全文获取类型
收费全文 | 23711篇 |
免费 | 2032篇 |
国内免费 | 2篇 |
出版年
2021年 | 181篇 |
2020年 | 138篇 |
2019年 | 166篇 |
2018年 | 408篇 |
2017年 | 426篇 |
2016年 | 465篇 |
2015年 | 427篇 |
2014年 | 534篇 |
2013年 | 860篇 |
2012年 | 1510篇 |
2011年 | 1646篇 |
2010年 | 883篇 |
2009年 | 613篇 |
2008年 | 1415篇 |
2007年 | 1456篇 |
2006年 | 1393篇 |
2005年 | 1253篇 |
2004年 | 1219篇 |
2003年 | 1119篇 |
2002年 | 1141篇 |
2001年 | 858篇 |
2000年 | 970篇 |
1999年 | 475篇 |
1998年 | 263篇 |
1997年 | 210篇 |
1996年 | 248篇 |
1995年 | 225篇 |
1994年 | 203篇 |
1993年 | 214篇 |
1992年 | 217篇 |
1991年 | 188篇 |
1990年 | 192篇 |
1989年 | 181篇 |
1988年 | 177篇 |
1987年 | 173篇 |
1986年 | 152篇 |
1985年 | 207篇 |
1984年 | 221篇 |
1983年 | 179篇 |
1982年 | 230篇 |
1981年 | 199篇 |
1980年 | 175篇 |
1979年 | 174篇 |
1978年 | 150篇 |
1977年 | 137篇 |
1976年 | 129篇 |
1975年 | 119篇 |
1974年 | 129篇 |
1973年 | 111篇 |
1972年 | 93篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
71.
72.
Characterization of a meta-Fluorotyrosine-Tolerant Cell Culture of Eschscholtzia californica Cham
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
A cell line of Eschscholtzia californica selected for meta-fluorotyrosine (MFT) tolerance was found to have 10-fold increased levels of phenylalanine and tyrosine compared to the parent line, while most other amino acids were only increased 2-fold. Tracer experiments with shikimic acid in the presence of MFT showed that the biosynthesis of the aromatic amino acids was not impaired in the tolerant line. Feeding experiments with phenylalanine, tyrosine, or shikimic acid also revealed a reduced turnover of the pools of the aromatic amino acids in the variant. Thus undisturbed de novo biosynthesis of the aromatic amino acids and dilution of toxic effects of MFT by the enlarged pool sizes seemed to be the main reason for the acquired tolerance. Despite the enlarged availability of the precursor tyrosine, formation of the benzophenanthridine alkaloids was enhanced neither in the growth nor in the production medium. 相似文献
73.
C. P. Werner A. P. Setter B. M. Smith J. Kubba M. J. Kearsey 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1989,77(4):527-534
Summary Performance of a random array of recombinant inbred lines derived by single seed descent from five different source populations of Brussels sprouts (Brassica oleracea var. gemmifera) is presented. A total of 2,356 lines were tested in trials during 1985 and 1986. Three of the source populations were derived from double crosses between F1 hybrids. These hybrids show a considerable heterotic advantage over their inbred parents for the most important agronomic traits. The recombinant inbred lines performed, on average, less well than the parental inbred material, indicating that additive x additive genie interactions may make a significant contribution to the performance of current inbred material. Nevertheless, the very large variation among the recombinant inbred lines permitted many lines to be identified which outperformed the best parental inbred for all traits. Two lines outperformed the reference F1 hybrid, Gower, for an index that included marketable yield and quality. Consideration was also given to the dangers of misinterpreting phenotypically based proportions. Accordingly, response equations were used to ascertain the real genetic progress that was made. Advance seemed small when compared with the large heterotic effect, which is consistent with the segregation of a large number of loci. The distribution of the recombinant inbred lines was compared to predictions made from early generation trials. There was broad agreement but significant discrepancies existed which, it is suggested, may arise from the effects of genotype-environment interactions. 相似文献
74.
