全文获取类型
收费全文 | 4600篇 |
免费 | 439篇 |
国内免费 | 1篇 |
专业分类
5040篇 |
出版年
2023年 | 34篇 |
2022年 | 59篇 |
2021年 | 90篇 |
2020年 | 58篇 |
2019年 | 90篇 |
2018年 | 99篇 |
2017年 | 81篇 |
2016年 | 162篇 |
2015年 | 225篇 |
2014年 | 282篇 |
2013年 | 290篇 |
2012年 | 401篇 |
2011年 | 362篇 |
2010年 | 241篇 |
2009年 | 200篇 |
2008年 | 255篇 |
2007年 | 242篇 |
2006年 | 235篇 |
2005年 | 249篇 |
2004年 | 236篇 |
2003年 | 227篇 |
2002年 | 194篇 |
2001年 | 35篇 |
2000年 | 35篇 |
1999年 | 46篇 |
1998年 | 67篇 |
1997年 | 41篇 |
1996年 | 43篇 |
1995年 | 42篇 |
1994年 | 31篇 |
1993年 | 29篇 |
1992年 | 41篇 |
1991年 | 23篇 |
1990年 | 23篇 |
1989年 | 12篇 |
1988年 | 24篇 |
1987年 | 21篇 |
1986年 | 13篇 |
1985年 | 21篇 |
1984年 | 22篇 |
1983年 | 14篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 8篇 |
1979年 | 16篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1974年 | 8篇 |
1970年 | 7篇 |
1960年 | 6篇 |
排序方式: 共有5040条查询结果,搜索用时 0 毫秒
1.
Methods for production of containerized seedlings ofAlnus species were developed which permit nitrogen-fixing nodules to form on the root systems prior to outplanting, in order to provide an early nitrogen input during seedling establishment. The methods are based on procedures for inoculating root systems with suspensions ofFrankia (Actinomycetales), applied either directly in the container cell as a soil drench at the time of seeding, or as a root dip for seedlings transplanted into the containers. Germination of dried, stored seed was enhanced by light and by presoaking for 16 h in water. Pretreatments to overcome seed dormancy or to eliminate fungal pathogens did not further enhance germination. Some loss of seedlings was recorded in the early stages of growth shortly after germination, which is a factor in calculating projected seedling yield. Nodulation and seedling growth were evaluated in terms of growth media characteristics. Seedlings performed well in peat-vermiculite, at soil pH between 5.5 and 8.0. 相似文献
2.
Chemokines and their ligands play a critical role in enabling chronic lymphocytic leukaemia (CLL) cells access to protective microenvironmental niches within tissues, ultimately resulting in chemoresistance and relapse: disruption of these signaling pathways has become a novel therapeutic approach in CLL. The tyrosine kinase inhibitor dasatinib inhibits migration of several cell lines from solid-organ tumours, but effects on CLL cells have not been reported. We studied the effect of clinically achievable concentrations of dasatinib on signaling induced by the chemokine CXCL12 through its'' receptor CXCR4, which is highly expressed on CLL cells. Dasatinib pre-treatment inhibited Akt and ERK phosphorylation in CLL cells upon stimulation with CXCL12. Dasatinib also significantly diminished the rapid increase in actin polymerisation observed in CLL cells following CXCL12 stimulation. Moreover, the drug significantly inhibited chemotaxis in a transwell assay, and reduced the percentage of cells able to migrate beneath a CXCL12-expressing murine stromal cell line. Dasatinib also abrogated the anti-apoptotic effect of prolonged CXCL12 stimulation on cultured CLL cells. These data suggest that dasatinib, akin to other small molecule kinase inhibitors targeting the B-cell receptor signaling pathway, may redistribute CLL cells from protective tissue niches to the peripheral blood, and support the investigation of dasatinib in combination strategies. 相似文献
3.
Over the past decade, studies on reproductive morphology in the Squamata (snakes and lizards) have expanded tremendously. With the accumulation of these studies and revisions of the terminology based on structural similarities and differences, it is imperative to review the work on tuataras to determine whether the structural organization fits the revised terminology of vertebrates. We investigated the morphology of the male reproductive system in the Tuatara, Sphenodon punctatus (Rhynchocephalia), the sister taxon to the Squamata. Previous studies on the Tuatara used a nomenclature for the testicular ducts different from the current terminology for amniotes. The reproductive system in the Tuatara is consistent with reports in the Squamata. Two rete testis tubules exit the testis within a connective tissue sheath similar to that shown in other squamate species and the protherian Echidna. Each rete testis divides into multiple ductuli efferentes that fuse with the epididymis. The epididymis transitions into the ductus deferens where the sperm become more concentrated into spherical bundles. The ductus deferens enters the cloacal urodeum separately from the ureter. An ampulla ureter or ampulla urogenital papilla was not observed, which differs from previous studies of lepidosaurians. Furthermore, a sexual segment of the kidney (SSK) was not observed, consistent with previous studies on the Tuatara. 相似文献
4.
Abstract. Contrary to current assumptions, the reflex blood of two-spot ladybirds, Adalia bipunctata , and seven-spot ladybirds, Coccinella septempunctata , contains haemocyte-like cells. Furthermore, DNA can be extracted and amplified from coccinellid reflex blood, confirming the presence of haemocyte-like cells and demonstrating a nondestructive method of DNA extraction. 相似文献
5.
6.
