全文获取类型
收费全文 | 1654篇 |
免费 | 155篇 |
出版年
2023年 | 14篇 |
2022年 | 27篇 |
2021年 | 53篇 |
2020年 | 47篇 |
2019年 | 56篇 |
2018年 | 40篇 |
2017年 | 42篇 |
2016年 | 74篇 |
2015年 | 130篇 |
2014年 | 113篇 |
2013年 | 123篇 |
2012年 | 164篇 |
2011年 | 143篇 |
2010年 | 78篇 |
2009年 | 63篇 |
2008年 | 81篇 |
2007年 | 72篇 |
2006年 | 60篇 |
2005年 | 43篇 |
2004年 | 41篇 |
2003年 | 26篇 |
2002年 | 38篇 |
2001年 | 16篇 |
2000年 | 11篇 |
1999年 | 17篇 |
1998年 | 14篇 |
1997年 | 13篇 |
1996年 | 15篇 |
1995年 | 14篇 |
1994年 | 5篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 11篇 |
1987年 | 14篇 |
1986年 | 8篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 6篇 |
1980年 | 8篇 |
1979年 | 5篇 |
1978年 | 8篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 3篇 |
1971年 | 7篇 |
排序方式: 共有1809条查询结果,搜索用时 15 毫秒
1.
2.
Summary In the initial phase of the geotropical reaction of the Chara rhizoid the growth difference postulated by Sievers (1967c) between the physically upper, slightly subapical flank and the lower one is demonstrated. In horizontal exposure the growth of the extreme cell apex is continued, while the growth of the lower flank is inhibited and that of the upper one is promoted. In the end phase the cell apex shows a damped oscillation until it finally reaches the vertical growth direction. The statoliths follow the oscillating growth of the cell tip from one flank to the opposite one until they are statistically equally redistributed in their normal position.—In vertical exposure under reduced turgor pressure the statoliths fall down into the extreme cell apex, where they inhibit the growth of this part of the cell wall, while the subapical wall grows transversally.—It is concluded that the statoliths inhibit the growth of the cell wall area which they cover.—The physical phase of the reaction chain, the susception, is the gravity-induced downward displacement of the statoliths. The physiological phase starts with the diversion of the acropetal transport of the Golgi vesicles to the upper part of the cell, which is caused by the block of statoliths (perception). The greater rate of vesicle incorporation into the upper flank in comparison to the lower one causes the subapical growth difference which results in the curvature (reaction).—In the case of the Chara rhizoid Golgi- and statolith-apparatus function as a self-regulating cellular system.
Herrn Prof. Dr. Dr. h. c. Kurt Mothes zum 70. Geburtstag. 相似文献
Herrn Prof. Dr. Dr. h. c. Kurt Mothes zum 70. Geburtstag. 相似文献
3.
Ewa A. Czy Carla Guilln Escrib Hendrik Wulf Andrew Tedder Meredith C. Schuman Fabian D. Schneider Michael E. Schaepman 《Ecology and evolution》2020,10(14):7419-7430
- The growing pace of environmental change has increased the need for large‐scale monitoring of biodiversity. Declining intraspecific genetic variation is likely a critical factor in biodiversity loss, but is especially difficult to monitor: assessments of genetic variation are commonly based on measuring allele pools, which requires sampling of individuals and extensive sample processing, limiting spatial coverage. Alternatively, imaging spectroscopy data from remote platforms may hold the potential to reveal genetic structure of populations. In this study, we investigated how differences detected in an airborne imaging spectroscopy time series correspond to genetic variation within a population of Fagus sylvatica under natural conditions.
- We used multi‐annual APEX (Airborne Prism Experiment) imaging spectrometer data from a temperate forest located in the Swiss midlands (Laegern, 47°28'N, 8°21'E), along with microsatellite data from F. sylvatica individuals collected at the site. We identified variation in foliar reflectance independent of annual and seasonal changes which we hypothesize is more likely to correspond to stable genetic differences. We established a direct connection between the spectroscopy and genetics data by using partial least squares (PLS) regression to predict the probability of belonging to a genetic cluster from spectral data.
- We achieved the best genetic structure prediction by using derivatives of reflectance and a subset of wavebands rather than full‐analyzed spectra. Our model indicates that spectral regions related to leaf water content, phenols, pigments, and wax composition contribute most to the ability of this approach to predict genetic structure of F. sylvatica population in natural conditions.
- This study advances the use of airborne imaging spectroscopy to assess tree genetic diversity at canopy level under natural conditions, which could overcome current spatiotemporal limitations on monitoring, understanding, and preventing genetic biodiversity loss imposed by requirements for extensive in situ sampling.
4.
J. Sievers H. P. Klemm† S. Jenner† H. G. Baumgarten† M. Berry‡ 《Journal of neurochemistry》1980,34(4):765-771
Abstract: Intracisternal administration of 100 μg 6-OHDA to newborn rats causes permanent defects, not only of the monoaminergic neuron system, but also of extraneuronal tissue elements. The long noradrenergic fibre tracts are irreversibly destroyed, while the short projections recover and regenerate after a transient period of injury. In the major noradrenergic cell group, the locus coeruleus, most of the cells in the caudal and middle parts degenerate, while a small dorsorostral group survives and forms the source of the regenerating fibres. Dopaminergic and serotonergic fibre tracts are also affected. The 6-OHDA treatment also damages granule and dial cells of the cerebellar cortex as well as the mesenchymal cells of the pial coverings of the cerebellum, leading to primitive foliation, absence of fissuration, and defective migration of granule cells and resulting in a marked reduction of cerebellar size, area, and granule cell number. 相似文献
5.
