全文获取类型
收费全文 | 1227篇 |
免费 | 36篇 |
国内免费 | 51篇 |
出版年
2023年 | 6篇 |
2022年 | 14篇 |
2021年 | 14篇 |
2020年 | 12篇 |
2019年 | 16篇 |
2018年 | 10篇 |
2017年 | 15篇 |
2016年 | 25篇 |
2015年 | 25篇 |
2014年 | 53篇 |
2013年 | 77篇 |
2012年 | 42篇 |
2011年 | 45篇 |
2010年 | 46篇 |
2009年 | 66篇 |
2008年 | 80篇 |
2007年 | 63篇 |
2006年 | 43篇 |
2005年 | 48篇 |
2004年 | 55篇 |
2003年 | 32篇 |
2002年 | 33篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 29篇 |
1998年 | 30篇 |
1997年 | 16篇 |
1996年 | 19篇 |
1995年 | 30篇 |
1994年 | 23篇 |
1993年 | 17篇 |
1992年 | 19篇 |
1991年 | 16篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 25篇 |
1986年 | 14篇 |
1985年 | 10篇 |
1984年 | 18篇 |
1983年 | 13篇 |
1982年 | 24篇 |
1981年 | 14篇 |
1980年 | 18篇 |
1979年 | 13篇 |
1978年 | 8篇 |
1977年 | 7篇 |
1975年 | 6篇 |
1973年 | 5篇 |
1963年 | 4篇 |
排序方式: 共有1314条查询结果,搜索用时 46 毫秒
1.
Summary Between 3 Nov. 1983 and 9 Apr. 1984, six applications of fertiliser N (ammonium, nitrate or urea) were given to four autumn
sown (26 Oct. 1983)Vicia faba L cultivars, Banner Winter (BW) and Maris Beagle (MBg), cold tolerant cultivars normally sown in the autumn, and Herz Freya
(HF) and Maris Bead (MBd), cold sensitive cultivars more commonly sown in the spring. The effects of additional N were determined
by comparison with plants given zero-N (controls).
Application of N, regardless of form, had no effect on % emergence at the first sampling (15 Dec. 1983); >90% for BW, MBg
and HF, but only 40–60% for MBd. At this time the dry weight, carbon content and nitrogen content of all cultivars was approximately
20% less than that of the seed on planting. No more plants emerged after 15 Dec. 1983. Between 15 Dec. 1983 and 20 Feb. 1984,
all cultivars, regardless of N treatment, showed little change in dry weight, carbon content and nitrogen content but the
proportion of total plant dry weight, carbon content and nitrogen content in the cotyledons decreased while the proportions
in root, stem and leaf tissue increased. On 20 Feb. 1984 there were no N effects. All cultivars but especially BW and MBg,
showed progressive increases in dry weight, carbon content and nitrogen content during the period 20 Feb. 1984 to 8 May 1984.
Pooled results for all four cultivars indicated that on 8 May 1984, plants given ammonium and urea had a greater dry weight,
carbon content and nitrogen content than controls.
At harvest (1–3 Sep. 1984), BW and MBg outyielded (g dw seed m−2) HF and MBd. Pooled results for all cultivars indicated that application of N regardless of form gave increased yield and
an increased N concentration (mg N g−1 dw) in the seed. 相似文献
2.
In mofette fields, natural carbon dioxide springs, organisms have to stand extreme CO2 concentrations up to 100%. These hostile conditions are spatially small-scaled and further influenced by earth tides, wind and temperature. The present project investigated the influence of increased atmospheric CO2 concentration on spiders as representatives of above-ground organisms by means of pitfall traps in three mofette fields, differing in habitat conditions in the Plesná valley, eastern Cheb Basin, Czech Republic.Among the 71 recorded spider species four were rarely found in the Czech Republic. A canonical correspondence analysis revealed significant influences of environmental parameters on the spider assemblages. Two groups of spiders are clearly distinguishable, one being positively influenced by humidity and the second by temperature. A cluster analysis showed distinct and congruent results: spider assemblages of pitfall traps at spots with a mean CO2 concentration above 7.6% grouped close together and this grouping was independent of site. At >7.6% CO2 significantly fewer individuals and species were found in comparison to areas with lower CO2 concentration. Between 2.5 and 10% CO2, spiders indicated increased CO2 concentrations much more sensitively than endogeic organisms (Nematoda, Collembola) in a nearby mofette field. Unlike in nematodes, collembolans and plants, no mofettovageous or mofettophilous spiders were detected. In contrast to humidity, CO2 concentration and temperature, the vegetation cover was not among the factors, which significantly influenced spiders. This is explained by the fact that mofettophilous plants occurred at spots where almost no spiders could live. In a field experiment, most Pardosa pullata males tested passed a 30 cm long corridor with increased carbon dioxide concentration. These results and that of pitfall traps showed that relatively large and wandering specimens respectively were able to transit moderately hostile spots. Further experiments are necessary to find out if there is any active avoidance of high-CO2 areas by spiders. 相似文献
3.
