全文获取类型
收费全文 | 2926篇 |
免费 | 38篇 |
国内免费 | 1篇 |
出版年
2014年 | 17篇 |
2013年 | 63篇 |
2012年 | 52篇 |
2011年 | 72篇 |
2010年 | 117篇 |
2009年 | 105篇 |
2008年 | 108篇 |
2007年 | 145篇 |
2006年 | 81篇 |
2005年 | 90篇 |
2004年 | 53篇 |
2003年 | 29篇 |
2002年 | 19篇 |
2001年 | 28篇 |
2000年 | 25篇 |
1999年 | 40篇 |
1998年 | 60篇 |
1997年 | 88篇 |
1996年 | 75篇 |
1995年 | 75篇 |
1994年 | 51篇 |
1993年 | 63篇 |
1992年 | 49篇 |
1991年 | 65篇 |
1990年 | 58篇 |
1989年 | 59篇 |
1988年 | 54篇 |
1987年 | 66篇 |
1986年 | 48篇 |
1985年 | 55篇 |
1984年 | 51篇 |
1983年 | 59篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 46篇 |
1979年 | 54篇 |
1978年 | 43篇 |
1977年 | 39篇 |
1976年 | 35篇 |
1975年 | 48篇 |
1974年 | 39篇 |
1973年 | 60篇 |
1972年 | 66篇 |
1971年 | 66篇 |
1970年 | 27篇 |
1969年 | 31篇 |
1968年 | 29篇 |
1967年 | 20篇 |
1966年 | 19篇 |
1957年 | 19篇 |
排序方式: 共有2965条查询结果,搜索用时 15 毫秒
991.
The fatty acid profiles and contents of protozoa from the rumen fluid of cattle varied according to the type of diet consumed by their host. Changing from a high-quality hay diet to a low-quality hay diet (DA) decreased the proportions of saturated acids and increased the proportions of the unsaturated acids 18:1 cis-9, 18:2 and 18:3 in neutral lipids (NL) and phospholipids (PL). Adding sucrose, urea and sulphur (SUS) to DA increased the proportions of branched chain acids in PL while addition of safflower oil increased polyunsaturated acids in PL and 18:1 trans-11 in NL. Diet did not alter the PL fatty acid content of protozoa but oil supplement of DA resulted in a 10-fold increase in the content of free fatty acids. The defaunating effect of oil supplement was partly reversed by SUS suggesting that factors other than the fatty acid content of cells are important in determining the toxicity of oil to rumen protozoa. The results indicate that the amounts of individual long-chain fatty acids taken up by rumen ciliates are largely determined by their concentrations in rumen digesta. 相似文献
992.
HANSEN ALEXANDER P.; PEOPLES MARK B.; GRESSHOFF PETER M.; ATKINS CRAIG A.; PATE JOHN S.; CARROLL BERNARD J. 《Journal of experimental botany》1989,40(7):715-724
Growth and symbiotic performance of soybean (Glycine max (L.)Merrill) cv. Bragg and three of its induced nodulation mutants(nod49, non-nodulating; ntsl 116, intermediate supernodulator;nts1007, extreme supernodulator) were compared throughout developmentunder different nitrogen regimes (0, 2, 5 and 10 mol nitratem3). Nitrogen fixation was assessed using 15N-isotopedilution and xylem sap analysis for ureide content. Both techniquesconfirmed a complete lack of N2 fixation activity in nod49.Plant reliance on nitrogen fixation by the other genotypes wasdependent on the nitrate regime and the developmental stage.The ntsl007 and ntsl 116 mutants fixed more nitrogen than theparent cultivar in the presence of 10 mol m3 nitratein the nutrient solution, but higher input of symbioticallyderived nitrogen was still insufficient to offset the amountof nitrogen removed in the harvested seed. However, the mutantsutilized less nitrate for growth than Bragg. Comparison of estimatesof N2 fixation derived from the 15N-dilution technique withthose based on relative ureide content of xylem sap indicatedthat the latter offered a simple and reliable procedure forevaluating the symbiotic performance of supernodulating plants. Key words: 15N-isotope dilution, supernodulation, ureides 相似文献
993.
EIMEAR M. NIC LUGHADHA JOHN A. N. PARNELL F.L.S. 《Botanical journal of the Linnean Society. Linnean Society of London》1989,100(4):337-354
NIC LUGHADHA, E. M. & PARNELL, J. A. N., 1989. Heterostyly and gene-flow in Menyanthes trifoliata L. (Menyanthaceae). Menyanthes trifoliata is shown to be a self-incompatible heterostylous species with a suite of associated pollen and gynoecium dimorphisms. These features may promote legitimate pollination. Analysis of a natural population showed substantial pollen inviability, a preponderance of illegitimate pollen on stigmas and a possible asymmetry of pollen flow. Mean pollen travel was estimated as 2.6 m by counting legitimate grains on stigmas whose distance from the nearest source of legitimate pollen was known. Direct field and laboratory measurement indicated mean seed travel of <0.2m. The density of flowering spikes varied greatly within the studied colony of Menyanthes , with a mean of 0.93 m-2 . Using suitable corrections the genetically effective density was estimated as between 0.04 and 4.4 m-2 . Crossing experiments indicated that Menyanthes is fully outcrossed (r=1). The neighbourhood area was estimated as 10–15 m2 while neighbourhood size ranged from less than one to sixty-five. Some published neighbourhood estimates are revised with the results for Menyanthes being at the lower end of the range. If the genetically effective density were estimated more accurately, neighbourhood estimates would likely be lower still. It is concluded that what is generally referred to as a population of Menyanthes , may actually represent many evolutionary units. 相似文献
994.