I Fabian Y Kletter I Bleiberg 《Proceedings of the Society for Experimental Biology and Medicine. Society for Experimental Biology and Medicine (New York, N.Y.)》1987,185(4):434-440
The modulatory effect of 1,25-dihydroxyvitamin D3 (vit D) on the growth of myeloid progenitors and on the composition of the stromal layer in human bone marrow long-term cultures was studied. Vit D (2 X 10(-8) M) caused an enhancement in myeloid progenitor cell (CFU-C) growth in the nonadherent and adherent layers during the entire 5-week incubation period. The vitamin did not alter the differentiation pattern of CFU-C (monocyte-macrophage progenitors CFU-M, granulocytic progenitors CFU-G, or monocyte-granulocyte progenitors CFU-GM). Vit D caused a marked increase in the percentage of lipid-containing cells in the adherent layer and an increase in the number of cells that specifically bound My4 monoclonal antibody (McAb), that reacted positively to fluoride-sensitive alpha-naphthyl acetate esterase, and that phagocytosed Candida albicans (CA). Concentrated supernatants harvested from control cultures showed significant levels of myeloid colony stimulating factor (CSF) activity. The addition of vit D to cultures for 5 weeks did not alter CSF levels. These results suggest that vit D may play a role in hematopoiesis by acting directly on the progenitor cells or via the stromal cell production of stimulatory factor(s). 相似文献
6.
Methoprene, a chemical analog of juvenile hormone, is toxic when applied to late third-instar larvae of Drosophila melanogaster. Using an ethyl methane sulfonate mutagenesis screen, we have selected two noncomplementing mutants, one of which is nearly 100 times more resistant than wild-type to either methoprene or juvenile hormone III topically applied or incorporated into the diet. The mutation, named methoprene-tolerant (Met), also confers resistance to methoprene-induced pseudotumor formation in larvae as well as to juvenile hormone III- or methoprene-induced vitellogenic oocyte development in adult females. Met adults show little or no cross-resistance to four other insecticides. The mutation was mapped by recombination to a location 35.4 on the X-chromosome and uncovered by chromosomes deficient for the region 10C2-10D4. Complementation was observed between Met and a lethal allele of the RNA polymerase II locus, which is also found in this region. Since the Met mutation also confers resistance to methoprene-induced abnormalities in adult cuticle formation, the autonomy of Met expression could be evaluated in flies mosiac for this mutation. Autonomous expression of Met was found both in abdominal cuticle as well as in external male genitalia. The characteristics of Met are consistent with those expected of a mutant having altered juvenile hormone reception in target tissue. 相似文献
7.
Iannotti E. L. Mueller R. E. Sievers D. M. Georgacakis D. G. Gerhardt K. O. 《Journal of industrial microbiology & biotechnology》1986,1(1):57-61
Summary The presence of phenylacetic acid (PAA) in an anaerobic swine manure digester was determined by gas chromatography of the butyl ester and confirmed by mass spectroscopy. PAA concentration increased during start-up of a digester and with low carbon, high nitrogen loading. Unlike acetate, propionate and butyrate, the concentration of PAA varied little through the day in a stable digester loaded once per day. The laboratory scale digester was loaded at 4 g of swine manure solids/liter digester volume per day. The retention time and temperature were 15 days and 37°C. PAA is a microbial intermediate which is produced by one group of anaerobic bacteria and converted to methane by other members of the bacterial community in the digester. As such, it may be a useful indicator of the relative metabolic activity of the bacterial groups and thus of the overall stability of the anaerobic process. 相似文献
8.
The structural polarity of statocytes of Lepidium sativum L. is converted to a physical stratification by a root-tip-directed centrifugal acceleration. Sedimentation of amyloplasts and nucleus to the centrifugal (distal) cell pole and the lateral displacement of the distal endoplasmic reticulum (ER) complex occur after centrifugation for 20 min at an acceleration of 50 g. With higher doses (20 min, 100-2,000 g), smaller organelles become increasingly displaced. From the centrifugal to the centripetal cell pole, the following stratification is observed: 1) amyloplasts with mitochondria; 2) nucleus with mitochondria and a few dictyosomes, as well as laterally located ER; 3) dictyosomes with a few mitochondria; 4) vacuoles; and 5) lipid droplets. Within the first 7.5 min, after the roots have been returned to 1 g, the original arrangement of the amyloplasts sedimented on the underlying ER complex is reestablished in 66% of the statocytes. When roots previously centrifuged in an apical direction are exposed in a horizontal position to 1 g, the latent period of the graviresponse is increased by 7.5 min relative to the non-centrifuged controls. The kinetics of the response are identical to the controls. Roots centrifuged first in an apical direction and then for 2 h in a lateral direction (1,000 g) have statocytes with a physical stratification perpendicular to the root axis. A gravitropic curvature does not take place during the lateral centrifugation. These results support the hypothesis that the distal ER complex is necessary and sufficient for graviperception. 相似文献
9.
J Wikman-Coffelt R Sievers R J Coffelt W W Parmley 《Biochemical and biophysical research communications》1983,111(2):450-455
Oscillations of cAMP with the cardiac cycle were demonstrated in the rat heart using a stimulator-triggered rapid freeze-clamp to decrease the temperature of the heart from 37 degrees C to -80 degrees C in 5 msec (20,000 degrees/sec) at a predetermined phase of the cardiac cycle. The nucleotide, cAMP, oscillated 60% with the cardiac cycle during normal working conditions, the higher cAMP value occurring during systole. 相似文献
10.
Allele-specific expression of the murine Ren-1 genes 总被引:5,自引:0,他引:5
J R Fabian L J Field R A McGowan J J Mullins C D Sigmund K W Gross 《The Journal of biological chemistry》1989,264(29):17589-17594