Rooted cuttings ofCeanothus griseus varhorizontalis were irrigated with 0, 10, 20, 50, 75 or 100ppm nitrogen as NH4NO3 for eight weeks prior to inoculation with infectiveFrankia. After inoculation, half of the plants for each treatment nitrogen level continued to be irrigated with the preconditioning nitrogen level and half were given no more supplemental nitrogen. For plants continuously receiving nitrogen, nodule initiation (nodule number) was inversely correlated with increasing supplemental nitrogen levels, and suppressed above 50 ppm N. Leaf nitrogen above 2% in continuous-N plants correlated with greatly reduced or suppressed nodulation. Plants maintained after inoculation without supplemental nitrogen showed influence of the prior nitrogen treatment on nodulation. Preconditioning at 50 ppm and above greatly reduced the number of nodules formed. The evidence suggests that stored internal nitrogen can regulate nodulation.Plant biomass accumulated maximally when nodulation was suppressed, at 75 and 100 ppm supplemental N applied continuously. Internode elongation during the nodulation period occurred only on nodulated plants, or in the presence of supplemental N (10 ppm and above). 相似文献
7.
Shrishailam Yemul Carole Berger Melissa Katz Alison Estabrook Richard Edelson Hagan Bayley 《Cancer immunology, immunotherapy : CII》1990,30(6):317-322
Summary Molecules such as antibodies that bind to cell surfaces can be used to deliver cytotoxic drugs to selected cells. To be effective the drug must usually be taken into the cells by endocytosis. In this study a T-cell line (CCRFCEM) was effectively killed by liposomes carrying a photosensitizer and bearing the antibody OKT4 (anti-CD4). The unconjugated antibody does not induce antigenic modulation in the target cells, an indication of the absence of endocytosis, and would therefore not normally have been selected as an agent for drug delivery. It cannot, however, be concluded with certainty that the conjugates act at the cell surface and several alternative explanations of their efficacy are offered. 相似文献
8.
Glycine decarboxylase is confined to the bundle-sheath cells of leaves of C3−C4 intermediate species
Christopher M. Hylton Stephen Rawsthorne Alison M. Smith D. Alan Jones Harold W. Woolhouse 《Planta》1988,175(4):452-459
Immunogold labelling has been used to determine the cellular distribution of glycine decarboxylase in leaves of C3, C3–C4 intermediate and C4 species in the genera Moricandia, Panicum, Flaveria and Mollugo. In the C3 species Moricandia foleyi and Panicum laxum, glycine decarboxylase was present in the mitochondria of both mesophyll and bundle-sheath cells. However, in all the C3–C4 intermediate (M. arvensis var. garamatum, M. nitens, M. sinaica, M. spinosa, M. suffruticosa, P. milioides, Flaveria floridana, F. linearis, Mollugo verticillata) and C4 (P. prionitis, F. trinervia) species studied glycine decarboxylase was present in the mitochondria of only the bundle-sheath cells. The bundle-sheath cells of all the C3–C4 intermediate species have on their centripetal faces numerous mitochondria which are larger in profile area than those in mesophyll cells and are in close association with chloroplasts and peroxisomes. Confinement of glycine decarboxylase to the bundle-sheath cells is likely to improve the potential for recapture of photorespired CO2 via the Calvin cycle and could account for the low rate of photorespiration in all C3–C4 intermediate species.Abbreviation and symbol kDa
kilodaltons
-
CO2 compensation point 相似文献
9.
-globin gene haplotypes were determined for 196 normal (-A) and 419 thalassaemia (-Th) chromosomes of individuals from four different regions of the Indian subcontinent; North-west Pakistan, Gujarat, Punjab and Sindh. Analysis of -A and -Th haplotypes and haplotype-mutation associations in each regional group along with a consideration of Indian history provided information about the origin and spread of -thalassaemia mutations on the Indian subcontinent. The data are consistent with relatively recent and local origins for most -thalassaemia mutations. The frequencies of particular alleles differ markedly in various regions and these may be useful population markers. Of the high frequency alleles, intervening sequence 1 (IVS-1) nucleotide 5 (G-C) and codons 41/42 (-CTTT) appear to be older as suggested by multiple haplotype associations and a widespread geographical distribution. The microepidemiology of -thalassaemia in this region reflects considerable ethnic diversity, gene flow from population migration and natural selection by malaria infection. 相似文献
10.
Mapping of the structural gene for S-adenosyl homocysteine hydrolase to mouse Chromosome 2, and related sequences to Chromosomes 8 and X 总被引:1,自引:0,他引:1
Alison Pilz Paul Le Tissier Heather Moseley Jo Peters Cathy Abbott 《Mammalian genome》1992,3(11):633-636
Comparative mapping studies in human and mouse have shown that, to date, human Chromosome (Chr) 20 is completely syntenic with distal mouse Chr 2. The structural locus for S-adenosyl-l-homocysteine hydrolase (EC 3.3.1.1) in human, AHCY, maps to 20 qterq13.1, and we report here that the homologous locus in the mouse, Ahcy, maps to distal mouse Chr 2 with gene order Pcna-Ahcy-Ada. Analysis of 123 progeny of an interspecific backcross between a laboratory stock, AN, and Mus spretus using a rat cDNA probe revealed the presence of at least two other Ahcy-related sequences segregating independently in the mouse genome. One, Ahcy-rs1, was mapped to Chr 8 in the BXH recombinant inbred strains, and the other, Ahcy-rs2, shows a pattern of inheritance consistent with X-linkage. 相似文献