ABSTRACT.
- 1 An account of the life-history with emphasis on phenology and number of instars is presented for Belgica antarctica Jacobs, the southernmost holometabolous insect.
- 2 Contrary to earlier reports, Belgica has four instars, in common with most other chironomids. Mean head capsule lengths varied between different populations but no overlap was found between discrete size classes of successive instars.
- 3 Belgica overwinters in all four instars.
- 4 Relative frequency of different instars from samples taken through the season indicates that a 2-year life-span is the commonest pattern.
- 5 Emergence of adults is relatively synchronous. Belgica exhibits protandry, which may be established at the time of pupation.
995.
ABSTRACT.
- 1 Tarsonemus mites phoretic on southern pine beetles were shown to be hypervectors of bluestain fungus, Ceratocystis minor (Hedgcock 1906).
- 2 Beetles with phoretic mites had significantly more C.minor than did mite-free beetles.
- 3 Over 90% of tarsonemid mites collected from beetle-infested inner bark and 59% of mites phoretic on emerging beetles carried C.minor spores.
- 4 This association is an interesting example of a three-way mutualism among insects, mites and fungi.
996.
TAYLOR JOHN S.; BLACKMAN SUSAN J.; YEUNG EDWAERD C. 《Journal of experimental botany》1982,33(3):495-505
Endogenous cytokinin and gibberellin-like activity were measuredby bioassay in developing fruit of the orchid Epidendrum ibaguense.Cytokinins decline during the first 30 d after pollination,then begin to accumulate, with very high levels (113µ g zeatin eq. g1 dry wt. ) in the mature fruitand seed. The major structural change in developing fruit duringthe first 30 d is the ongoing cell division in the fruit wall.By day 30 most ovules have been fertilized and embryo developmentbegins. The increase in cytokinin activity thus coincides withthe onset of embryo development. Gibberellin levels declinein the fruit throughout development, although high activity(0.9 µ g GA3 eq. g1 dry wt. ) is observed in themature seed. The mature embryo shows no obvious structural differentiationinto embryonic axis and cotyledon and no endosperm develops. 相似文献
997.
The Osmotic Pressure of Concentrated Solutions of Polyethylene Glycol 6000, and its Variation with Temperature 总被引:1,自引:0,他引:1
The vapour pressures of aqueous solutions of polyethylene glycol6000 have been measured (by equilibration with sucrose solutions)up to the saturation point at 25 °C (1.45 g g1 water).The reduced-osmotic-pressure (/c), when plotted versus concentration(c), rapidly and linearly increased up to a concentration ofabout 0.8 g g1 (crossing the similar plot for sucrose).Above this concentration, the reduced-osmotic-pressure rosemore slowly, but still more rapidly than sucrose. The maximumosmotic pressure achieved at saturation was nearly 18 MPa. Usingthe virial equation: /c= RT/M + RTA2c, the calculated secondvirial coefficient (A2) for the linear part is 4.5 x 103mol g1, a value slightly greater than most literaturevalues at 25 °C. Data are cited showing that A2 varies linearlyfrom 56 x 10x3 at 0 °C, to zero at 8090 °C 相似文献
998.
999.
Live trapping at 0.9 m of alate aphid vectors of barley yellow dwarf virus (BYDV) at Aberystwyth from 1970 to 1979 showed that ten species transmitted the virus to oat test plants. Conversion of percentage infective at 0.9 m to numbers infective based on continuous trapping at 1.2 m showed Rhopalosiphum padi and R. insertum to be the main vector species in most years, whilst Metopolophium dirhodum and Sitobion auenae were normally of minor importance. The data obtained suggest that epiphytotics of BYDV in autumn-sown cereals were caused by numerous infective vectors flying late in the year and transmitting severe strains of the virus. Evidence is presented that gynoparae and males of R. padi are involved in the autumn spread of BYDV and that three further aphid species, Anoecia corni, Metopolophium albidum and M. frisicum are BYDV vectors. The use of live and continuous trapping techniques in forecasting BYDV epiphytotics is discussed. 相似文献
1000.
JOHN J. WILLE JR. EARL WEIDNER W. L. STEFFENS 《The Journal of eukaryotic microbiology》1981,28(2):223-227
A trypanosomatid flagellate, Leptomonas sp., develops and multiplies in the macronucleus (only) of natural and laboratory-reared populations of the ciliate Euplotes. Up to 90% of the natural populations of Euplotes in our test pond had such nuclear infections. Laboratory infections were transmitted to this ciliate by feeding it liberated parasites. Paramecium resisted infections. All laboratory-induced infections were lethal to Euplotes, while control clones of the uninfected ciliates remained viable. This leptomonad, unlike Leptomonas karyophilus (found in Paramecium), shows no leishmanial forms in its several ciliate hosts and shows a varied pattern of locomotion. 相似文献