全文获取类型
收费全文 | 7105篇 |
免费 | 694篇 |
国内免费 | 1篇 |
专业分类
7800篇 |
出版年
2023年 | 37篇 |
2022年 | 87篇 |
2021年 | 164篇 |
2020年 | 80篇 |
2019年 | 123篇 |
2018年 | 138篇 |
2017年 | 124篇 |
2016年 | 200篇 |
2015年 | 346篇 |
2014年 | 348篇 |
2013年 | 469篇 |
2012年 | 617篇 |
2011年 | 555篇 |
2010年 | 375篇 |
2009年 | 335篇 |
2008年 | 430篇 |
2007年 | 466篇 |
2006年 | 433篇 |
2005年 | 378篇 |
2004年 | 334篇 |
2003年 | 333篇 |
2002年 | 264篇 |
2001年 | 84篇 |
2000年 | 67篇 |
1999年 | 70篇 |
1998年 | 75篇 |
1997年 | 36篇 |
1996年 | 53篇 |
1995年 | 45篇 |
1994年 | 38篇 |
1993年 | 41篇 |
1992年 | 39篇 |
1991年 | 27篇 |
1990年 | 39篇 |
1989年 | 39篇 |
1988年 | 37篇 |
1987年 | 30篇 |
1986年 | 26篇 |
1985年 | 26篇 |
1984年 | 28篇 |
1983年 | 25篇 |
1982年 | 24篇 |
1981年 | 19篇 |
1980年 | 24篇 |
1979年 | 16篇 |
1977年 | 25篇 |
1975年 | 19篇 |
1974年 | 37篇 |
1973年 | 18篇 |
1968年 | 18篇 |
排序方式: 共有7800条查询结果,搜索用时 0 毫秒
1.
Summary Worldwide, invasive weeds threaten agricultural, natural and urban ecosystems. In Australia's agricultural and grazing regions, invasive species often establish across extensive areas where weed management is hampered by an inability to detect the location and timing of an outbreak. In these vast landscapes, an effective detection and monitoring system is required to delineate the extent of the invasion and identify spatial and temporal factors associated with weed establishment and thickening. In this study, we utilize a time series of remote sensing imagery to detect the spatial and temporal patterns of Prickly Acacia ( Acacia nilotica ) invasion in the Mitchell grass plains of North Queensland. We develop a spectral index from Landsat images which is applied to images from 1989 to 2004, in combination with a classification mask, to identify locations and monitor changes in Prickly Acacia density across 29 000 km2 of Mitchell grass plains. The approach identified spectral and temporal signatures consistent with Prickly Acacia infestation on 1.9% of this landscape. Field checking of results confirmed presence of the weed in previously unrecorded locations. The approach may be used to evaluate future spread, or outcomes of management strategies for Prickly Acacia in this landscape and could be employed to detect and monitor invasions in other extensive landscapes. 相似文献
2.
3.
4.
Summary Previous work has shown that the monoclonal antibody 22/18 identifies progenitor cells (blastemal cells) which depend on the nerve for their division in the early stages of limb regeneration in the newt,Notophthalmus viridescens. This antibody also reacts with cultured cells derived from the newt limb, and the intensity of immunoreactivity appears related to cell density and differentiation into myotubes. We report here that the monoclonal antibody 22/18 recognizes a polypeptide (22/18 antigen) which is intracellular and filamentous. Double staining of cells with 22/18 monoclonal antibody and antibodies against various cytoskeletal components indicates that the epitope is expressed on an intermediate filament component. Although this antibody is specific for blastemal cells in cryostat sections of the regenerating limb, its reactivity on immunoblots is not confined to this tissue. The 22/18 antigen is differentially affected by aldehyde fixatives distinguished by the spacing of their reactive groups. While formaldehyde fixation impairs detection of the antigen, ethylene glycol-bis[succinic acid n-hydroxysuccinimide ester] reveals the antigen in sections of normal and regenerating limbs in a distribution that is consistent with the one obtained from immunoblots. We suggest that the 22/18 monoclonal antibody detects a change in protein conformation, probably related to changes in the physiological state of the cell, that occurs transiently during regeneration and possibly during development. 相似文献
5.
Ian D. Hodkinson Jeremy M. Bird Jane K. Hill Robert Baxter . 《Ecological Entomology》2001,26(4):376-387
1. Salix lapponum host plants at an upper altitudinal site differed significantly in size, structural density, phenology, growth performance, and spatial isolation from those growing at a lower site. 2. Plant differences were paralleled by significant differences in psyllid population density and phenology parameters, with psyllid population density, percentage of catkins occupied, and phenological development relatively lower or retarded at the upper site. Population densities at the upper site, nevertheless, remained high. 3. Plant measurements were good predictors of insect density, often explaining up to 73% of the variance in abundance among plants at a given site. 4. Sets of four plant characters identified by best subsets regression were better predictors of psyllid density and development than single factors, although differences were often not great and the combinations of characters selected by multiple regression sometimes differed from the best single predictors. 5. Best single predictors of psyllid density on catkins were measurements of plant size, particularly height, length, and basal stem diameter. Shoot density and catkin phenology were occasionally important but plant isolation and prior growth performance were less important. 6. By contrast with density, age structure of the psyllid population was predicted best from plant phenological measurements, notably catkin phenology. 相似文献
6.
7.
Jeremy Elston 《Plant, cell & environment》1987,10(9):783-784
Books reviewed in this article:
A FUNCTIONAL BIOLOGY OF CROP PLANTS. By V. P. G utschick
MOLECULAR BIOLOGY OF CROP IMPROVEMENT. A Case Study of Wheat, Oilseed Rape, and Faba Beans. By R. B. A ustin , R. B. F lavell , I. E. H enson & H. J. B. L owe 相似文献
A FUNCTIONAL BIOLOGY OF CROP PLANTS. By V. P. G utschick
MOLECULAR BIOLOGY OF CROP IMPROVEMENT. A Case Study of Wheat, Oilseed Rape, and Faba Beans. By R. B. A ustin , R. B. F lavell , I. E. H enson & H. J. B. L owe 相似文献
8.
A sensitive method for detecting bacterial elastase production in growing cultures is described. A variety of commonly isolated clinically relevant aerobic and anaerobic bacteria have been shown to produce the enzyme. 相似文献
9.
A new method of visualizing the angioarchitecture of tissues has been developed that uses blood components in nonperfused materials. Tissue blocks are fixed in 4% paraformaldehyde and cut with a vibratome into 50-60 μm sections. Endogenous peroxidase in red blood cells is then reduced in the presence of hydrogen peroxide with the resultant oxidation of the chromogen 3,3'-diaminobenzidine (DAB). This generates a dark, highly insoluble reaction product throughout the vascular system. The visualization of vascular components can be further enhanced by exposing the sections to peroxidase-conjugated IgG to increase the background staining of the blood plasma. The technique minimizes preparation artifact and permits the application of morphometric analytical methods, thus allowing parameters such as the volume density of the vascular bed to be quantified. 相似文献