全文获取类型
收费全文 | 886篇 |
免费 | 40篇 |
国内免费 | 33篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 11篇 |
2021年 | 15篇 |
2020年 | 15篇 |
2019年 | 19篇 |
2018年 | 14篇 |
2017年 | 23篇 |
2016年 | 18篇 |
2015年 | 34篇 |
2014年 | 37篇 |
2013年 | 47篇 |
2012年 | 21篇 |
2011年 | 33篇 |
2010年 | 38篇 |
2009年 | 55篇 |
2008年 | 40篇 |
2007年 | 42篇 |
2006年 | 53篇 |
2005年 | 45篇 |
2004年 | 26篇 |
2003年 | 25篇 |
2002年 | 30篇 |
2001年 | 18篇 |
2000年 | 12篇 |
1999年 | 23篇 |
1998年 | 29篇 |
1997年 | 26篇 |
1996年 | 28篇 |
1995年 | 19篇 |
1994年 | 16篇 |
1993年 | 20篇 |
1992年 | 13篇 |
1991年 | 16篇 |
1990年 | 13篇 |
1989年 | 15篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
排序方式: 共有959条查询结果,搜索用时 31 毫秒
1.
Takashi Sugawara 《Journal of plant research》1987,100(4):335-348
The floral vascular anatomy of 12 species representing each ofAsarum s. str.,Asiasarum, Geotaenium, Heterotropa andHexastylis are compared to clarify intergeneric relationships. The five genera have basically similar structures in floral morphology
and vasculature, and consistently have a six-carpelled compound ovary and the associated similar placental vasculature. They
show, however, a significant difference in the position and the constituent of the “ventral” carpellary bundles in the placental
axis betweenAsiasarum-Heterotropa-Hexastylis andAsarum-Geotaenium. InAsiasarum, Heterotropa andHexastylis the ventral bundles of each carpel are basically free and antilocular as expected in the least specialized compound ovary
of angiosperms; in contrast, inAsarum andGeotaenium the ventral carpellary bundles are antiseptal and heterogenous (i.e., formed by the lateral fusion of ventral bundles of
adjacent carpels). Shared probable apomorphic floral vasculature, as well as shared single style-column, suggests the closest
mutual relationships betweenAsarum andGeotaenium. In terms of floral morphology and anatomy,Asiasarum, Heterotropa andHexastylis retain plesiomorphies. Possible chromosomal evolution in the related genera is also discussed. 相似文献
2.
Culex annulirostris Skuse mosquitoes (Brisbane strain) were reared at 20 degrees C or 27 degrees C and the adult females were experimentally infected by feeding Murray Valley encephalitis virus (MVE). They were then maintained (a) in the insectary at 20 degrees C, after rearing at either 20 degrees C or 27 degrees C; (b) at ambient outdoor temperatures, range 12.2-28.9 degrees C, mean 19.6 degrees C; or (c) at 27 degrees C after rearing at 27 degrees C. There was no significant difference in rates of MVE infection or transmission when mosquitoes were reared and maintained constantly at 20 degrees C or 27 degrees C. However, for females kept at reduced temperature (i.e. mean = 19.6 degrees C or 20 degrees C after rearing at 27 degrees C), the infection and transmission rates of MVE were significantly reduced (2 x 8 replicates). This investigation illustrates that vector competence is depressed by decreasing temperatures for adult mosquitoes compared with those they experienced during development. Similar patterns were evident with previously published work on Japanese and St Louis encephalitis, dengue and yellow fever. The process appears to be reversible, i.e. increased temperature raises virus infection and transmission rates. It is concluded that, without incubation at warmer temperatures, flavivirus recovery from overwintering mosquitoes will be negatively biased. 相似文献
3.
In the selection of acid-tolerantRhizobium meliloti, procedures for the collection and isolation of rhizobia, and the assessment of acid tolerance, have not been critically
evaluated. Such procedures form the basis of this study. Root nodules were collected fromMedicago spp. found growing on acid soil in Sardinia. Their encumbent bacteria were isolated directly onto media adjusted over a range
of pH values, and then assessed for acid tolerance in both the laboratory and field. Strains ofRhizobium meliloti isolated onto low pH media were, in general, more acid-tolerant than sister isolates from high pH media, when tested in both
the laboratory and field. Dilution (10 or 100 fold) of the inocula used in the laboratory assessment did not greatly influence
the rating derived, although there was some effect of bacterial colony type on growth rating. The link between polysaccharide
production and acid tolerance was not strong. There was a poor correlation between the growth ratings derived from the laboratory
screening and acid tolerance as expressed in the field. 相似文献
4.
