全文获取类型
收费全文 | 6906篇 |
免费 | 492篇 |
国内免费 | 912篇 |
出版年
2024年 | 20篇 |
2023年 | 126篇 |
2022年 | 129篇 |
2021年 | 190篇 |
2020年 | 249篇 |
2019年 | 298篇 |
2018年 | 221篇 |
2017年 | 235篇 |
2016年 | 268篇 |
2015年 | 221篇 |
2014年 | 305篇 |
2013年 | 401篇 |
2012年 | 226篇 |
2011年 | 412篇 |
2010年 | 190篇 |
2009年 | 396篇 |
2008年 | 441篇 |
2007年 | 418篇 |
2006年 | 345篇 |
2005年 | 302篇 |
2004年 | 271篇 |
2003年 | 260篇 |
2002年 | 205篇 |
2001年 | 160篇 |
2000年 | 159篇 |
1999年 | 159篇 |
1998年 | 148篇 |
1997年 | 119篇 |
1996年 | 113篇 |
1995年 | 123篇 |
1994年 | 101篇 |
1993年 | 85篇 |
1992年 | 84篇 |
1991年 | 104篇 |
1990年 | 63篇 |
1989年 | 63篇 |
1988年 | 57篇 |
1987年 | 73篇 |
1986年 | 38篇 |
1985年 | 77篇 |
1984年 | 92篇 |
1983年 | 40篇 |
1982年 | 68篇 |
1981年 | 63篇 |
1980年 | 60篇 |
1979年 | 44篇 |
1978年 | 17篇 |
1977年 | 15篇 |
1976年 | 23篇 |
1973年 | 12篇 |
排序方式: 共有8310条查询结果,搜索用时 15 毫秒
171.
Determination of substrate utilization patterns of soil microbial communities: An approach to assess population changes after hydrocarbon pollution 总被引:13,自引:0,他引:13
Abstract: Changes in hydrocarbon content in soils resulted in characteristic shifts of the substrate utilization patterns as tested with the Biolog system. The altered patterns of substrate utilization corresponded to similar changes in abundance of hydrocarbon-utilizing bacteria and the occurrence of specific bacterial groups in the soils. Substrate utilization patterns as recorded with the Biolog system are suitable for rapidly assessing dynamics of autochthonous soil communities and evaluating their biodegradative potential. 相似文献
172.
Alberto M. Martelli Lucia Manzoli Silvia Rubbini Anna Maria Billi Renato Bareggi Lucio Cocco 《Biology of the cell / under the auspices of the European Cell Biology Organization》1995,83(1):15-22
Summary— Using two-dimensional polyacrylamide gels stained with Coomassie blue we have studied the protein composition of the nuclear matrix obtained from mouse erythroleukemic nuclei kept at O°C throughout the isolation procedure to prepare the high ionic strength resistant fraction (control matrix) or stabilized in vitro or in vivo by different procedures prior to subfractionation (ie 37°C incubation of isolated nuclei; sodium tetrathionate exposure of purified nuclei; heat shock of intact cells). When the matrix obtained from 37°C incubated nuclei was compared with the control matrix, striking differences in the polypeptide pattern were seen if the protein was obtained in both cases from an equivalent number of nuclei. On the other hand, if the same amount of protein for both the samples was applied to the gels the differences were less evident. Sodium tetrathionate stabilization of isolated nuclei and heat shock of intact cells produced a matrix protein pattern that was very similar and differed from that of the in vitro heat-exposed matrix. Using specific polyclonal antisera, we demonstrate that nucleolar proteins B23/numatrin and C23/nucleolin were very abundant in the matrix obtained from chemically-treated nuclei or in vivo heat-stabilized nuclei but were recovered in very small amounts (B23) or completely absent (C23) in the matrix prepared from nuclei heated to 37°C in vitro. Differences were seen also in the recovery of nuclear lamins, and especially lamin B, that was poorly represented in the sodium tetrathionate-stabilized matrix. The results demonstrate that in mouse erythroleukemia cells the increased recovery of nuclear matrix protein that is seen after in vitro heating of isolated nuclei is predominantly due to an additional recovery of the same types of polypeptides that are detected also in the absence of such a treatment. The data also indicate that in vivo heat shock of intact cells produces a nuclear matrix protein pattern that is more similar to the pattern seen after stabilization of purified nuclei with sodium tetrathionate and differs significantly from that obtained by exposing nuclei to 37°C in vitro, unlike to that what previous reports have indicated. 相似文献
173.
Martin Sprung 《Hydrobiologia》1995,304(2):117-132
Shell growth of tagged zebra mussels (Dreissena polymorpha) was monitored in lakes at 3 sites over 1 1/2 years. It varied greatly with the season and was almost absent during winter
months. Shell growth was significantly correlated with seston concentration, but not with water temperature. The theoretical
maximum size (L∞) of the v. Bertalanffy growth equation did not vary seasonally.