PurposeTo determine the field output correction factors of the radiophotoluminescence glass dosimeter (RPLGD) in parallel and perpendicular orientations with reference to CC01, the ionization chamber.MethodsThe dose to a small water volume and the sensitive volume of the RPLGD and the IBA-CC01 were determined for 6-MV, 100-cm SAD, 10-cm depth using egs_chamber user-code. The RPLGD in perpendicular and parallel orientations to the beam axis were studied. The field output correction factors of each detector for 0.5 × 0.5 to 10 × 10 cm2 field sizes were determined. These field output correction factors were validated by comparing field output factors against data determined from IAEA-AAPM TRS-483 code of practice.ResultsThe field output correction factors of all detectors were within 5% for field sizes down to 0.8 × 0.8 cm2. For 0.5 × 0.5 cm2, the field output correction factors of CC01, RPLGD in perpendicular and parallel orientations differed from unity by 14%, 19%, and 5%, respectively. The percentage difference between field output factors determined using RPLGD and CC01 data, corrected using the field output correction factors determined in this work and measurements with CC01 data corrected using TRS-483, was less than 3% for all field sizes, except for the smallest field size of RPLGD in perpendicular orientation and the CC01.ConclusionsThe field output correction factors of RPLGD and CC01 are reported. The validation proves that RPLGD in parallel orientation combined with the field output correction factors is the most suitable for determining the field output factors for the smallest field used in this study. 相似文献
4.
The influence of earthworms (Lumbricidae) on the nitrogen dynamics in the soil litter system of a deciduous forest 总被引:2,自引:0,他引:2
S. Scheu 《Oecologia》1987,72(2):197-201
Summary The influence of earthworms (Aporrectodea caliginosa (Savigny) and Lumbricus castaneus (Savigny)) on the rate of nitrogen net mineralization of the soil was studied in the laboratory and in the field. The additional mineralization of nitrogen cause by the burrowing activity of the substrat feeding earthworm A. caliginosa (N
L
)was directly correlated to the biomass of the lumbricids independently of their number. A rise in temperature caused an exponential increase in N
L
values. The Q
10 value of this process (2.18) was found to be much higher than that of the nitrogen mineralization without earthworms (Q
10=1.22). At 15°C the N
L
value caused by A. caliginosa was calculated to be about 250 g N g-1 fresh body wt d-1. Using the experimentally determined exponential relationship between temperature and N
L
values, the additional nitrogen mineralization caused by a population of A. caliginosa in a beechwood on limestone was calculated to be 4.23 kg ha-1 a-1.In contrast to A. caliginosa the litter dwelling species L. castaneus lost considerable amounts of biomass (56%) during the 4 week incubation period. Only 1/3 of the nitrogen equivalent to the weight loss of the animals was recovered in the mineral nitrogen pool.The addition of litter (old beech leaf litter, freshly fallen beech and ash leaf litter) had a pronounced effect on both nitrogen net mineralization and N
L
values of the soil. Presence of old beech leaves caused an increase in both values, wheres the other litter types effected a decrease in nitrogen net mineralization. Fragmented ash litter was found to have the most distinct effect on N
L
values (-69%) and nitrogen net mineralization (-74%). 相似文献
5.
The effect of pretransplant inoculation with VA mycorrhizal fungi on the subsequent growth of leeks in the field 总被引:1,自引:1,他引:0
Summary Leek plants were preinoculated with a mixed inoculum ofGlomus caledonium, Glomus fasciculatum andGlomus sp., and transplanted to Dazomet disinfected and untreated field plots of moderate P deficiency. Successive harvests were
made until 99 days after transplanting. Preinoculated leeks attained marketable weights 25 days earlier than uninoculated
leeks from untreated soil and their final dry matter yields were 5.7 and 1.5 times as high as those of uninoculated leeks
from disinfected and untreated soil, respectively. Phosphorus concentration in preinoculated leeks remained highest for at
least 22 and 75 days after transplanting in untreated and disinfected soil, respectively. Preinoculation had a similar, although
smaller, influence on Cu and Zn concentrations. Infection levels produced by introduced and indigenous VA endophytes in leeks
reached plateaus of 90% and 40%, respectively, 47 days after transplanting. It is concluded that VAM is essential to leeks
grown in moderately P deficient soils, and the potential for inoculating seedlings in commercial leek production is discussed. 相似文献
6.
Sukhada Mohandas 《Plant and Soil》1987,98(2):295-297
Summary The response of tomato (Lycopersicon esculentum Mill) to inoculation with the vasicular arbuscular mycorrhizal (VAM) fungusGlomus fasiculatum andAzotobacter vinelandii singly and in combination was tested in the field. It was found thatG. fasiculatum as well asA. vinelandii significantly increased leaf area, shoot dry weight, nitrogen content phosphorus content and yield in respect to uninoculated
control. While, VAM fungal treatment alone could bring about substantial increase in growth, nitrogen content, phosphorus
content and yield, its combination withA. vinelandii produced additional effects on leaf area, shoot dry weight, phosphorus content and yield.
Contribution No. 304/83 of Indian Institute of Horticultural Research, Bangalore-89. 相似文献
7.
