首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1301篇
  免费   60篇
  2021年   12篇
  2020年   7篇
  2019年   5篇
  2018年   12篇
  2017年   12篇
  2016年   17篇
  2015年   35篇
  2014年   36篇
  2013年   139篇
  2012年   44篇
  2011年   58篇
  2010年   36篇
  2009年   42篇
  2008年   64篇
  2007年   80篇
  2006年   68篇
  2005年   75篇
  2004年   74篇
  2003年   57篇
  2002年   79篇
  2001年   18篇
  2000年   12篇
  1999年   14篇
  1998年   13篇
  1997年   7篇
  1996年   12篇
  1995年   11篇
  1994年   12篇
  1993年   20篇
  1992年   17篇
  1991年   13篇
  1990年   20篇
  1989年   24篇
  1988年   18篇
  1987年   17篇
  1986年   13篇
  1985年   14篇
  1984年   27篇
  1983年   18篇
  1982年   10篇
  1981年   13篇
  1980年   8篇
  1979年   8篇
  1978年   11篇
  1976年   8篇
  1975年   6篇
  1974年   6篇
  1973年   6篇
  1967年   5篇
  1961年   4篇
排序方式: 共有1361条查询结果,搜索用时 15 毫秒
951.
Cell culture flasks applicable for altered gravity conditions, such as centrifugation, clino-rotation or microgravity in space, were manufactured for trial. The flask has flat polystyrene surface for monolayer culture and gas-permeable film window on the opposite face. The space in-between consists the culture chamber to be filled with liquid medium. To reduce the water loss and bubble formation in the culture fluid, another gas permeable window was placed on top to form a space where distilled water may be filled. The double-decker culture flask can be used for both space and ground-based experiments in common.  相似文献   
952.
The early ontogeny of the pinnately, palmately, and ternately compound leaves in the Lardizabalaceae was studied by SEM. The leaf primordium of each of the three leaf types emerges as an identical short protrusion on the shoot apex; the leaf primordium produces the first leaflet initials laterally on its margin. Successive acropetal growth of the leaf axis and the following inception of the leaflet primordia are responsible for the pinnately compound leaf, whereas short basipetal growth accompanied with initiation of two or more pairs of leaflet initials results in a palmately compound leaf. If no elongation of the leaf axis nor additional inception of leaflet primordia occur during early ontogeny, a ternate leaf ensues.  相似文献   
953.
An immunocytochemical study was performed to examine the expression of cellular c-myc protein in the heart of 30-, 120- and 180-day-old cardiomyopathic Syrian UM-X7.1 hamsters. The heart of age- and sex-matched BIO-RB hamster was used as normal control. In paraffin sections, an immunostaining for c-myc was markedly increased in cytoplasm of cells from the UM-X7.1 heart as compared with that of the BIO-RB heart which showed a weak staining. However, c-myc was localized in nuclei of cells in frozen sections of the heart. Specific cell types of the heart were differentiated with anti-vimentin, and we found that the increased expression of c-myc was present in nuclei of muscle cells of the UM-X7.1 myocardium. A quantitative study of c-myc-positive nuclei of muscle and nonmuscle cells was carried out by a video micrometer. The mean number of c-myc-positive nuclei of muscle cells was significantly higher in the cardiomyopathic heart than in the control heart from hamsters of all ages studied. These results suggest that the increase of c-myc protein may relate to the pathological state or pathogenesis of the hereditary cardiomyopathy.  相似文献   
954.
Placental 20 alpha-hydroxysteroid dehydrogenase (20 alpha-HSD) activity was studied in order to evaluate the mechanism of continuation of pregnancy and initiation of labor. The placentas obtained at various gestational weeks were homogenized and fractionated into "nuclear", "mitochondrial", "microsomal" and "supernatant" fractions. Each fraction was incubated with 14C-progesterone and a hydrogen donor. Enzymatic activity was measured by the conversion of progesterone to 20 alpha-dihydroprogesterone. The highest activity of 20 alpha-HSD for progesterone was found to be localized in "microsomal" fraction. The Km constant of 20 alpha-HSD was 4.5 X 10(-6)M for progesterone in "microsomal" fraction. It was found that placental microsomal 20 alpha-HSD required NADPH as well as NADH. 20 alpha-HSD activity for progesterone increased as gestational weeks advanced. The addition of DHA-sulfate and DHA inhibited 20 alpha-HSD activity for progesterone significantly, suggesting that the steroid produced by the feto-placental unit may be involved in the metabolism of progesterone in human placenta.  相似文献   
955.
