全文获取类型
收费全文 | 3079篇 |
免费 | 50篇 |
国内免费 | 2篇 |
出版年
2015年 | 20篇 |
2014年 | 20篇 |
2013年 | 70篇 |
2012年 | 73篇 |
2011年 | 117篇 |
2010年 | 168篇 |
2009年 | 171篇 |
2008年 | 155篇 |
2007年 | 197篇 |
2006年 | 108篇 |
2005年 | 117篇 |
2004年 | 56篇 |
2003年 | 34篇 |
2002年 | 32篇 |
2001年 | 28篇 |
2000年 | 23篇 |
1999年 | 34篇 |
1998年 | 67篇 |
1997年 | 85篇 |
1996年 | 70篇 |
1995年 | 61篇 |
1994年 | 58篇 |
1993年 | 63篇 |
1992年 | 57篇 |
1991年 | 64篇 |
1990年 | 64篇 |
1989年 | 66篇 |
1988年 | 57篇 |
1987年 | 65篇 |
1986年 | 60篇 |
1985年 | 54篇 |
1984年 | 41篇 |
1983年 | 41篇 |
1982年 | 25篇 |
1981年 | 41篇 |
1980年 | 43篇 |
1979年 | 51篇 |
1978年 | 37篇 |
1977年 | 25篇 |
1976年 | 23篇 |
1975年 | 34篇 |
1974年 | 35篇 |
1973年 | 41篇 |
1972年 | 63篇 |
1971年 | 40篇 |
1970年 | 15篇 |
1955年 | 14篇 |
1954年 | 15篇 |
1953年 | 15篇 |
1951年 | 17篇 |
排序方式: 共有3131条查询结果,搜索用时 15 毫秒
1.
Growth and Osmotic Adjustment of Cultured Suspension Cells from Alternanthera philoxeroides (Mart.) Griseb After an Abrupt Increase in Salinity 总被引:2,自引:0,他引:2
BALAGTAS-BUROW GLORIA E.; MORONEY JAMES V.; LONGSTRETH DAVID J. 《Journal of experimental botany》1993,44(3):673-679
We have developed a cell suspension culture from alligator weed(Alternanthera philoxeroides [Mart.] Griseb), a C3 member ofthe Amaranthaceae. Intact plants of alligator weed can growat 400 mol m3 NaCl. Growth of alligator weed suspensionswas compared to growth of tobacco (Nicotiana tabacum L. cv.Wisconsin 38) suspensions after subculture to 200 mol m3NaCl. Fresh weight and cell density of salt-treated alligatorweed suspensions more than doubled by 7 d after subculture,but the fresh weight of salt-treated tobacco suspensions didnot double during the 21 d experiment. Correspondingly, cellviability dropped from about 90% to 77% in alligator weed andto 41% in tobacco, at 1 d after subculture to 200 mol m3NaCl. The symplastic volume of alligator weed cells declined36% by 2 h after subculture to 200 mol m3 NaCl, but cellcontents became iso-osmotic with the media at this point. Between2 h and 6 h there was a further decrease in osmotic potential,an increase in turgor potential and a partial recovery of symplasticvolume. Turgor potential was similar to that in control cellsby 24 h, indicating significant osmotic adjustment. Turgor potentialsremained similar in both treatments from 24 h through 21 d butthe average symplastic volume of salt-treated cells was 11 %less than in control cells. Therefore, alligator weed suspensioncells exhibit a rapid recovery of water balance and cell growthafter an abrupt and substantial increase in salinity. Key words: Cell culture, growth, osmotic adjustment, salinity, turgor potential 相似文献
2.
