全文获取类型
收费全文 | 5594篇 |
免费 | 141篇 |
国内免费 | 291篇 |
出版年
2023年 | 43篇 |
2022年 | 60篇 |
2021年 | 79篇 |
2020年 | 65篇 |
2019年 | 107篇 |
2018年 | 107篇 |
2017年 | 67篇 |
2016年 | 88篇 |
2015年 | 94篇 |
2014年 | 255篇 |
2013年 | 291篇 |
2012年 | 195篇 |
2011年 | 293篇 |
2010年 | 197篇 |
2009年 | 231篇 |
2008年 | 264篇 |
2007年 | 283篇 |
2006年 | 228篇 |
2005年 | 240篇 |
2004年 | 174篇 |
2003年 | 162篇 |
2002年 | 142篇 |
2001年 | 130篇 |
2000年 | 106篇 |
1999年 | 141篇 |
1998年 | 114篇 |
1997年 | 108篇 |
1996年 | 95篇 |
1995年 | 110篇 |
1994年 | 94篇 |
1993年 | 109篇 |
1992年 | 112篇 |
1991年 | 109篇 |
1990年 | 86篇 |
1989年 | 88篇 |
1988年 | 71篇 |
1987年 | 87篇 |
1986年 | 63篇 |
1985年 | 86篇 |
1984年 | 99篇 |
1983年 | 86篇 |
1982年 | 73篇 |
1981年 | 66篇 |
1980年 | 71篇 |
1979年 | 57篇 |
1978年 | 47篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1975年 | 27篇 |
1973年 | 24篇 |
排序方式: 共有6026条查询结果,搜索用时 15 毫秒
51.
Kirsten Brandt 《Plant Cell, Tissue and Organ Culture》1991,26(3):195-201
A method for estimating the oxygen availability in plant cell cultures grown in stationary liquid media (e.g. many protoplast cultures) was developed. The method is based on short-term measurements of respiration rate versus oxygen concentration on a sample of cells, suspended in liquid media. From such data it is possible to estimate the oxygen concentration at the bottom of a stagnant liquid culture, by calculating the amount of oxygen reaching the cells by diffusion. As an example, rape (Brassica napus L. cv. Omega) hypocotyl protoplasts were grown with different oxygen concentrations at the site of the cells, obtained by varying the cell density, the height of the liquid layer and the oxygen content of the gas phase. The number of surviving calli was positively correlated with the estimated oxygen availability in the range between 60 and 350 M O2, below 60 M all cells died. This indicates that oxygen availability can be a limiting factor in the range usually encountered in protoplast cultures, and that the method can be useful when designing optimal growth conditions for stationary cultures of plant cells.Abbreviations C1
bulk oxygen concentration in agitated medium
- Co
oxygen concentration in medium at the gas-liquid interface, in equilibrium with the gas
- Cx
oxygen concentration at cell level
- D
diffusion constant of oxygen in water
- KLa
oxygen transfer rate
- l
height of liquid above cells
- n
number of cells per ml
- Rx
respiration rate per cell 相似文献
52.
Rhizome induction and plantlet regeneration of Cymbidium goeringii from flower bud cultures in vitro
Apical flower buds of Cymbidium goeringli Reichenbach fil. (ca 2 mm long) exeised from infloreseences (ca 5 cm long) were explanted on modified Murashige & Skoog medium (=MS medium) supplemented with N6-benzyladenine (BA) and -naphthaleneacetic acid (NAA). Within 107 days of culture, swelling growth, chlorophyll synthesis, and subsequent rhizome differentiation were observed. MS medium containing 0.1 mg l-1 BA and 10 mg l-1 NAA was found to be optimal for initiating rhizome development and subsequent plantlet regeneration.Explants cultured on MS medium supplemented with 1 mg l-1 NAA alone formed a mass of rhizome branches. Multiple shoots of rhizome branches were induced from apical segments when rhizomes were transferred to MS medium containing 0.1 mg l-1 BA and 10 mg l-1 NAA.Abbreviations NAA
-naphthaleneacetic acid
- BA
N6-benzyladenine 相似文献
53.
