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991.
Andrej Halabuk 《Biologia》2006,61(19):S266-S269
In the Parí? creek catchment (southwestern part of Slovakia), the influence of different vegetation types on selected soil properties in alluvial topsoils was studied. Specifically, the effect on saturated hydraulic conductivity considered as indicator of water transport process and the effect on soil bulk density considered as indicator of soil structure were analysed. Due to the mutual influence of plant roots on soil properties, the root biomass was also estimated and its relationship to the studied soil properties was explored. Reed and tall-sedge wetlands and alluvial wet meadows represented the studied vegetation types. Adjacent arable lands (former grasslands) with corn were included for comparison. In total, 64 samples were used for comparative analysis. A standard methodology for measurement of the saturated hydraulic conductivity, the so-called falling head technique was used on 250 cm3 soil cores. Undisturbed soil samples were taken from the depth of 5 cm. Analysis of variance, mutual comparison of mean values and correlation matrix were used for statistical analyses. Measurements showed significantly higher values of saturated hydraulic conductivity for topsoils in wetlands (6.2 m day?1 on average) compared to mown grasslands (1.47 m day?1) and arable land (0.79 m day?1). The results indicated a specific significance of wetlands in relation to water transport processes in alluvial topsoils. 相似文献
992.
993.
In the Baltic Sea, herring (Clupea harengus membras) spawns in the littoral zone, where its eggs are attached to algae or vascular plants. Field studies indicate that egg mortality
can be very high (up to 100%) in eggs that are attached to red algae (Rajasilta et al., 1989, 1993). Because high mortality can be due to allelochemical effects of the algae, we studied the mortality of herring eggs on different
algal substrates experimentally. Four types of substrates were tested: fresh Cladophora and Furcellaria, and Furcellaria that had decomposed six days or 23 days. The incubation time in the experiments was 3 days and incubation temperature 12–13 oC (ca. 700–800 h-degrees). The results were in accordance with observations made in field studies and indicated significant
differences among the substrate types. In eggs attached to fresh Cladophora, mortality was significantly lower (mean=2.8%; n=20) than in those attached to Furcellaria, independently of the treatment of the algae. The highest values of mortality (mean=14.4%; n=20) were found in eggs attached to Furcellaria that had decomposed over a six days’ period. This suggested that Furcellaria contain some chemical substances, which can cause mortality in herring embryos and the effect seems to be dependent on the
state of decomposition of the algae. 相似文献
994.
Trophic Levels and Functional Feeding Groups of Macroinvertebrates in Neotropical Streams 总被引:3,自引:2,他引:1
Feeding strategies are typical traits reflecting the adaptation of species to environmental conditions. This concept is currently
developed in some water quality systems (e.g. Index of Trophic completeness) and the structure of functional feeding groups
(FFGs) could form part of a unified measure across communities differing in taxonomic composition. However, in South America,
information about the FFG classification of invertebrates in streams is almost absent and existing studies using FFG structure
follows classification from North America. But even taxonomically related species may have different diets in tropical and
temperate areas and therefore, studies about FFG structure in neotropics could be biased. For this reason, we determined diet
composition, trophic level and FFGs, using gut contents analysis and mouthpart observations of 49 macroinvertebrate taxa (mostly
at genus level) from neotropical streams. We observed that practically all macroinvertebrates fed upon fine detritus which
indicates the importance of this food resource in neotropical streams. As the assignment to a single FFG does not accurately
reflect the functional profile of taxa, we transcribed the affinity of taxa to each FFG using fuzzy codes. Finally, we published
the coding of diet composition and FFG of the taxa examined, which could be used in future community analyses of lotic ecosystems
in the Neotropical zone. 相似文献
995.
R. Jan Stevenson Steven T. Rier Catherine M. Riseng Richard E. Schultz Michael J. Wiley 《Hydrobiologia》2006,561(1):149-165
Responses of stream algal biomass to nutrient enrichment were studied in two regions where differences in hydrologic variability
cause great differences in herbivory. Around northwestern Kentucky (KY) hydrologic variability constrains invertebrate biomass
and their effects on algae, but hydrologic stability in Michigan (MI) streams permits accrual of high herbivore densities
and herbivory of benthic algae. Multiple indicators of algal biomass and nutrient availability were measured in 104 streams
with repeated sampling at each site over a 2−month period. Many measures of algal biomass and nutrient availability were positively
correlated in both regions, however the amount of variation explained varied with measures of biomass and nutrient concentration
and with region. Indicators of diatom biomass were higher in KY than MI, but were not related to nutrient concentrations in
either region. Chl a and % area of substratum covered by Cladophora were positively correlated to nutrient concentrations in both regions. Cladophora responded significantly more to nutrients in MI than KY. Total phosphorus (TP) and total nitrogen (TN) explained similar
amounts of variation in algal biomass, and not significantly more variation in biomass than dissolved nutrient concentrations.
Low N:P ratios in the benthic algae indicated N as well as P may be limiting their accrual. Most observed responses in benthic
algal biomass occurred in nutrient concentrations between 10 and 30 μg TP l−1 and between 400 and 1000 μg TN l−1. 相似文献
996.