The highly complicated differentiation of the spermatid in the pseudoscorpion Diplotemnus sp. is accomplished without the presence of microtubules. Instead membranous tubes measuring approximately 50 nm in diameter and closely associated with endoplasmic reticulum are found from early to mid spermatids. The lumen of the tube is devoid of electron dense contents but a fluffy material is attached to the cytoplasmic side. They run straight or slightly bent and are in open connection with the cell membrane. First appearing near the cell bridge of the interconnected spermatids they form a bundle in the longitudinal axis during a transitory phase of elongation. When the cell rounds off again the tubules together with the endoplasmic reticulum disappear. The arrangement of the tubes and their presence during abortive elongation of the spermatid suggest a supportive function commonly attributed to microtubules. Moreover, the open connection with the cell membrane and their close association with the endoplasmic reticulum may indicate their participation also in transport. 相似文献
75.
Päivi Heikkilä Arvi I. Kahri Christian Ehnholm Petri T. Kovanen 《Cell and tissue research》1989,255(3):487-494
Summary The choroid plexus consists of the choroidal epithelium, a derivative of the neural tube, and the choroidal stroma, which originates from the embryonic head mesenchyme. This study deals with epithelio-mesenchymal interactions of these two components leading to the formation of the organ. Grafting experiments of the prospective components have been performed using the quail-chicken marker technique. Prospective choroidal epithelium of quail embryos, forced to interact with mesenchyme of the body wall of chicken embryos, gives rise to a choroid plexus showing normal morphogenesis and differentiation. The choroidal epithelium induces the differentiation of organtypical fenestrated capillaries, which are highly permeable to intravenously injected horseradish peroxidase. The choroidal epithelium of the grafts constitutes a blood-cerebrospinal fluid barrier. On top of the choroidal epithelium, there are epiplexus cells displaying a typical ultrastructure. The experimental results show that these cells do not originate from the transplanted neural epithelium. Prospective choroidal stroma of chicken embryos does not exert a choroid plexus-inducing influence upon a quail embryo's neural epithelium isolated from parts of the brain that normally do not develop a choroid plexus. The experiments show that the choroidal epithelial cells are determined at least three days before the first organ anlage is detectable.This work was supported by the Deutsche Forschungsgemeinschaft (grant Ch 44/7-1) 相似文献
76.
Gerd Gäde 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1989,159(5):589-596
Summary Extracts of corpora cardiaca from two members of the family Tenebrionidae,Zophobas rugipes andTenebrio molitor, from one member of the Chrysomelidae,Leptinotarsa decemlineata, and from three members of the Scarabaeidae,Pachnoda marginata, P. sinuata andMelolontha hippocastani, were assayed for adipokinetic and hypertrehalosaemic activity in acceptor locusts (Locusta migratoria) and cockroaches (Periplaneta americana), respectively. All corpus cardiacum material tested, except that from the cockchafer,M. hippocastani, gave positive bioassay results. Biological activities of corpus cardiacum extracts from all species investigated can be resolved on reversed-phase high performance liquid chromatography (RP-HPLC). Gland extracts from the two tenebrionid species each show a single peak of biological activity associated with a single peak of UV absorbance having an identical retention time in both species. The two biologically active fractions from the corpora cardiaca of the potato beetle,L. decemlineata, coelute with exogenous (synthetic) hypertrehalosaemic hormones I and II of the American cockroach. The two species of the genusPachnoda contain two active compounds in their glands; compound I of each species is more abundant and elutes just ahead of the (synthetic) hypertrehalosaemic hormone of the cockroachBlaberus discoidalis. The gland material ofM. hippocastani exhibits and absorbance peak with the same retention time as the major peak from thePachnoda-species; however, this peak material does not elicit biological activity in the assays used here. After fractionation by RP-HPLC the main biologically active compounds were subjected to amino acid analyses. All factors are peptidic and contain 8 amino acid residues. The peptides from the tenebrionid species have the amino acid residues Asx(2), Glx(1), Ser(1), Pro(1), Leu(1), Phe(1) and Trp(i), whereas the main peptide from corpora cardiaca ofP. marginata contains the residues Asx(2), Glx(1), Ser(1), Pro(1), Tyr(1), Leu(1) and Trp(1). Amino acid composition analyses of the two active fractions fromL. decemlineata reveal the residues Asx(2), Glx(1), Ser(1), Pro(1), Val(1), Phe(1) and Trp(1) for compound I and Asx(1), Glx(1), Thr(2), Pro(1), Leu(1), Phe(1) and Trp(1) for compound II. 相似文献
77.
