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In common cypress, Cupressus sempervirens L., the megagametophyte persists in mature seeds as a polyploid endosperm containing cells with even and odd series of DNA
contents: 1C, 2C, 3C, 4C, 5C etc., where C is the amount of DNA in the haploid genome. In this study, cytometrical, histological
and cytochemical investigations were performed in order to determine the behavior of megagametophyte nuclei during the reproductive
cycle. Unexpected nuclear alterations due to a continuous process of nuclear fusion were observed in the megagametophyte,
leading to polyploidization and consequently to intense food-reserve synthesis. During the free nuclear stage, the megagametophyte
exhibited only sporadic nuclear fusion and limited food-reserve production. When cellularization took place, multinucleated
compartments were observed in which nuclei fused, producing odd and even series of DNA contents as proved by flow-cytometric
analysis. This polyploidization process considerably increased after fertilization and during embryo development, and was
accompanied by increased food-reserve synthesis. During these later stages, fusion mainly involved nuclei of contiguous cells
and was preceded by the disintegration of their adjacent walls. Mitoses with incomplete phragmoplast differentiation were
also observed to yield polyploid nuclei. Finally, in mature seeds the endosperm still exhibited multinucleate cells and fusion
nuclei, and contained high amounts of storage products. The results are interpreted as an alteration of DNA contents in the
megagametophyte cells in relation to specific metabolic activity during seed development.
Received: 2 September 1998 / Accepted: 31 December 1998 相似文献
948.
Salwa A. Maksoud M. Nabil El Hadidi Wafaa Mahrous Amer 《Vegetation History and Archaeobotany》1994,3(4):219-224
Physical and chemical analyses of beer residues recovered from a vat site at Hierakonpolis (Upper Egypt) were carried out. Radiocarbon dates of the residues suggest a dating of 3500–3400 cal B.C. and are believed to represent the oldest known beer in the world. Macroscopic and microscopic examinations of the residues revealed the presence of intact remains of grains and spikelets of wheat and barley, as well as fragments of dates and grape pips. Chemical analyses included percentages of sample ingredients, pH and total soluble ions, quantitative determinations of sugars, carboxylic acids and free amino acids. A total of 25 compounds were identified, which are components of fermentation processes that are believed to have formed in connection with the preparation of what is called Nekhen-Hoffman beer. 相似文献
949.
Hydrobiologia - The species composition, abundance and biomass of autotrophic picoplankton and larger phytoplanktonic organisms were analysed in 17 non-stratified lakes of Pomerania. Water samples... 相似文献
950.
Different capacity for phenotypic variation of Pseudomonas aurantiaca and P. fluorescens in populations of cyst-like resting cells (CRC) during their germination on solid media, was shown to be a characteristic trait of biodiversity for the dormant forms of these bacteria. This biodiversity manifests itself as qualitative and quantitative differences in the spectra and emergence frequency of phenotype variants, obtained by plating of CRC, and depends on the conditions of CRC formation and storage time. In P. aurantiaca, the variation was associated with transition of the wild-type S-colonial phenotype into the R-type or the more pigmented P-type. These transitions were most pronounced for the CRC obtained under nitrogen depletion (a twofold N limitation), as well as under the influence of a chemical analogue of microbial anabiosis autoinducers, C12-AHB. In the latter case, the frequency of S?R and S?P transitions (up to 70% and 80%, respectively) depended on the C12-AHB concentration (1.0 × 10?4 M and 2.5 × 10?4 M) and on the storage time of CRC suspensions (from 3 days to 1.3 months). In the CRC populations grown in nitrogen-deficient media, R-type appeared with a frequency of up to 45% after at least four months of storage. In the case of P. fluorescens, S?R transitions depended not only on the storage time of CRC and C12-AHB concentrations, but also on the composition of the solid medium used for plating. Differences were shown between the R-, P-, and S-variants of P. aurantiaca in such morphological, physiological, and biochemical characteristics as the growth rate (μmax) in a poor medium, biomass yield (Y max), resistance to streptomycin and tetracycline (LD50), and the productivity in extracellular proteases. The R-and S-variants of P. fluorescens differed in their growth characteristics, resistance to high salinity and oxidative stress, as well as in their sensitivity to exogenous introduction of chemical analogues of microbial autoregulators (C12-AHB and C7-AHB). Hence, both the formation of dormant forms of the various morphological types [1] and intrapopulation phenotypic variability observed during their germination are important for the survival strategy of pseudomonads under unfavorable environmental conditions. 相似文献