全文获取类型
收费全文 | 106480篇 |
免费 | 8227篇 |
国内免费 | 9243篇 |
专业分类
123950篇 |
出版年
2024年 | 274篇 |
2023年 | 1508篇 |
2022年 | 3473篇 |
2021年 | 5646篇 |
2020年 | 3851篇 |
2019年 | 4755篇 |
2018年 | 4641篇 |
2017年 | 3375篇 |
2016年 | 4725篇 |
2015年 | 6849篇 |
2014年 | 8054篇 |
2013年 | 8424篇 |
2012年 | 10020篇 |
2011年 | 9066篇 |
2010年 | 5384篇 |
2009年 | 5052篇 |
2008年 | 5717篇 |
2007年 | 4892篇 |
2006年 | 4221篇 |
2005年 | 3383篇 |
2004年 | 2813篇 |
2003年 | 2548篇 |
2002年 | 2085篇 |
2001年 | 1714篇 |
2000年 | 1538篇 |
1999年 | 1576篇 |
1998年 | 915篇 |
1997年 | 965篇 |
1996年 | 882篇 |
1995年 | 833篇 |
1994年 | 717篇 |
1993年 | 594篇 |
1992年 | 715篇 |
1991年 | 576篇 |
1990年 | 474篇 |
1989年 | 353篇 |
1988年 | 285篇 |
1987年 | 234篇 |
1986年 | 193篇 |
1985年 | 222篇 |
1984年 | 131篇 |
1983年 | 128篇 |
1982年 | 67篇 |
1981年 | 25篇 |
1980年 | 22篇 |
1979年 | 19篇 |
1976年 | 2篇 |
1975年 | 3篇 |
1974年 | 2篇 |
1973年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
蔷薇科植物体细胞胚胎发生及影响因素研究进展 总被引:1,自引:0,他引:1
总结了近30年来蔷薇科植物体细胞胚胎发生及影响因素的研究进展。蔷薇科植物体胚发生多数是直接发生途径和间接发生途径同时存在,但以间接发生途径为主。合子胚作为外植体明显好于营养器官作为外植体。诱导体胚发生的植物生长素类调节剂以NAA、2,4-D为主,细胞分裂素类调节剂以6-BA为主,少数植物种类的体胚诱导需要添加KT。冷处理对蔷薇科植物的体胚分化有效。光照对蔷薇科植物的体胚发生没有显著的影响,有时光照会抑制体胚发生。今后应逐步开展对蔷薇科植物体细胞胚胎发生的生理、生化及分子机理的研究,这在蔷薇科植物的新品种培育、遗传改良、优良单株的离体扩殖等具有重要意义。 相似文献
143.
小麦秆锈抗性遗传及抗性基因研究进展 总被引:1,自引:0,他引:1
目前国际上已发现近80个小麦抗秆锈基因,其中45个抗秆锈基因已被正式定名,58个抗秆锈基因已定位在小麦特定染色体上,其中12个基因被标记。本文对小麦抗秆锈病基因抗源、抗秆锈性遗传、分子标记研究现状及存在问题加以综述,并对抗秆锈分子遗传前景进行展望。 相似文献
144.
145.
146.
Rumi Zhang Peiyu Zhang Colin Dalton Graham A. Jullien 《Biomechanics and modeling in mechanobiology》2010,9(1):77-86
In this paper, we apply mixture theory to quantitatively predict the transient behavior of drug delivery by using a microneedle
array inserted into tissue. In the framework of mixture theory, biological tissue is treated as a multi-phase fluid saturated
porous medium, where the mathematical behavior of the tissue is characterized by the conservation equations of multi-phase
models. Drug delivery by microneedle array imposes additional requirements on the simulation procedures, including drug absorption
by the blood capillaries and tissue cells, as well as a moving interface along its flowing pathway. The contribution of this
paper is to combine mixture theory with the moving mesh methods in modeling the transient behavior of drug delivery into tissue.
Numerical simulations are provided to obtain drug concentration distributions into tissues and capillaries. 相似文献
147.
Controllable storage and release of solar energy has always been a highlighted scientific issue for its benefit of mankind. Solar thermal fuels (STFs) supply a closed cycle and renewable energy‐storage strategy by transforming solar energy into chemical energy stored in the conformation of molecular isomers, such as cis/trans‐azobenzene, and releasing it as heat under various stimuli. Although the potential high energy density of the STFs which are based on the hybrids of azobenzene derivatives and carbon nanomaterials has been reported the solvent‐assistant charging hinders their practicability. In this study, a solid‐state STF device is designed and fabricated by compositing one photoliquefiable azobenzene (PLAZ) derivative with a flexible fabric template. The photoinduced phase transition of the PLAZ derivative enables the charging of the flexible STFs to be totally solvent‐free. Interestingly, the energy‐storage capacity (energy density ≈201 J g?1) of flexible PLAZ STFs has been improved by the soft fabric template. The exothermic situation is monitored with one infrared camera, which shows 4 °C temperature difference between charged and discharged samples under blue light stimulus. The flexible STFs are may be used in practice as heating equipment. 相似文献
148.
DNA methylation changes in photoperiod-thermo-sensitive male sterile rice PA64S under two different conditions 总被引:1,自引:0,他引:1
Epigenetic modification can occur at a high frequency in crop plants and might generate phenotypic variation without changes in DNA sequences. DNA methylation is an important epigenetic modification that may contribute to environmentally-induced phenotypic variations by regulating gene expression. Rice Photoperiod-Thermo-Sensitive Genic Male Sterile (PTGMS) lines can transform from sterility to fertility under lower temperatures and short-day (SD) conditions during anther development. So far, little is known about the DNA methylation variation of PTGMS throughout the genome in rice. In this study, we investigated DNA cytosine methylation alterations in the young panicles of PTGMS line PA64S under two different conditions using methylation sensitive amplified polymorphism (MSAP) method. Compared with the DNA methylation level of PA64S under lower temperatures and SD conditions (fertility), higher methylation was observed in PA64S (sterility). The sequences of 25 differentially amplified fragments were successfully obtained and annotated. Three methylated fragments, which are homologous to D2, NAD7 and psaA, were confirmed by bisulfite sequencing and their expression levels were also evaluated by qPCR. Real time quantitative PCR analysis revealed that five of the six selected methylated genes were downregulated in PA64S (sterility). These results suggested that DNA methylation may be involved in the sterility–fertility transition of PA64S under two different environmental conditions. 相似文献
149.
150.