全文获取类型
收费全文 | 317篇 |
免费 | 14篇 |
国内免费 | 17篇 |
专业分类
348篇 |
出版年
2023年 | 4篇 |
2022年 | 7篇 |
2021年 | 5篇 |
2020年 | 10篇 |
2019年 | 3篇 |
2018年 | 6篇 |
2017年 | 5篇 |
2016年 | 6篇 |
2015年 | 6篇 |
2014年 | 7篇 |
2013年 | 27篇 |
2012年 | 13篇 |
2011年 | 13篇 |
2010年 | 6篇 |
2009年 | 10篇 |
2008年 | 12篇 |
2007年 | 22篇 |
2006年 | 8篇 |
2005年 | 9篇 |
2004年 | 10篇 |
2003年 | 8篇 |
2002年 | 7篇 |
2001年 | 11篇 |
2000年 | 8篇 |
1999年 | 9篇 |
1998年 | 11篇 |
1997年 | 9篇 |
1996年 | 7篇 |
1995年 | 9篇 |
1994年 | 6篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 9篇 |
1989年 | 12篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 2篇 |
排序方式: 共有348条查询结果,搜索用时 15 毫秒
81.
We investigated radial patterns of sap flux density and wood properties in the sapwood of young loblolly pine (Finns taeda L.), mature white oak (Quercus alba L.) and sweetgum (Liquidambar styraciflua L.), which represent three major classes of wood anatomy: non-porous (coniferous), ring-porous and diffuse-porous. Radial measurements of xylem sap flux density were made in sections of xylem extending to 20 mm and 20–40 mm from the cambium. These measurements were compared with measurements of the relative water content (Rs) and sapwood specific gravity (ρr) of corresponding radial sections. In both hardwood species, sap flow differences were rarely significant between the two depth intervals. In pine, a 59% reduction in daily sap flux density from outer to inner sapwood was found. This could not be accounted for by a 3% drop in Rs; rather, an accompanying 9% reduction in ρr indicated a transition between the depth intervals from mature to juvenile sapwood, and is the probable cause of the lower flux rate in the inner xylem of pine. 相似文献
82.
We developed an efficient plant regeneration system from protoplasts for poplar (Populus alba L.). Protoplasts were isolated from 4-day-old suspension cultures derived from seed-induced calli with a yield of 6.96× 106 cells/g fresh weight cells and then cultured at a concentration of 2.5×105 cells/ml in NH4NO3-free Murashige and Skoog (MS) medium supplemented with 5 μM 2,4-dichlorophenoxyacetic acid (2,4-D), 0.05 μM thidiazuron (TDZ) and 0.5 M glucose as a osmoticum. The plating efficiency of the cultured protoplasts was calculated at 26.5% at day 7 and 31.7% at
day 14. Cell colonies were observed after culturing for 4 weeks. Regenerated colonies were propagated through subculture in
liquid MS medium supplemented with 5 μM 2,4-D. Buds were induced from regenerated calli on MS medium containing 10 μM kinetin or 1 μM TDZ. Regenerated shoots were rooted on half-strength MS medium, and the plantlets were transplanted in soil. Randomly amplified
polymorphic DNA analysis did not detect any DNA polymorphism among the regenerated plants.
Received: 7 March 1997 / Revision received: 16 June 1997 / Accepted: 5 July 1997 相似文献
83.
Sex ratio polymorphism has been extensively studied in Silene latifolia, but it is neither known whether inbreeding (which is likely to occur under field conditions) affects it, nor which of the proposed mechanisms (Y degeneration, X-linked drive) is more important. Both mechanisms predict reduced pollen performance. In this study, females were crossed with pollen from related and unrelated males in single-donor and two-donor crosses, and the sex ratio of offspring (n = 866, 60 crosses), sons'in vitro pollen germination and sex ratios in parental families were scored. Flowers receiving only unrelated pollen produced a significant excess of sons. Sex ratios were not significantly correlated between generations. Sons'in vitro pollen germination was significantly negatively correlated with the 'sex-ratio phenotype' of maternal grandfathers, but not of fathers. This generation leap may be consistent with X-linked determinants because Y-linked determinants alone cannot explain it (grandfathers, fathers and sons share the same Y chromosome). Further work is required, but inbreeding and limited dispersal may lead to local accumulation of biasing factors, a process potentially countered by conditional shifts to produce more sons in pure outbred crosses. 相似文献
84.
