全文获取类型
收费全文 | 2040篇 |
免费 | 169篇 |
国内免费 | 4篇 |
专业分类
2213篇 |
出版年
2023年 | 15篇 |
2022年 | 21篇 |
2021年 | 48篇 |
2020年 | 12篇 |
2019年 | 34篇 |
2018年 | 44篇 |
2017年 | 35篇 |
2016年 | 41篇 |
2015年 | 78篇 |
2014年 | 98篇 |
2013年 | 126篇 |
2012年 | 132篇 |
2011年 | 132篇 |
2010年 | 92篇 |
2009年 | 79篇 |
2008年 | 115篇 |
2007年 | 109篇 |
2006年 | 105篇 |
2005年 | 72篇 |
2004年 | 77篇 |
2003年 | 64篇 |
2002年 | 53篇 |
2001年 | 36篇 |
2000年 | 47篇 |
1999年 | 40篇 |
1998年 | 25篇 |
1997年 | 22篇 |
1996年 | 19篇 |
1995年 | 19篇 |
1994年 | 25篇 |
1993年 | 15篇 |
1992年 | 37篇 |
1991年 | 25篇 |
1990年 | 22篇 |
1989年 | 23篇 |
1988年 | 20篇 |
1985年 | 23篇 |
1984年 | 11篇 |
1983年 | 13篇 |
1982年 | 13篇 |
1979年 | 12篇 |
1977年 | 12篇 |
1976年 | 13篇 |
1975年 | 13篇 |
1974年 | 14篇 |
1973年 | 9篇 |
1972年 | 16篇 |
1971年 | 12篇 |
1967年 | 11篇 |
1966年 | 14篇 |
排序方式: 共有2213条查询结果,搜索用时 15 毫秒
1.
2.
3.
A palaeocommunity of large Conichnus conicus, a conical, cone-in-cone shaped burrow, created by sea anemones, occurs in medium-grained, crossbedded, well-sorted sandstone in the middle part of the Cretaceous Guneri Member of the Bhuj Formation in India. The trace fossil Conichnus is considered to be a common element of the Skolithos ichnofacies and is interpreted to reflect equilibrium movement in response to substrate aggradation. In the present study, three different varieties of Conichnus conicus are distinguished based on morphology and internal fabric. Community dynamics and burrowing behavior are revealed by inter-burrow relationships, burrow initiation levels and sedimentology. Three types of behavior are envisaged: retrusive equilibrium, protrusive equilibrium response, and escape behavior. Palaeocommunity dynamics show that the tracemakers consisted of only adult organisms that initiated burrows during neap tides and are adapted to feed effectively during weak flow conditions. The occurrence of Conichnus palaeocommunity in the Guneri Member indicates the tidal conditions in a fully marine setting. Results presented herein may aid in the understanding of palaeocommunity dynamics in other shallow marine sequences. 相似文献
4.
The isolation,characterization and application in the Triticeae of a set of wheat RFLP probes identifying each homoeologous chromosome arm 总被引:8,自引:0,他引:8
P. J. Sharp S. Chao S. Desai M. D. Gale 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1989,78(3):342-348
Summary To investigate the use of RFLP analysis in the Triticeae, a set of low copy number probes has been isolated from a wheat cDNA library. The probes identify each of the 14 homoeologous chromosome arms of wheat as determined by analysis of DNA fragments hybridizing to the probes in aneuploid lines of Chinese Spring. These probes can be used in RFLP analyses both for the assignment of homoeology of alien chromosomes or arms added to wheat, and for the determination of chromosome dosage in wheat aneuploids. Different chromosomes from various Triticeae species can therefore be followed in a wheat genetic background using a single technique. The potential uses of the set in facilitating the transfer of alien segments into wheat are outlined. 相似文献
5.
6.
