全文获取类型
收费全文 | 9723篇 |
免费 | 972篇 |
国内免费 | 381篇 |
出版年
2024年 | 22篇 |
2023年 | 162篇 |
2022年 | 153篇 |
2021年 | 304篇 |
2020年 | 327篇 |
2019年 | 359篇 |
2018年 | 395篇 |
2017年 | 336篇 |
2016年 | 299篇 |
2015年 | 336篇 |
2014年 | 590篇 |
2013年 | 884篇 |
2012年 | 418篇 |
2011年 | 451篇 |
2010年 | 350篇 |
2009年 | 405篇 |
2008年 | 396篇 |
2007年 | 425篇 |
2006年 | 391篇 |
2005年 | 312篇 |
2004年 | 305篇 |
2003年 | 304篇 |
2002年 | 224篇 |
2001年 | 185篇 |
2000年 | 148篇 |
1999年 | 163篇 |
1998年 | 193篇 |
1997年 | 158篇 |
1996年 | 151篇 |
1995年 | 132篇 |
1994年 | 161篇 |
1993年 | 147篇 |
1992年 | 134篇 |
1991年 | 135篇 |
1990年 | 116篇 |
1989年 | 80篇 |
1988年 | 101篇 |
1987年 | 80篇 |
1986年 | 87篇 |
1985年 | 124篇 |
1984年 | 130篇 |
1983年 | 89篇 |
1982年 | 112篇 |
1981年 | 87篇 |
1980年 | 66篇 |
1979年 | 42篇 |
1978年 | 42篇 |
1977年 | 17篇 |
1976年 | 16篇 |
1973年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Sumihiro Koyama 《Cytotechnology》2007,55(2-3):125-133
Establishing tissue cultures derived from deep-sea multicellular organisms has been extremely difficult because of the serious
damage they sustain upon decompression and exposure to the high temperature of surface seawater. We developed a novel pressure-stat
aquarium system for the study of living deep-sea multicellular organisms under pressure. Using this system, we have succeeded
in maintaining a variety of deep-sea multicellular organisms under pressure and atmospheric conditions after gradual, slow
decompression. Furthermore, we successfully cultivated and freeze-stocked pectoral fin cells of the deep-sea eel Simenchelys parasiticus collected at a depth of 1,162 m under atmospheric pressure conditions. This review describes novel capture and maintenance
devices for deep-sea organisms and cell culture studies of the organisms under atmospheric and pressure conditions. 相似文献
992.
J. Waterhouse G. Atkinson T. Reilly H. Jones B. Edwards 《Biological Rhythm Research》2007,38(3):181-194
The development of ambulatory blood pressure monitoring devices and the beat-by-beat measurement of heart rate have enabled it to be established that there are circadian rhythms in heart rate and blood pressure in subjects living normally. Investigations of these variables have led to quantification of their fall at night, and rapid rise on awakening and becoming active in the morning. These changes are of particular interest insofar as abnormalities in them are associated with cardiovascular problems and morbidity in patients and also act as risk factors in otherwise healthy individuals. It has also been shown that there are many other variables of the cardiovascular system. The causes of the circadian rhythms in heart rate and blood pressure are outlined, with particular stress upon the role of the autonomic nervous system, as assessed from low- and high-frequency components of the variation in heart rate measured beat-by-beat. Activity increases blood pressure, but there is evidence that this “reactivity” varies with time of day, and this also might be related to cardiovascular morbidity. Based upon data from several sources, including night work, resting subjects and bed-ridden patients, it is concluded that the contribution of the “body clock” to producing the circadian rhythm in heart rate and blood pressure is relatively small. A bias towards an exogenous cause applies also to most other circadian rhythms in the cardiovascular system. Knowledge of circadian rhythmicity in cardiovascular system, together with an understanding of its causes, provides a rationale for advice to reduce cardiovascular risk and to assess the efficacy of therapies. 相似文献
993.
Iterative applications are known to run as slow as their slowest computational component. This paper introduces malleability, a new dynamic reconfiguration strategy to overcome this limitation. Malleability is the ability to dynamically change the
data size and number of computational entities in an application. Malleability can be used by middleware to autonomously reconfigure
an application in response to dynamic changes in resource availability in an architecture-aware manner, allowing applications
to optimize the use of multiple processors and diverse memory hierarchies in heterogeneous environments.
The modular Internet Operating System (IOS) was extended to reconfigure applications autonomously using malleability. Two
different iterative applications were made malleable. The first is used in astronomical modeling, and representative of maximum-likelihood
applications was made malleable in the SALSA programming language. The second models the diffusion of heat over a two dimensional
object, and is representative of applications such as partial differential equations and some types of distributed simulations.