The effect of microelements on the induction of embryogenic callus from epiblast and scutellum of wheat (Triticum aestivum L.) embryos was studied by the sequential omission of each of the microelements from Murashige & Skoog medium. Omission of iron caused a marked decrease in yield and poor shoot formation from embryogenic callus. The yield of embryogenic callus on medium without added manganese was also reduced. Omission of boron, copper-cobalt, iodine, and molybdenum had little effect on the induction of embryogenic epiblast callus. By contrast there was a marked increase in the formation of white structures on the medium without any microelements or, specifically without addition of zinc. Since the formation of typical embryoids of wheat is associated with the formation of white structures, our result highlights the importance of certain microelements on somatic embryogenesis of wheat.Abbreviations 2,4-D
2,4-dichlorophenoxyacetic acid
- MS medium
Murashige & Skoog medium 相似文献
54.
Xiao-yan Ding Wallace L. McKeehan Jianming Xu Horst Grunz 《Development genes and evolution》1992,201(6):334-339
Summary Mesoderm formation is a result of cell-cell interactions between the vegetal and animal hemisphere and is thought to be mediated by inducing peptide growth factors including members of the FGF and TGF superfamilies. Our immunochemical study analyses the distribution of FGF receptors coded by the human flg gene during embryogenesis of Xenopus laevis. Immunostaining was detected in the dorsal and ventral ectoderm and also in the marginal zone of early cleavage, blastula and gastrula stages. Signals were very strong in the mid and late blastula (stage 8 and 9) and declined slightly in the early gastrula (stage 10). A dramatic decrease was observed up to the late gastrula (stage 11+). In stage 13 embryos, immunostaining was only found in cells around the blastopore. Isolated ectoderm cultured in vitro showed a similar temporal expression and decrease of the signal as the normal embryos. These results indicate that receptor expression is independent of the interaction of the animal cells with the vegetal part of the embryo. Of interest is the fact that the signal cannot only be found at or near the cell surface but also within the cell. This suggests the presence of an intracellular isoform of the receptor resulting from the endogenous expression of splice variants and the internalization of transmembrane receptor. Taken together our results suggest that the loss of competence (for bFGF around stage 10) is not directly correlated with the presence of receptors. The possible roles of heparan sulphate glucosaminoglycans (low affinity receptors) and control mechanisms in the intracellular signalling pathway downstream of the receptor level should be taken into consideration. 相似文献
55.
R Benali F Dupuit M Chevillard J Jacquot B Haye E Puchelle 《Biology of the cell / under the auspices of the European Cell Biology Organization》1991,73(1):49-56
Bovine tracheal gland (BTG) cells in culture show an epithelial-fibroblastoid transition after several passages. To investigate these BTG cell phenotype changes, we studied the effects of both the culture medium and passage number on the expression of epithelial cytoskeletal proteins and glandular serous cell markers. We also analyzed the intracellular cAMP level in the basal state and after adrenergic stimulation. Three culture media were used: 1) serum-free defined medium (SFDM); 2) medium supplemented with 2% Ultroser G; and 3) medium supplemented with 10% fetal calf serum (FCS). Using immunofluorescence microscopy, we showed that, in the first 4 passages whatever the culture conditions, BTG cells expressed immunoreactivities to cytokeratin filaments and desmoplakins I and II, whereas vimentin filaments were not detected. After four passages, BTG cells cultured in 10% FCS or 2% Ultroser G became progressively fibroblastoid and showed immunoreactivities to both vimentin and cytokeratin intermediate filaments. No immunoreactivity to vimentin filaments was observed on BTG cells cultured in a SFDM. Using biochemical analysis, we showed that basal levels of cAMP in cultured BTG cells and lysozyme secretion by these cells vary according to the culture medium and passage number. It was higher in BTG cells cultured in a SFDM compared to that recovered from cells cultured in medium supplemented with Ultroser G or FCS. Whatever the culture medium, BTG cells responded to stimulation by isoproterenol. However, the results of stimulation in a SFDM were higher than in Ultroser G or FCS supplemented medium. We conclude that the BTG epithelial cell organization and the regulation of biosynthesis of secretory proteins by these cells in culture depend on both the culture medium and passage number.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
56.