Ent-kaurene (ent-kaur-16-ene) and squalene were analyzed in extracts of the shoots of three cultivars of rice (Oryza sativa L.) of 14 and 28 days of age by gas chromatography-mass spectrometry (GS-MS) and GC-selected ion monitoring (GS-SIM).Ent-kaurene occurred at approximate concentrations of <1 to 13 ng/g f.w. in 14-day-old plants and 26 to 147 ng/g f.w. in 28-day-old plants. Shoots of the dwarf cultivar Waito-C contained much lessent-kaurene than the other two cultivars at both developmental stages. The level ofent-kaurene in the dwarf cultivar Tan-ginbozu was similar to that in the normal cultivar Nihonbare at 14 days but was lower than in Nihonbare at 28 days. Trace amounts ofent-isokaurene (ent-kaur-15-ene) were also found in the extracts of all three cultivars of shoots at 28 days. Squalene occurred in approximate concentrations from as low as 19 ng/g f.w. in 28-day-old Waito-C shoots to as much as 626 ng/g f.w. in 14-day-old Nihonbare shoots. Both Tan-ginbozu and Waito-C shoots contained less squalene than Nihonbare shoots at both developmental stages.  相似文献   
956.
Flower-inducing factors in extracts of flowering Lemna gibbaG3 were investigated using Lemna paucicostata 151 as the bioassayplant. Fractions with flower-inducing activity were obtainedafter several purification steps. Two of the active substanceswere identified as L-pipecolic acid and nicotinamide by MS andNMR analyses. Both L-pipecolic acid and nicotinamide exhibited flower-inducingactivity in L. paucicostata 151 grown on one-tenth-strengthM medium containing benzyladenine, the former being ten timesas active as the latter. L-Pipecolic acid was active even at0.01 ppm (7.8 ? 10–8 M). The effect of L-pipecolic acidon flowering strongly depended upon the presence of exogenouscytokinin. The coexistence of cytokinin seemed to be essentialfor L-pipecolic acid to exhibit flower-inducing activity. Incontrast, the effect of nicotinamide on flowering was basicallythe same as that of benzoic acid or nicotinic acid. (Received February 9, 1987; Accepted May 21, 1987)  相似文献   
957.
It has been reported that both c-fos and c-myc mRNAs are induced in NIH/3T3 cells after 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. We have studied the effect of TPA on the expression of c-fos and c-myc in EJ-ras-transformed NIH/3T3 and its nontumorigenic flat revertant R1 cells. Although TPA treatment induces c-myc mRNA, as in the case of NIH/3T3 cells, the induced level of c-fos mRNA is greatly reduced not only in slow-growing EJ-ras-transformed NIH/3T3 but also in quiescent R1 cells. In addition, serum-induced c-fos expression is also reduced in EJ-ras-transformed NIH/3T3 and R1 cells. These observations suggest that the pathway from TPA to c-fos gene is different from that to c-myc gene and that the former pathway is down-regulated in association not with the transformed phenotype, but with EJ-ras expression, and it is possible that this reduced induction of c-fos is not specific to TPA.  相似文献   
958.
Helodermin is a peptide from the venom of the lizard Heloderma suspectum (Gila Monster) showing a high degree of sequence similarity with VIP, PHI and secretin in its N-terminal moiety. The present data support the presence of peptide(s) closely related to helodermin in the brain, gut and salivary glands of rat. In our radioimmunoassays, we routinely used one of the three specific antisera obtained from rabbits that were immunized against lizard helodermin coupled to bovine serum albumin with carbodiimide. Heat- and acid-stable immunoreactive helodermin-like material was more abundant in striatum, hippocampus and anterior pituitary than in cerebral cortex and hypothalamus. High levels of helodermin-like material were also present in salivary glands, duodenum and jejunum. When submitted to gel permeation chromatography on a TSK-G 2000 SW column, the apparent molecular radius of most of the immunoreactive material ranged from 6 to 12 KDa.  相似文献   
959.
In the present study, Methylobacterium sp. FD1 utilizing formaldehyde was isolated from soil. The resting cells of FD1 degraded high concentrations of formaldehyde (~2.7 M) and produced formic acid and methanol that were molar equivalents of one-half of the degraded formaldehyde. This result suggests that formaldehyde degradation by FD1 is caused by formaldehyde dismutase. The optimal temperature and pH for formaldehyde degradation by the resting cells of FD1 were 40 °C and 5–7, respectively. The lyophilized cells of FD1 also degraded high concentrations of formaldehyde. The formaldehyde degradation activity of the lyophilized cells was maintained as the initial activity at 25 °C for 287 days. These results suggest that the lyophilized cells of FD1 are useful as formaldehyde degradation materials.  相似文献   
960.
Filamentous phage, fs1, was obtained from Vibrio cholerae O139. The lysogenized strains produced a large amount of fs1 phage in the culture supernatant. This phage was previously reported as novel fimbriae of that organism. The genome of the phage was a 6.5 kb single-stranded DNA. The capsid of fs1 consists of a small molecule peptide (about 2.5 kDa).  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号