Non-Advertized does not Mean Concealed: Body Odour Changes across the Human Menstrual Cycle 总被引:1,自引:0,他引:1
Jan Havlíek† Radka DvoákovᆠLudk Barto‡ & Jaroslav Flegr§ 《Ethology : formerly Zeitschrift fur Tierpsychologie》2006,112(1):81-90
Females of a number of primate species display their fertile period by behavioural and/or morphological changes. Traditionally, the fertile period in human females has been considered to be concealed. However, this presumption has rarely been tested. One of the possible mechanisms for assessing menstrual cycle phase is through the sense of smell. In this study possible changes in odour across the menstrual cycle were investigated. Samples of body odour were acquired from 12 women (aged 19–27 yr), none of whom were using hormonal contraceptives. Samples were collected using cotton pads worn in the armpit for 24 h, from the menstrual, follicular and luteal cycle phases. Our experimental sample of 42 males (age 19–34 yr) repeatedly rated these odour samples for their intensity, pleasantness, attractiveness and femininity. Raw subjective smell ratings from each man were transformed to z‐scores. Subsequently, these z‐scores were tested by the general linear mixed‐model analysis (PROC MIXED, SAS) with the female's ID nested within the subject's ID as a random factor to account for the repeated measures of the subjects. Significant changes across the cycle were found for ratings of pleasantness [F(2,689) = 702; p = 0.001], attractiveness [F(2,546) = 6.35; p = 0.002] and intensity [F(2,530) = 3.57; p = 0.028]. Odour from women in the follicular (i.e. fertile) phase was rated as the least intense and the most attractive. Subsequent post hoc analysis revealed significant differences in intensity, pleasantness and attractiveness between the menstrual phase and the follicular phase, and in pleasantness and attractiveness between the menstrual and luteal phases. Significant difference between the follicular and the luteal phase was found only for attractiveness. Our results suggest that men can potentially use smell as a mechanism for monitoring menstrual cycle phase in current or prospective sexual partners. Therefore, the fertile period in humans should be considered non‐advertized, rather than concealed. 相似文献
997.
dsRNA-mediated resistance to Beet Necrotic Yellow Vein Virus infections in sugar beet (Beta vulgaris L. ssp. vulgaris) 总被引:1,自引:0,他引:1
Britt-Louise Lennefors Eugene I. Savenkov Jan Bensefelt Elisabeth Wremerth-Weich Petra van Roggen Stig Tuvesson Jari P. T. Valkonen Jan Gielen 《Molecular breeding : new strategies in plant improvement》2006,18(4):313-325
Rhizomania, one of the most devastating diseases in sugar beet, is caused by Beet Necrotic Yellow Vein Virus (BNYVV) belonging to the genus Benyvirus. Use of sugar beet varieties with resistance to BNYVV is generally considered as the only way to maintain a profitable yield on rhizomania-infested fields. As an alternative to natural resistance, we explored the transgenic expression of viral dsRNA for engineering resistance to rhizomania. Transgenic plants expressing an inverted repeat of a 0.4 kb fragment derived from the BNYVV replicase gene displayed high levels of resistance against different genetic strains of BNYVV when inoculated using the natural vector, Polymyxa betae. The resistance was maintained under high infection pressures and over prolonged growing periods in the greenhouse as well as in the field. Resistant plants accumulated extremely low amounts of transgene mRNA and high amounts of the corresponding siRNA in the roots, illustrative of RNA silencing as the underlying mechanism. The transgenic resistance compared very favourably to natural sources of resistance to rhizomania and thus offers an attractive alternative for breeding resistant sugar beet varieties. 相似文献
998.
The effect of nutrient enrichment on the structure and carbon flow in the pelagic microbial food web was studied in mesocosm experiments using seawater from the northern Baltic Sea. The experiments included food webs of at least four trophic levels; (1) phytoplankton–bacteria, (2) flagellates, (3) ciliates and (4) mesozooplankton. In the enriched treatments high autotrophic growth rates were observed, followed by increased heterotrophic production. The largest growth increase was due to heterotrophic bacteria, indicating that the heterotrophic microbial food web was promoted. This was further supported by increased growth of heterotrophic flagellates and ciliates in the high nutrient treatments. The phytoplankton peak in the middle of the experiments was mainly due to an autotrophic nanoflagellate, Pyramimonas sp. At the end of the experiment, the proportion of heterotrophic organisms was higher in the nutrient enriched than in the nutrient-poor treatment, indicating increased predation control of primary producers. The proportion of potentially mixotrophic plankton, prymnesiophyceans, chrysophyceans and dinophyceans, were significantly higher in the nutrient-poor treatment. Furthermore, the results indicated that the food web efficiency, defined as mesozooplankton production per basal production (primary production + bacterial production − sedimentation), decreased with increasing nutrient status, possibly due to increasing loss processes in the food web. This could be explained by promotion of the heterotrophic microbial food web, causing more trophic levels and respiration steps in the food web. 相似文献
999.
Angel Rivera-Calzada Rémi Fronzes Christos G Savva Vidya Chandran Pei W Lian Toon Laeremans Els Pardon Jan Steyaert Han Remaut Gabriel Waksman Elena V Orlova 《The EMBO journal》2013,32(8):1195-1204
Type IV secretion (T4S) systems are able to transport DNAs and/or proteins through the membranes of bacteria. They form large multiprotein complexes consisting of 12 proteins termed VirB1‐11 and VirD4. VirB7, 9 and 10 assemble into a 1.07 MegaDalton membrane‐spanning core complex (CC), around which all other components assemble. This complex is made of two parts, the O‐layer inserted in the outer membrane and the I‐layer inserted in the inner membrane. While the structure of the O‐layer has been solved by X‐ray crystallography, there is no detailed structural information on the I‐layer. Using high‐resolution cryo‐electron microscopy and molecular modelling combined with biochemical approaches, we determined the I‐layer structure and located its various components in the electron density. Our results provide new structural insights on the CC, from which the essential features of T4S system mechanisms can be derived. 相似文献