Tissue weight underwent a pronounced seasonal cycle. In animals of 20 mm shell length, minimum weights recorded in September
only corresponded to one quarter to one third of the maxima in late spring. Tissue weight of animals from the 3 sites was
distinctly different.
Carbohydrate content of the tissue stayed below 10 per cent and tended to be highest in spring. Spring maxima of lipids were
very pronounced. These lipids were primarily located in the digestive gland.
These data were combined with data of gonad size by Borcherding (1991). Results imply that production of reproductive tissue
even continues at a retarded rate during winter months, if food conditions were favourable. With poor food conditions, no
production of reproductive tissue was estimated during winter; this, however, was followed by an elevated rate of reproductive
tissue production in spring. Weight specific production decreased with a weight exponent of −0.24. Shell and byssus production
contributed only in the range of 10 per cent to total production.
Reproductive effort showed maxima of 30 to 45%. It increased with shell length at 2 sites and decreased at the other site
with the largest animals. 相似文献
174.
A comparison of natural and human determinants of phytoplankton communities in the Kentucky River basin,USA 总被引:1,自引:1,他引:0
Physical, chemical, and biological characteristics of the Kentucky River and its tributaries were assessed for one year to compare effects of seasonal, spatial, and human environmental factors on phytoplankton. Phytoplankton cell densities were highest in the fall and summer and lowest in the winter. Cell densities averaged 1162 (± 289 SE) cells m1–1. Cell densities were positively correlated to water temperature and negatively correlated to dissolved oxygen concentration and to factors associated with high-flow conditions (such as, suspended sediment concentrations). Chrysophytes, diatoms, and blue-green algae dominated winter, spring, and summer assemblages, respectively. Ordination analyses (DCCA) indicated that variation in taxonomic composition of assemblages was associated with stream size as well as season.Spatial variation in phytoplankton assemblages and effects of humans was investigated by sampling 55 sites in low flow conditions during August. Phytoplankton density increased with stream size. Assemblages shifted in composition from those dominated by benthic diatoms upstream to downstream communities dominated by blue-green algae and small flagellates. Human impacts were assumed to cause higher algal densities in stream basins with high proportions of agricultural or urban land use than in basins with forested/mined land use. While density and composition of phytoplankton were positively correlated to agricultural land use, they were poorly correlated to nutrient concentrations. Phytoplankton diversity changed with water quality: decreasing with nutrient enrichment and increasing with conditions that probably changed species composition or inhibited algal growth. Human impacts on phytoplankton in running water ecosystems were as great or greater than effects by natural seasonal and spatial factors. Our results indicated that phytoplankton could be useful indicators of water quality and ecosystem integrity in large river systems. 相似文献
175.
一种水稻卷叶性状的遗传分析 总被引:15,自引:0,他引:15
对水稻卷叶品种流岗卷叶粳与4个平展叶品种及1个卷叶标志基因(rl3)系的杂交或回交后代进行了考察。结果表明,流岗卷叶粳的卷叶特性以单基因不完全显性方式遗传; 该基因与rl3基因不等位,当rl3位点处于隐性纯合时两者以累加方式发生互作。这一等位基因可作标志基因使用,暂定名为Rl(t)。
Abstract:Liugangjuanyejing is a rolled leaf mutant of rice.A genetic study on the rolled-leaf character was carried out by crossing it with four flat-leaf cultivars and a genetic marker line (with a rolled-leaf allele rl3).The results showed that this character was controlled by an incomplete dominant gene which was non-allelic to rl3 locus and that there existed additive effect between the two loci when the rl3 locus was homozygous recessiveness.This new rolled-leaf allele was provisionally named as Rl(t) and could be used as a genetic marker of rice. 相似文献
176.
R. L. Wolff Bernard Comps Anne M. Marpeau Laurent G. Deluc 《Trees - Structure and Function》1997,12(2):113-118
The fatty acid compositions of the seed oils from ten pine species have been established by capillary gas-liquid chromatography
of the methyl esters. With regard to either normal fatty acids or Δ5-olefinic acids, the general pattern of fatty acids did
not differ from that of other pine seed oils reported previously. The main fatty acid was linoleic (9,12–18:2) acid (44.4–57.1%),
followed by either oleic (9–18:1) acid (13.4–24.5%) or pinolenic (5,9,12–18:3) acid (1.5–25.2%). When applying multivariate
analyses to the chemometric data (13 variables) of 49 pine species (ca. 40% of the living pine species), it was possible to
distinguish between several sections: Pinea, Longifolia, Halepensis, Ponderosa-Banksiana, Sylvestris, and Cembra. The latter section was clearly divided into two sub-groups. A few species that presented a low overall content of Δ5-olefinic
acids, and that grow in warm-temperate regions, were isolated from the bulk of other pine species. It is hypothesized that
Δ5-olefinic acids might be related to cold-acclimation.