Applications of a thermal imaging technique in the study of the ascent of sap in woody species 总被引:10,自引:0,他引:10
T. ANFODILLO G. B. SIGALOTTI M. TOMASI P. SEMENZATO R. VALENTINI 《Plant, cell & environment》1993,16(8):997-1001
We present applications of infrared thermograph to the direct observation of water transport in stems of woody species. Presently, the method gives only qualitative information on the path of sap movement along the stem, and therefore, does not replace any of the common quantitative methods of sap flow measurement. Nevertheless, the thermal imaging technique provides a novel approach to the study of the ascent of sap and could have a role in supporting more quantitative methods. Thermography permits visualization of the thermal variations of a fairly large area of the stem in real time, and hence, is well suited for spatial analysis of sap movement. Three experiments were carried out during the summers of 1991 and 1992, with the following objectives: to define the sap flow distribution in the active annual rings; to identify grain anomalies in the sap wood; and to study the consequences of induced embolism on the path of sap flow. Altogether, we tested more than 10 woody species (broad-leaves and conifers) either in natural conditions or in the nursery. We found considerable differences in the spatial distribution of sap flow not only between species and individuals but also within a single tree. Grain anomalies or embolized zones in the xylem, which are easily demonstrated, often further modify the path of sap flow. The irregularity of sapwood dimensions and of sap flow among tree rings requires careful evaluation of the positioning of the probes when using methods for quantitative measurements. 相似文献
85.
Abstract Mycoflora of Silver fir (Abies alba Mill.) stumps. - This work is a first contribution to the knowledge of the mycoflora colonizing Silver fir (Abies alba Mill.) stumps in Italy. Autochthonous fungal species were isolated from the wood of A. alba stumps from three different sites in the Tuscan Apennines. Frequency values for the isolated species and taxonomic groups were calculated. Deuteromycetes (57–82.5%) occurred more frequently than basidiomycetes (5–36%), phycomycetes (0.7–11.6%) and ascomycetes (0–1%). The most common genera were found to be: Aleurodiscus, Penicillium, Phellinus, Phialophora, Scytalidium, Sistotrema and Trichoderma. An effective method of isolation from wood is also described. 相似文献
86.
E. Wagner 《Biological Rhythm Research》2013,44(4):313-332
Abstract The period length of the locomotor activity rhythm of Drosophila melanogaster wild form is under conditions of continuous weak red light 23.38 h, whereas die eye mutants Ly3 with a 23.71 h mean period and JK 84 with 23.14 h differ significantly. This might be due to a changed perception of light and not the result of a change in the circadian pacemaker by the mutation. The mutant sine oculis exhibits a normal activity rhythm if the complex eyes are not completely reduced. If this is the case, the activity pattern is either less rhythmic, composed of several rhythms with different periods or truely arrhythmic depending on the individual fly. Since the mutation in sine oculis affects in addition to the complex eye the distal part of die medulla and the lamina of the optic lobe, it is suggested that the circadian pacemakers for the locomotor activity rhythm is localized in these parts. 相似文献
87.
Addition of lysine to the culture medium of rumen ciliates increased the amount of pipecolate in the medium to the level over the control.l-Lysine- U-14C was partly incorporated into ciliate protein unchanged, and partly converted to radioactive pipecolate. The autoradiogram of amino acids in supernatant fluid of the medium revealed the traces of two unidentified spots other than lysine and pipecolate as a main metabolite.Since the radioactive pipecolate was not changed further more by ciliates, it seemed to be the end product in lysine degradation.After addition of l-lysine-α,ε-15N to the medium, 15N was detected mainly in the fractions of pipecolate and ammonia and a little in that of glutamate, alanine and aspartate of the medium. 相似文献
88.
We describe the features of the root system and the gravitropism of roots produced bySonneratia alba. The root system consists of four root types with different growth directions: (a) Pneumatophores, which are negatively orthogravitropic
and their statocytes are very large (922 μm2) and the statolith is located near the proximal wall, (b) Cable roots and (c) Feeding roots which are both diagravitropic
and their statoliths are settled along the longitudinal wall, and (d) Anchor roots which are positively orthogravitropic.
The statocyte cells are the smallest (420 μm2) and statoliths settled at the distal wall. We found that all roots with marked gravitropism have statoliths that settle
along different walls of the statocyte. This implies that the statoliths sensing of gravity is done by gravity on mass, and
that they are denser than surrounding cytoplasm and this position is related to root growth direction. This finding matches
the statoliths sediment under the effect of gravity. Irrespective of statolith, position and direction of growth may be stable. 相似文献
89.
90.
Claudia Albrecht María G. Pellarin José Baronetti María J. Rojas Inés Albesa Alberto J. Eraso 《Luminescence》2011,26(6):424-428
This study was undertaken to elucidate the antioxidant effect of Zizyphus mistol and Prosopis alba, with the hypothesis that these fruits can counteract the induction of reactive oxygen species (ROS) caused by toxins produced by Escherichia coli. In the search of nutrients effective against the Hemolytic Uremic Syndrome (HUS), we detected by chemiluminescence a protective role of both plants, due to their natural antioxidants significantly decreasing the levels of ROS induced by toxins from E. coli in blood. The ferric reducing antioxidant power (FRAP) was found to be higher in Z. mistol than in P. alba. The chemical analyses of the phenols and flavonoids present in the fruit extracts indicated that the FRAP correlated with the amount of phenolic compounds, but not with the flavonoids analyzed. Both fruits studied reduce the induction of ROS, and in this way help to prevent the development of complications related to oxidative stress generated in the blood of patients with HUS. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献