Linus Svensson 《Plant Systematics and Evolution》1992,180(3-4):157-180
The variation in two sets of morphological characters of the flowers of the highly inbreedingScleranthus annuus (Caryophyllaceae) was assessed using 15–20 plants from each of 20 natural populations from the southernmost region of Sweden. The stamen fertility data set consisted of 10 characters describing the degree of fertility of the ten stamens/staminoids, while the sepal/gynoecium data set comprised 10 sepal characters, together with style and stigma length. Substantial variation was found in both the degree of development and the fertility of the stamens, the degree of variability in stamen fertility being related to stamen position within the flower. Considerable variation was found in the characters of the sepal and the gynoecium. Hierarchical analyses of variance indicated that 29% of the variation in total male reproductive effort was distributed among populations, 28% among plants within populations and 43% represented within-plant variation. The corresponding averages for the characters from the sepal/gynoecium data set are 26, 38, and 35%: a greater proportion of the total variance in female reproductive characters is accounted for by among individual variation than is the case with the male reproductive characters. Significance tests of Mahalanobis distances derived by canonical variate analyses indicated that all populations were significantly separated using the sepal/gynoecium data set, while only 50% of the pairwise comparisons on the basis of the stamen fertility data set were significant. Cluster analysis did not reveal any aggregation of the populations. The incongruence of the two data sets and their ability to discriminate between the populations is discussed. 相似文献
5.
Reproductive structures and phylogenetic significance of extant primitive Angiosperms 总被引:2,自引:0,他引:2
Peter K. Endress 《Plant Systematics and Evolution》1986,152(1-2):1-28
A brief survey is presented on fossil reproductive structures of early Angiosperms from the Lower and mid-Cretaceous and at the same time on the reproductive structures of those extant Angiosperms which resemble most closely these fossils and which seem to be especially primitive also on other grounds: a first group (Degeneriaceae, Himantandraceae, Eupomatiaceae, Austrobaileyaceae) possessing relatively complicated and conspicuous flowers with elaborated inner staminodes, a second group (Chloranthaceae, Trimeniaceae, Amborellaceae) possessing small and relatively simple, inconspicuous flowers with peculiar features in the carpels, and a third group (Winteraceae) possessing flowers with unusual variability in organ number and size. The three groups exhibit a certain diversity in pollination biology, although cantharophily seems to prevail, however different the cantharophily character syndromes may be between the groups. In the extant primitiveMagnoliidae variability occurs on other morphological levels than in the higher advanced Angiosperms. This has to be taken into consideration in evaluations of the systematic relationships of the various groups of theMagnoliidae. Presumably often their relationships are closer than it may appear at first sight. This is also true for the three groups here discussed. 相似文献
6.
Motomi Ito 《Journal of plant research》1986,99(2):169-184
Floral morphology ofBrasenia schreberi Gmel. andCabomba caroliniana A. Gray was observed chiefly from an anatomical point of view. The receptacle ofB. schreberi is rather flat and a vascular plexus is observable in the mature flower. The vasculature in this plexus is so complex taht
it is not easy to trace its structure in detail. by observation on small buds, it can be seen that the receptacular vasculature
consists of a girdling bundle in the basal area and usually nine receptacular strands from which traces to the petals and
stamens branch off. The vasculature in the receptacle is reconstructed and diagramatically shown as though split longitudinally
and spread out in one plane.
Floral vasculature inCabomba caroliniana is simpler, and is probably related to the smaller number of stamens and carpels. It also has a girdling bundle at the bottom
of receptacle and this vasculature is suggested to be derived by simplification from aBrasenia-type vasculature.
Evidence from floral anatomy suggests that these two genera are closely related. InNymphaea, a vascular plexus in the receptacle is also observed (Moseley, 1961; Ito 1983). The plexus ofBrasenia andNymphaea are not the same in their construction. Nevertheless, their fundamental floral vasculature is comparable and it is preferable
to place them in the same family or same order. 相似文献
7.
8.