Received: 5 June 1997 / Accepted: 17 August 1997 相似文献
177.
Transgenic animals that over- or underexpress a protein of interest have been used to study obesity development, prevention, and susceptibility to diet-induced obesity such as a high-fat diet. Several transgenic models are resistant to diet-induced obesity including those that overexpress the insulin-sensitive glucose transporter, GLUT4, in adipose tissue only. In this animal there is increased adipose tissue mass but the animal maintains its insulin sensitivity. The overexpression of lipoprotein lipase (LPL) in skeletal muscle and the elimination of a protein kinase A subunit both resulted in lean and obesity resistant animals. By directing the production of the diphtheria toxin A chain to adipose tissue only the resulting animals not only had less adipose tissue mass but were resistant to MSG-induced obesity. Conversely, transgenic models with decreased brown adipose tissue or its function have all resulted in obese animals, highlighting the importance of thermoregulation in body weight maintenance. The use of transgenic technology in the field of obesity has emphasized the regional differences among fat pads as well as the dissimilarity between genders in fuel metabolism. Several transgenic models have separated obesity from insulin resistance allowing the importance of each state to be studied individually. Results using transgenic animals have re-emphasized that obesity is a polygenic disease. 相似文献
178.
On the production,elemental composition (C,N, P) and distribution of photosynthetic organic matter in the Southern Black Sea 总被引:2,自引:1,他引:1
Yılmaz Ayşen Tuğrul Süleyman Polat Çolpan Ediger Dilek Çoban Yeşim Morkoç Enis 《Hydrobiologia》1997,363(1-3):141-155
Chemical oceanographic understanding of the southernBlack Sea has been improved by recent measurements ofthe optical transparency,
phytoplankton biomass (interms of chlorophyll-a and particulate organic matter)and primary productivity. During the spring-autmunperiod
of 1995–1996, light generally penetrated onlyinto the upper 15–40 m, with an attenuation coefficientvarying between 0.125
and 0.350 m2122;1. The averagechlorophyll-a (Chl-a) concentrations for the euphoticzone ranged from 0.1 to 1.5 μg l2122;1. Coherentsub-surface Chl-a maxima were formed near the base ofthe euphotic zone only in summer. Production rate variedbetween
247 and 1925 in the spring and between 405 and687 mgC m2122;2 d2122;1 in the summer-autumn period.The average POM concentrations in the euphotic zonevaried regionally and seasonally between 3.8
and28.6 μm for POC, 0.5 and 3.1 μm for PON and0.02 and 0.1 μm for PP. Atomic ratios of C/N, C/Pand N/P, derived from the regressions
of POM data,ranged between 7.5 and 9.6, 109 and 165, and 11.2 and16.6, respectively. In the suboxic/anoxic interface,the elemental
ratios change substantially due to anaccumulation of PP cohering to Fe and Mn oxides. Thechemocline boundaries and the distinct
chemicalfeatures of the oxic/anoxic transition layer (the so-called suboxic zone) are all located at specificdensity surfaces;
however, they exhibit remarkablespatial and temporal variations both in their positionand in their magnitude, which permit
the definition of long-term changes in the biochemical properties of theBlack Sea upper layer.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
179.
180.
Several aspects of community organization wereanalyzed comparatively in a small side-arm of theParaná River (Correntoso) and a shallowfloodplain lake (El Tigre) (31° 41 S and60° 42 W), in relation to the hydrology of thesystem. Taxonomic and morphological composition inthe river differed from that in the lake: the riverhad lower species richness (151 vs 218),different contributions of some Classes to totalspecies number (higher Cyano-, Zygo- andDiatomophyceae vs higher Chlorophyceae), anddiffent proportions of nannoplanktonic algae (67.5%vs 80.7%) and netplanktonic filamentousspecies (18.2% vs 4.2%). Phytoplanktonbiomass, higher in the lake than in the river due tothe retention time, was mostly dominated bynannoplankton and netplankton. Loticphytoplankton was dominated by typical fluvialspecies of Diatomophyceae (R-strategists). Riverconditions seem to maintain a subclimacticcommunity, which was little impacted by the flushingof populations from floodplain lakes. Water levelwas the main factor controlling phytoplanktonbiomass, species diversity (H), evenness (E) andcommunity change rate () in the river. Inthe lake, phytoplankton had an autogenicsuccessional sequence during the isolation phase (C-to S-strategists) and other responses todisturbance, mainly during the flood(R-strategists). Frequent changes in phytoplanktoncomposition, biomass, H, E and , revealed aenvironmental instability in the lake, which may beexplained by interactions of external factors(hydrology and climatology) and those of internalorigin, such as nutrients and grazing. 相似文献