Versions of the heat application were made malleable both in SALSA and MPI. Algorithms for concurrent data redistribution
are given for each type of application. Results show that using malleability for reconfiguration is 10 to 100 times faster
on the tested environments. The algorithms are also shown to be highly scalable with respect to the quantity of data involved.
While previous work has shown the utility of dynamically reconfigurable applications using only computational component migration,
malleability is shown to provide up to a 15% speedup over component migration alone on a dynamic cluster environment.
This work is part of an ongoing research effort to enable applications to be highly reconfigurable and autonomously modifiable
by middleware in order to efficiently utilize distributed environments. Grid computing environments are becoming increasingly
heterogeneous and dynamic, placing new demands on applications’ adaptive behavior. This work shows that malleability is a
key aspect in enabling effective dynamic reconfiguration of iterative applications in these environments.
相似文献
Carlos A. VarelaEmail: |
994.
A mathematical model of high shear wet granulation is proposed, where granule breakage, and not growth, is the dominant process.
The energy required for granule breakage is assumed to be provided by the impact of granules between themselves and the granulator
parts, and the extent of granule breakage determined by the balance between the impactenergy and the work of adhesion between
the agglomerating particles. A specific volume of dry powder per unit crack surface area was allowed to reattach to the surface
of broken granules to account for granule growth. To verify proposed model conditions, lactose monohydrate was granulated
with a relatively low amount (6%) of the binder phase, polyvinyl-pyrrolidone and water, and was added to the powder before
granulation.
The trend in granule size distribution during the experiment closely follwed the predicted model with an initial increase
in the weight fraction of the larger granules. This increase was possibly due to extensive breakage of weaker granules and
less extensive breakage, as if by attrition, of stronger granules, accompanied by the attachment of dry powder to the cracked
surfaces. Eventually, larger granules experience increased impact energy and break. When excess binder is added and, higher
volumes of powder reattach to the crack surface, more large granules form leading to granule overgrowth. This model highlights
the importance of the probability of impact per unit time interval (ie, the rate of impact), the strength of the granules
and the volume of powder that could attach to the cracked surface in high shear granulation processes where significant granule
breakage is encountered.
Published: August 10, 2007 相似文献
995.
Karlia Meitha Dennis Konnerup Timothy D. Colmer John A. Considine Christine H. Foyer Michael J. Considine 《Annals of botany》2015,116(4):703-711
Background and Aims Plants regulate cellular oxygen partial pressures (pO2), together with reduction/oxidation (redox) state in order to manage rapid developmental transitions such as bud burst after a period of quiescence. However, our understanding of pO2 regulation in complex meristematic organs such as buds is incomplete and, in particular, lacks spatial resolution.Methods The gradients in pO2 from the outer scales to the primary meristem complex were measured in grapevine (Vitis vinifera) buds, together with respiratory CO2 production rates and the accumulation of superoxide and hydrogen peroxide, from ecodormancy through the first 72 h preceding bud burst, triggered by the transition from low to ambient temperatures.Key Results Steep internal pO2 gradients were measured in dormant buds with values as low as 2·5 kPa found in the core of the bud prior to bud burst. Respiratory CO2 production rates increased soon after the transition from low to ambient temperatures and the bud tissues gradually became oxygenated in a patterned process. Within 3 h of the transition to ambient temperatures, superoxide accumulation was observed in the cambial meristem, co-localizing with lignified cellulose associated with pro-vascular tissues. Thereafter, superoxide accumulated in other areas subtending the apical meristem complex, in the absence of significant hydrogen peroxide accumulation, except in the cambial meristem. By 72 h, the internal pO2 gradient showed a biphasic profile, where the minimum pO2 was external to the core of the bud complex.Conclusions Spatial and temporal control of the tissue oxygen environment occurs within quiescent buds, and the transition from quiescence to bud burst is accompanied by a regulated relaxation of the hypoxic state and accumulation of reactive oxygen species within the developing cambium and vascular tissues of the heterotrophic grapevine buds. 相似文献
996.
MARTIN D. VENTURAS EVAN D. MACKINNON ANNA L. JACOBSEN R. BRANDON PRATT 《Plant, cell & environment》2015,38(6):1060-1068
Xylem resistance to water stress‐induced cavitation is an important trait that is associated with drought tolerance of plants. The level of xylem cavitation experienced by a plant is often assessed as the percentage loss in conductivity (PLC) at different water potentials. Such measurements are constructed with samples that are excised underwater at native tensions. However, a recent study concluded that cutting conduits under significant tension induced cavitation, even when samples were held underwater during cutting. This resulted in artificially increased PLC because of what we have termed a ‘tension‐cutting artefact’. We tested the hypothesized tension‐cutting artefact on five species by measuring PLC at native tension compared with after xylem tensions had been relaxed. Our results did not support the tension‐cutting artefact hypothesis, as no differences were observed between native and relaxed samples in four of five species. In a fifth species (Laurus nobilis), differences between native and relaxed samples appear to be due to vessel refilling rather than a tension‐cutting effect. We avoided the tension‐cutting artefact by cutting samples to slightly longer than their measurement length and subsequent trimming of at least 0.5 cm of sample ends prior to measurement. 相似文献
997.