Effects of platelet-derived growth factor and fibroblast growth factor on free intracellular calcium and mitogenesis 总被引:2,自引:0,他引:2
Although increased free intracellular calcium (Cai) may be one of the main regulators of cell growth and differentiation, studies in cell populations have implied that not all growth factors produce Cai increases. In order to examine in more detail whether Cai increases were related to mitogenesis, we used digital image analysis of intracellular Fura-2 fluorescence to measure Cai in individual BALB/c 3T3 cells stimulated with either platelet-derived growth factor (PDGF) or fibroblast growth factor (FGF). We found that PDGF induced larger and more prolonged Cai increases than FGF did, but that both growth factors induced an initial rapid increase in Cai (less than 2 min) followed by a later sustained increase (greater than 20 min). Only the prolonged Cai increase required extracellular calcium. Following PDGF treatment (1-8 units/ml), the percentage of cells with a large peak Cai increase (greater than twofold) correlated with the percentage of cells made competent (subsequent growth in 1% platelet-poor-plasma). In contrast, purified bovine basic FGF (200-800 pg/ml) and recombinant human acidic FGF (10-300 ng/ml) produced peak Cai increases that were not directly correlated with mitogenesis. In addition, concentrations of intracellular Quin 2 that inhibited Cai transients also inhibited PDGF stimulation but not FGF stimulation of mitogenesis. Thus, Cai increases are necessary for mitogenesis in BALB/c 3T3 cells stimulated by PDGF, but not that stimulated by FGF. 相似文献
9.
Philip J. Bassford Jr. 《Journal of bioenergetics and biomembranes》1990,22(3):401-439
The export of the maltose-binding protein (MBP), themalE gene product, to the periplasm ofEschericha coli cells has been extensively investigated. The isolation of strains synthesizing MalE-LacZ hybrid proteins led to a novel genetic selection for mutants that accumulate export-defective precursor MBP (preMBP) in the cytoplasm. The export defects were subsequently shown to result from alterations in the MBP signal peptide. Analysis of these and a variety of mutants obtained in other ways has provided considerable insight into the requirements for an optimally functional MBP signal peptide. This structure has been shown to have multiple roles in the export process, including promoting entry of preMBP into the export pathway and initiating MBP translocation across the cytoplasmic membrane. The latter has been shown to be a late event relative to synthesis and can occur entirely posttranslationally, even many minutes after the completion of synthesis. Translocation requires that the MBP polypeptide exist in an export-competent conformation that most likely represents an unfolded state that is not inhibitory to membrane transit. The signal peptide contributes to the export competence of preMBP by slowing the rate at which the attached mature moiety folds. In addition, preMBP folding is thought to be further retarded by the binding of a cytoplasmic protein, SecB, to the mature moiety of nascent preMBP. In cells lacking this antifolding factor, MBP export represents a race between delivery of newly synthesized, export-competent preMBP to the translocation machinery in the cytoplasmic membrane and folding of preMBP into an export-incompetent conformation. SecB is one of threeE. coli proteins classified as molecular chaperones by their ability to stabilize precursor proteins for membrane translocation. 相似文献
10.
Eva-Marià Kupsch Dominique Aubel Carol P. Gibbs Andreas F. Kahrs Thomas Rudel Thomas F. Meyer 《Molecular & general genetics : MGG》1996,250(5):558-569
A versatile shuttle system has been developed for genetic complementation with cloned genes of transformable and non-transformableNeisseria mutants. By random insertion of a selectable marker into the conjugativeNeisseria plasmidptetM25.2, a site within this plasmid was identified that is compatible with plasmid replication and with conjugative transfer of plasmid. Regions flanking the permissive insertion site of ptetM25.2 were cloned inEscherichia coli and served as a basis for the construction of the Hermes vectors. Hermes vectors are composed of anE. coli replicon that does not support autonomous replication inNeisseria, e.g. ColE1, p15A, orori
fd, fused with a shuttle consisting of a selectable marker and a multiple cloning site flanked by the integration region of ptetM25.2. Complementation of a non-transformableNeisseria strain involves a three-step process: (i) insertion of the desired gene into a Hermes vector; (ii) transformation of Hermes into aNeisseria strain containing ptetM25.2 to create a hybrid ptetM25.2 via gene replacement by the Hermes shuttle cassette; and (iii) conjugative transfer of the hybrid ptetM25.2 into the finalNeisseria recipient. Several applications for the genetic manipulation of pathogenicNeisseriae are described. 相似文献