970年 | 5篇 |
排序方式: 共有2456条查询结果,搜索用时 15 毫秒
991.
Xi‐Ping Dong Kelly Vargas John A. Cunningham Huaqiao Zhang Teng Liu Fang Chen Jianbo Liu Stefan Bengtson Philip C. J. Donoghue 《Palaeontology》2016,59(3):387-407
Fossilized embryos afford direct insight into the pattern of development in extinct organisms, providing unique tests of hypotheses of developmental evolution based in comparative embryology. However, these fossils can only be effective in this role if their embryology and phylogenetic affinities are well constrained. We elucidate and interpret the development of Olivooides from embryonic and adult stages and use these data to discriminate among competing interpretations of their anatomy and affinity. The embryology of Olivooides is principally characterized by the development of an ornamented periderm that initially forms externally and is subsequently formed internally, released at the aperture, facilitating the direct development of the embryo into an adult theca. Internal anatomy is known only from embryonic stages, revealing two internal tissue layers, the innermost of which is developed into three transversally arranged walls that partly divide the lumen into an abapertural region, interpreted as the gut of a polyp, and an adapertural region that includes structures that resemble the peridermal teeth of coronate scyphozoans. The anatomy and pattern of development exhibited by Olivooides appears common to the other known genus of olivooid, Quadrapyrgites, which differs in its tetraradial, as opposed to pentaradial symmetry. We reject previous interpretations of the olivooids as cycloneuralians, principally on the grounds that they lack a through gut and introvert, in embryo and adult. Instead we consider the affinities of the olivooids among medusozoan cnidarians; our phylogenetic analysis supports their classification as total‐group Coronata, within crown‐Scyphozoa. Olivooides and Quadrapyrgites evidence a broader range of life history strategies and bodyplan symmetry than is otherwise commonly represented in extant Scyphozoa specifically, and Cnidaria more generally. 相似文献
992.
993.
Yuan Zhang Xiaohuan Yuan Wenjun Teng Chao Chen Juying Wu 《Plant Molecular Biology Reporter》2016,34(6):1181-1192
Pennisetum species are widely used as ornamental grasses and may be a valuable genetic resource for their breeding to broaden its genetic basis. At present, new ornamental Pennisetum cultivars are primarily bred via somaclone, which increases the number of variants. It is difficult to estimate whether the suspected variants are authentic in genetic features by morphological traits because of their many limitations. Moreover, although the phylogenetic classification of the Pennisetum genus has been approved in some morphological and cytological studies, genetic evidence is lacking. In the present study, we developed 15 specific simple sequence repeat (SSR) markers with a large amount of polymorphisms and strong distinguishing abilities for Pennisetum ornamental grasses using magnetic bead enrichment. These markers, together with the other 11 reported polymorphic SSRs, were further used for the identification of a broad collection of 55 Pennisetum samples, including nine original taxa and 46 suspected variants. After comparing the genetic characteristics between each variant and its corresponding original taxon, we verified 20 suspected variants that possess the potential to become new, commercially desirable cultivars. The nine original taxa and the 20 verified variants were identified based on the polymorphisms of six core loci, and unique molecular identities with 15 denary digits for each taxon were further established. The rationality of the traditional phylogenetic classification system of the Pennisetum genus was further verified using 147 polymorphic alleles. The present study promotes the protection, registration, breeding, and international communication of Pennisetum ornamental grasses. 相似文献
994.
995.
996.
997.
998.
999.
Ascorbate accumulation during sulphur deprivation and its effects on photosystem II activity and H2 production of the green alga Chlamydomonas reinhardtii
下载免费PDF全文
![点击此处可从《Plant, cell & environment》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Valéria Nagy André Vidal‐Meireles Roland Tengölics Gábor Rákhely Győző Garab László Kovács Szilvia Z. Tóth 《Plant, cell & environment》2016,39(7):1460-1472
In nature, H2 production in Chlamydomonas reinhardtii serves as a safety valve during the induction of photosynthesis in anoxia, and it prevents the over‐reduction of the photosynthetic electron transport chain. Sulphur deprivation of C. reinhardtii also triggers a complex metabolic response resulting in the induction of various stress‐related genes, down‐regulation of photosynthesis, the establishment of anaerobiosis and expression of active hydrogenase. Photosystem II (PSII) plays dual role in H2 production because it supplies electrons but the evolved O2 inhibits the hydrogenase. Here, we show that upon sulphur deprivation, the ascorbate content in C. reinhardtii increases about 50‐fold, reaching the mM range; at this concentration, ascorbate inactivates the Mn‐cluster of PSII, and afterwards, it can donate electrons to tyrozin Z+ at a slow rate. This stage is followed by donor‐side‐induced photoinhibition, leading to the loss of charge separation activity in PSII and reaction centre degradation. The time point at which maximum ascorbate concentration is reached in the cell is critical for the establishment of anaerobiosis and initiation of H2 production. We also show that ascorbate influenced H2 evolution via altering the photosynthetic electron transport rather than hydrogenase activity and starch degradation. 相似文献
1000.