Wataru Teshirogi 《Hydrobiologia》1986,132(1):207-216
Experiments on 1) regeneration of the cave-adapted planarian, Sphalloplana zeschi, 2) induction of sexuality in an asexual strain of Dugesia japonica japonica by feeding, and 3) culture of dissociated planarian cells, show that neoblasts originate from intestinal cells, i.e. phagocytic cells and granular clubs. 相似文献
57.
Summary We used specific binding of phlorizin to the intact intestinal mucosa in order to measure glucose transport site density in intestines of mice fed a high-carbohydrate or no-carbohydrate diet. Nonspecific binding varied with intestinal position but showed only modest dependence on diet. Specific binding to glucose transporters was 1.9 times greater in jejunum of high-carbohydrate mice than of no-carbohydrate mice; this ratio was the same as the ratio for Vmax values of actived-glucose uptake between the two diet groups. The gradient in specific binding of phlorizin along the intestine paralleled the gradient in Vmax of glucose transport. These results directly demonstrate that the increase in intestinal glucose transport caused by a high-carbohydrate diet is due to induction of glucose transporter. They also indicate that the normal positional graident in glucose transport along the intestine arises from a gradient in transporters, induced by the normal gradient in luminal glucose concentration. 相似文献
58.
59.
Noritsugu Yabe Yutaka Matsuya Isao Yamane Mitsuru Takada 《In vitro cellular & developmental biology. Plant》1986,22(7):363-368
Summary A newly developed, serum-free medium (NYSF-404) selects for antibody-producing hybridomas after fusion of antigen-sensitized
mouse spleen cells with myeloma cell lines P3-X63-Ag8-U1 (P3-U1), P3-X63-Ag8-6.5.3 (Ag8.653), or P3-NSI/1-Ag4-1 (NS-1). Without
the need for hypoxanthine-aminopterinthymidine (HAT) selection of hybrid cells, frequency of hybridoma formation in medium
NYSF-404 is higher (twice) than that in serum- and HAT-containing medium. Colonies developed upon limiting dilution in the
presence of the mortal parent myeloma cells in medium NYSF-404 and pure culture of antibody-secreting cells could be subsequently
established. The results suggest that fusions can be done in serum-free medium and that the clonal growth of hybridomas is
dependent on factors produced by parent myeloma cells under serum-free culture conditions. Such factors seem deficient in
serum- and HAT-containing medium or are masked by serum. 相似文献
60.
Summary The tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA) was examined for its ability to induce endogenous retrovirus from a high-passage clone
of Kirsten sarcoma virus-transformed Balb/c (K-Balb) mouse cells. TPA activated virus in a concentration-dependent manner
(0.0016 to 4.0 μM). Exposure to 1mM actinomycin D inhibited virus induction, suggesting that cellular RNA synthesis is required de novo by this inducer. A broad-spectrum
neutralizing antibody to murine type C virus, gp70, was shown to neutralize the infectivity of the induced virus. The activated
virus had the host range of the xenotropic Balb virus:2, and after removal of the inducer, the activated state decayed rapidly.
TPA stimulated DNA, RNA, and protein synthesis in K-Balb cells, indicating that the mechanism of inducation may be different
from that of previously identified virus inducers. The effects observed using the well-defined K-Balb system offer an opportunity
to study the modulation of retrovirus gene expression by TPA.
This work was conducted while the authors were with the Biological Carcinogenesis Program, Frederick Cancer Research Facility,
Frederick, MD 21701, and was supported under Contract NO1-CO-75380 with the National Cancer Institute, National Institutes
of Health, Bethesda, MD 20205. 相似文献