R Dahiya B Boyle B C Goldberg W H Yoon B Konety K Chen T S Yen W Blumenfeld P Narayan 《Biochimie et biologie cellulaire》1992,70(7):548-554
Metastatic variants of human prostatic adenocarcinoma cell lines (DU-145, LNCaP, and ND-1) were studied by using soft agar colony forming efficiency, nude mice tumorigenicity, in vitro invasion assay, and type IV collagenase assay. The DU-145 and ND-1 cell line showed higher metastatic potential than LNCaP. Lipids from DU-145, ND-1, and LNCaP cells were extracted and analyzed by thin-layer chromatography and gas-liquid chromatography. The major lipids were phosphatidylcholine, phosphatidylethanolamine, sphingomyelin, fatty acids, and cholesterol. The sphingomyelin level was significantly higher in highly metastatic cells (DU-145 and ND-1) compared with the lower metastatic variant (LNCaP). The increase in the synthetic pathway and decrease in degradation pathway of sphingomyelin in microsomal fractions was sufficient to account for the measured increase in sphingomyelin in DU-145 cells compared with LNCaP cells. The major fatty acids of these lipids were palmitic (16:0), stearic (18:0), oelic (18:1), and arachidonic acid (20:4). The arachidonic acid level was significantly decreased in DU-145 and ND-1 compared with LNCaP cells. Electron microscopic studies showed no significant changes in the morphology of DU-145, ND-1, and LNCaP cells. The results of these investigations demonstrate for the first time that sphingomyelin and arachidonic acid contents are different in high and low metastatic variants of human prostatic adenocarcinoma cell lines. 相似文献
7.
PACAP, a VIP-like peptide, in neurons of the esophagus. 总被引:1,自引:0,他引:1
R Uddman A Luts A Absood A Arimura M Ekelund H Desai R H?kanson G Hambreaus F Sundler 《Regulatory peptides》1991,36(3):415-422
The lower esophagus of guinea-pig, cat, sheep and man was analyzed for pituitary adenylate cyclase activating peptide (PACAP), a novel vasoactive intestinal peptide (VIP)-like peptide, using immunocytochemistry and radioimmunoassay. PACAP-immunoreactive nerve fibers were numerous in the longitudinal and circular muscle layers of sheep and man, moderate in numbers in cat, while being few in the esophagus of guinea-pig. A few PACAP-immunoreactive nerve cell bodies and numerous nerve fibers were seen in the myenteric ganglia of the esophagus of cat, sheep and man. In the lower esophagus of cat, sheep and man all PACAP-containing nerve cell bodies and nerve fibers stored VIP. The results of radioimmunoassay of PACAP in extracts of specimens from man were in good agreement with the immunocytochemical findings. High performance liquid chromatography revealed one major peak of PACAP-like immunoreactivity in extracts of human esophagus. We suggest that neuronal PACAP may serve to modulate motor activity and secretion in the lower esophageal sphincter region. 相似文献
8.
9.
The present communication deals with a histological study of the pituitary gland of the teleost fish Mugil cephalus , found in the estuarine waters of Cochin area. Six different cell types were identified in the pituitary gland on the basis of their grouping, distribution and staining properties. The prolactin and the TSH cells (thyroid stimulating hormone producing cells or thyrotrops) were identified in the rostral pars distalis and the ACTH cells (adrenocorticotropic hormone producing cells or corticotrops) in the interphase between the neurohypophysis and the rostral pars distalis. The STH cells (somatotropic hormone producing cells or somatotrops) and the gonadotropic cells were distinguished in the proximal pars distalis and the MSH cells (melanin stimulating hormone producing cells or melanotrops) in the pars intermedia. 相似文献
10.
Following illumination with wavelengths longer than 700 nm, the intensity of light emission from Pothos aurea leaf falls for 1 min and then increases to a maximum after 2 min in the dark. The spectrum of this minute-range liminescence matches that of prompt fluorescence excited at the same wavelength, but differs from that of prompt or minute-range delayed emission excited by wavelengths shorter than 700 nm. This emission is less sensitive to heat damage than millisecond delayed emission, and may originate from photosystem I. 相似文献