Maya Schuldiner Einat Zalckvar 《Biology of the cell / under the auspices of the European Cell Biology Organization》2015,107(4):89-97
In recent years, high‐throughput experimentation with quantitative analysis and modelling of cells, recently dubbed systems cell biology, has been harnessed to study the organisation and dynamics of simple biological systems. Here, we suggest that the peroxisome, a fascinating dynamic organelle, can be used as a good candidate for studying a complete biological system. We discuss several aspects of peroxisomes that can be studied using high‐throughput systematic approaches and be integrated into a predictive model. Such approaches can be used in the future to study and understand how a more complex biological system, like a cell and maybe even ultimately a whole organism, works. 相似文献
998.
为了探讨齐口裂腹鱼(Schizothorax prenanti)血细胞发生发育的模式,采用Wright’s、碘酸雪夫氏(PAS)和苏丹黑B(SBB)染色方法对齐口裂腹鱼的头肾、中肾、脾和肝组织印片进行染色,了解其发生的具体组织和发育的一般过程。结果表明:齐口裂腹鱼血细胞可分为红细胞系、淋巴细胞系、单核细胞系、粒细胞系和其他细胞,且这些细胞系的发育均经过3个阶段,即原始阶段、幼稚阶段、成熟阶段。通过观察和统计不同阶段各种血细胞的形态、大小、比例及染色特征,发现原始阶段的血细胞体积较大,其中原始单核细胞最大,大小为(18.90±1.59)μm×(16.32±0.70)μm,在4种组织中原始阶段的红细胞和单核细胞在头肾中所占比例最大,分别为0.64%和0.59%,原粒细胞和原淋巴细胞分别在中肾和脾中比例最大,在肝中少量存在,此外在各组织印片中均发现血栓细胞的存在,在肝中发现巨噬细胞。因此头肾、中肾和脾是齐口裂腹鱼的主要造血组织,少量细胞在肝中产生。粒细胞发育过程中,除原始粒细胞PAS染色阴性外,其他阶段均呈阳性,且阳性随不断成熟逐渐增强;单核细胞从原单核细胞到成熟的单核细胞,PAS阳性逐渐增强;而SBB染色发现,粒细胞、淋巴细胞及单核细胞均呈阳性,未成熟的细胞染色程度不一致,成熟的细胞阳性染色最为强烈。在红细胞系发育过程中经历了细胞由大变小再变大的过程,而粒细胞和淋巴细胞发育过程仅出现由大变小的过程,在红细胞的发育过程中还出现了染色质固缩和血红蛋白增加。 相似文献
999.
1000.
Shereen Shawki Taghrid Gaafar Hadeel Erfan Engy El Khateeb Ahmad El Sheikhah Rabab El Hawary 《Microbiology and immunology》2015,59(6):348-356
Umbilical cord blood (UCB) is of great interest as a source of stem cells for use in cellular therapies. The immunomodulatory effect of mesenchymal stem cells (MSCs) originating from bone marrow, adipose tissue and amniotic membrane has previously been reported. In this study, MSCs were isolated from UCB with the aim of evaluating their immunomodulatory effects on proliferation of PB lymphocytes by two different techniques; namely, 5‐bromo‐2‐deoxyuridine ELISA and a carboxy fluorescein diacetate succinimidyl ester flow cytometric technique. MSCs were isolated from UCB, propagated until Passage four, and then characterized for cell surface markers by flow cytometry and ability to differentiate towards osteocytes and adipocytes. Immunosuppressive effects on PB lymphocytes were examined by co‐culturing mitomycin C‐treated UCB MSCs with mitogen‐stimulated lymphocytes for 72 hr. Thereafter, proliferation of lymphocytes was detected by CFSE flow cytometry and colorimetric ELISA. The titers of cytokines in cell culture supernatant were also assayed to clarify possible mechanisms of immunomodulation. UCB MSCs suppressed mitogen‐stimulated lymphocyte proliferation, which occurs via both cell‐cell contact and cytokine secretion. Titers of transforming growth factor beta and IL 10 increased, whereas that of IFN‐γ decreased in the supernatants of co‐cultures. Thus, UCB MSCs suppress the proliferation of mitogen‐stimulated lymphocytes. However further in vivo studies are required to fully evaluate the immunomodulatory effects of UCB MSCs. 相似文献