Neuromedin U-immunoreactivity in the nervous system of the small intestine of the pig and its coexistence with substance P and CGRP 总被引:2,自引:0,他引:2
Jean-Pierre Timmermans Prof. Dietrich W. Scheuermann Werner Stach Dirk Adriaensen Marie H. A. De Groodt-Lasseel Julia M. Polak 《Cell and tissue research》1989,258(2):331-337
Summary In the small intestine of the pig, neuromedin U (NMU)-immunoreactivity was mainly confined to the nerve plexus of the inner submucosal and mucosal regions. After colchicine treatment, a high number of immunoreactive nerve cell bodies was observed in the plexus submucosus internus (Meissner), whereas only a low number was found in the plexus submucosus externus (Schabadasch). The plexus myentericus as well as the aganglionic nerve meshworks in the circular and longitudinal smooth muscle layers almost completely lacked NMU-immunoreactivity. Double-labeling experiments demonstrated the occurrence of distinct NMU-containing neuron populations in the plexus submucosus internus: (1) relatively large type-II neurons revealing immunoreactivity for NMU and calcitonin gene-related peptide (CGRP) and/or substance P (SP); (2) a group of small NMU- and SP-immunoreactive neurons; (3) a relatively low number of small neurons displaying immunoreactivity for NMU but not for SP. Based on its distributional pattern, it is concluded that NMU plays an important role in the regulation and control of mucosal functions. 相似文献
78.
79.
Toxicity and isolation of the cyanobacterium Nodularia spumigena from the southern Baltic Sea in 1986 总被引:1,自引:1,他引:0
Three water bloom samples were collected in August 1986 from the southern Baltic Sea. Acute toxicity of the samples was determined by mouse bioassay and the toxins were further studied by HPLC. The bloom samples contained equal amounts of cyanobacteria Nodularia spumigena and Aphanizomenon flos-aquae and were hepatotoxic. Two hepatotoxic Nodularia spumigena strains were isolated from the samples. The isolates produce a toxic peak indistinguishable from the bloom material in the HPLC analysis. The toxicity of the fractions was verified by mouse bioassay. Thus the toxicity of the bloom samples was in all likelihood caused by Nodularia spumigena. 相似文献
80.
Helmut Krüger Werner Schr?der Klaus Buchner Ferdinand Hucho 《Journal of Protein Chemistry》1990,9(4):467-473
Inhibition of protein kinase C (PKC) by calmodulin is investigated and we describe the localization of inhibitory sequences within the calmodulin molecule. We present evidence that calmodulin inhibits PKC through an inhibition of the activation of PKC associated with lipid membranes: Binding of PKC to lipid vesicles is not affected, but activation is abolished. The potent calmodulin antagonist R24571 (calmidazol) did not affect the inhibition of PKC by calmodulin at concentrations up to 10–5 M. Two tryptic fragments of calmodulin were isolated which inhibited PKC. They were only slightly less potent than intact calmodulin with an IC50 of 6 µ M compared to 1 µ M of intact calmodulin. They were identified as Ser38-Arg74 and His107-Lys148. Each of the inhibiting fragments contains an intact Ca2+-binding domain with complete helix-loop-helix structure (EF hand). Other calmodulin peptides showed only weak inhibitory activity. Both fragments did not stimulate cAMP phosphodiesterase even at concentrations 100-fold higher than the calmodulin concentration needed for maximal stimulation. None of the fragments acted as a calmodulin antagonist. 相似文献