Heimo Haikala 《Journal of neurochemistry》1987,49(4):1033-1041
Abstract: A novel type of rotating disc electrode and a flow cell with laminar flow pattern were developed and applied to the electrochemical detection of dopamine, 3,4-dihy-droxyphenylacetic acid, homovanillic acid, 3-methoxytyra-mine (3-MT), noradrenaline, 3-methoxy-4-hydroxyphenyl-ethyleneglycol (MOPEG), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid after HPLC of these compounds. The active surface of the rotating disc working electrode was made from solid paraffin (40%; wt/wt) and graphite powder (60%; wt/wt). The sensitivity of the detector was proportional to the square root of the angular velocity and was practically independent of the flow rate of the mobile phase. The surface of the working electrode was very large (radius = 12 mm), and so the percentage of oxidation was 24–67%; (flow rate = 1.0 ml/min), depending on the compound. Electrical noise between 20 and 40 pA and background current of 20–60 nA were observed. In practice, the sensitivity for the detection of the compounds examined here was 8–16 nA/ng, and so a detection limit of 5 pg/injection could be achieved, when the detector was combined with reversed-phase HPLC. Supernatants obtained from the extracts of the tissue samples (nine brain parts of rat brain were studied) were purified by using Sephadex G-10 gel chromatography. Before this procedure, the proteins of the tissue extracts were precipitated by 0.2 M HC1O4, and the excess of HC1O4 was precipitated by KOH/HCOOH buffer. Simultaneously, the pH of the extracts was set to 2.4 by the above buffer. Adjustment of the pH was necessary so that elution of 5-HT from the Sephadex G-10 columns in the same fraction with 3-MT was avoided. If these compounds were in the same solution, their peaks would overlap on HPLC. MOPEG sulfate was purified by diethylaminoethyl-Sephadex A-25 (anion exchange resin) from the first fraction collected from the Sephadex G-10 columns. The contents of the compounds under investigation in nine brain parts agreed with those found by other investigators. 相似文献
8.
Motohiko Takemura Hiroyuki Fukui Yohichi Yamamoto Naohiro Hosomi Hiroshi Wada 《Journal of neurochemistry》1987,48(4):1185-1190
The nitrendipine receptor associated with the voltage-dependent calcium channel in rat brain was solubilized by detergent extraction and sonication. The detergent solution used for extraction consisted of 10 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), 0.25% (wt/vol) polyoxyethylene 20 cetyl ether (Brij 58), and 0.025% (wt/vol) polyoxyethylene 17 cetyl stearyl ether (Lubrol WX) in the presence of 30% (wt/vol) glycerol as a stabilizer. The molecular weight of the receptor was estimated to be 1,800K by Sephacryl S-500 gel filtration and 800K by sucrose density gradient sedimentation. The equilibrium dissociation constant of [3H]nitrendipine to the solubilized receptors was 5.6 nM, which is approximately 10 times that of the membrane-bound receptor. The binding of nitrendipine to the receptor was inhibited noncompetitively by the structurally unrelated calcium channel inhibitors verapamil and prenylamine; their concentrations for 50% inhibition were both 1.0 X 10(-7) M, and they caused maximal inhibitions of 70 and 100%, respectively. 相似文献
9.
Levels and Distribution of the Calcium-Modulated Proteins S100 and Calmodulin in Rat C6 Glioma Cells 总被引:5,自引:3,他引:2
To understand better the mechanisms involved in the transduction of a calcium signal into an intracellular response via multiple calcium-modulated proteins, we have examined the calcium-modulated proteins, S100 and calmodulin, and their intracellular targets in rat C6 glioma cells. Subconfluent, confluent, and postconfluent C6 cells contain predominantly, if not exclusively, the S100 beta polypeptide. The level of S100 beta in C6 cells increases approximately 20-fold from subconfluency to postconfluency whereas the level of calmodulin increases only about two-fold. The subcellular distribution of S100 beta and calmodulin in mitotic cells is similar. However, the subcellular distribution of these proteins in interphase cells is different and appears to change with cell density. Gel overlay analysis demonstrated that the S100- and calmodulin-binding protein profiles are significantly different and that some of the binding proteins appear to change in intensity with cell density. These data demonstrate that S100 beta is the predominant S100 polypeptide in C6 cells and suggest that changes in S100 beta and S100 beta-binding proteins may be involved in regulating S100-mediated intracellular processes in C6 cells. Our studies also suggest that the levels of S100 and calmodulin may be differentially regulated in C6 cells. 相似文献
10.
虫生真菌双生座壳孢的研究 总被引:1,自引:0,他引:1
双生座壳孢(Aschersonia duplex Berk.)系日本卷毛蚧的病原真菌。此菌在人工培养基和自然基物上生长好,并能产生大量分生孢子。对 C 源中的蔗糖、葡萄糖、甘露醇、山梨糖;N 源中的蛋白胨,天冬素、L-酪氨酸、KNO_3利用好。生长适温22—26℃。病原流行季节相对湿度在80%以上。田间接种,对寄主致病性强,有应用潜力。 相似文献