Potential Increase in Photosynthetic Efficiency from the Redistribution of Solar Radiation in a Crop 总被引:2,自引:0,他引:2
The efficiency of the conversion of photosynthetically activeradiation by C3 plants falls off with increasing intensity.Hypothesis: an increase in the productivity of direct solarenergy will be achieved if, by redistribution, it is interceptedat a more uniform and lower intensity by a greater proportionof the leaf area of a crop. A model is developed which usesestimates of the proportions of clear and overcast conditionsfrom site records of solar radiation to calculate the resultantphotosynthetic productivity. The amounts of diffuse light anddirect light are estimated for clear conditions. The model predictsthat redistributing direct solar radiation over twice the leafarea at half the intensity would give an increase of 22% inannual productivity. The model gives reasonable values for theproductivity reductions reported for two shading regimes. Tomato plants were grown for 21 d in three cabinets under regimesthat differed from each other only in the distribution of PARenergy over the daily photoperiod: (a) 103 W m2 for halfthe photoperiod followed by 13 W m2, (b) 13 W m2followed by 103 W m2 and (c) 58 W m2 for the wholephotoperiod. The dry matter increase of plants under the uniformregime was 33% greater than the average of those in the twoasymmetric regimes. It is suggested that, in protected cultivation, screens of partiallyreflective material could be used to redistribute solar radiationfrom leaves exposed to high intensities on to shaded leavesand so raise the photosynthetic efficiency. Assuming an absorptionof direct light by the screens of 0.10, the increase in productivityis estimated to be 17%. Key words: Photosynthesis, C3, canopy, solar energy, solar redistribution, light interception, partial reflection, growth cabinet, glasshouse, tomato 相似文献
3.
The effects on auxin-stimulated growth and cell wall formationof 8-(N, N-diethylamino)-octyl-3, 4, 5-trimethoxybenzoate.HCI(TMB-8), an intracellular Ca2+ antagonist, were investigatedin abraded stem segments from aetiolated seedlings of Pisumsativum L. cv. Alaska. Incubation of segments at pH 6.0 with200 mmol m3 TMB-8 resulted in a 50% inhibition of auxin-stimulatedgrowth. Added Ca2+ did not restore normal auxin-stimulated growth,presumably because of its well-known stiffening effect on thecell wall. In segments incubated at a pH (72) which preventedelongation, auxin promoted the incorporation of [3H]glucoseinto the cell wall relative to total uptake of label. TMB-8abolished about 60% of the total incorporation of label intocell walls in the presence of auxin, but was not effective inthe absence of auxin. Exogenous CaCl2 reversed the inhibitoryeffect of TMB-8 on relative cell wall incorporation in a parabolicmanner, with a 50% reversal at about 100 mmol m3 andcomplete reversal at 1.0 mol m3 Ca2+. Other ions tested(Mg2+, Mn2+, Cu2+, Zn2+) were without substantial effect atconcentrations of 0.5 mol m3. Both apparent uptake ofCa2+ and consequent reversal of TMB-8 inhibition of cell wallincorporation were blocked by the Ca2+ channel blockers verapamiland La3+. The data provide further evidence that auxin-stimulatedgrowth is dependent upon continued cell wall incorporation,and suggest that a Ca2+ messenger system may be involved inthe promotory actions of auxin on cell wall synthesis and long-termgrowth. Key words: Auxin, calcium, cell wall synthesis 相似文献
4.
5.
Infiltration of a facultative ant–plant mutualism by the introduced Argentine ant: effects on mutualist diversity and mutualism benefits 下载免费PDF全文
1. Ant–plant mutualisms have been the focus of considerable empirical research, but few studies have investigated how introduced ants affect these interactions. Using 2 years of survey data, this study examines how the introduced Argentine ant [Linepithema humile (Mayr)] differs from native ants with respect to its ability to protect the extrafloral nectary‐bearing coast barrel cactus (Ferocactus viridescens) in Southern California. 2. Eighteen native ant species visited cacti in uninvaded areas, but cacti in invaded areas were primarily visited by the Argentine ant. The main herbivore of the coast barrel cactus present at the study sites is a leaf‐footed bug (Narnia wilsoni). 3. Herbivore presence (the fraction of surveys in which leaf‐footed bugs were present on individual cacti) was negatively related to ant presence (the fraction of surveys in which ants were present on individual cacti). Compared with cacti in uninvaded areas, those in invaded areas were less likely to have herbivores and when they did had them less often. 4. Seed mass was negatively related to herbivore presence, and this relationship did not differ for cacti in invaded areas versus those in uninvaded areas. 5. Although the Argentine ant might provide superior protection from herbivores, invasion‐induced reductions in ant mutualist diversity could potentially compromise plant reproduction. The cumulative number of ant species on individual cacti over time was lower in invaded areas and was associated with a shortened seasonal duration of ant protection and reduced seed mass. These results support the hypothesis that multiple partners may enhance mutualism benefits. 相似文献
6.
7.
DAVID A. MORRISON GEOFFREY J. CARY STUART M. PENGELLY DAVID G. ROSS BRUCE J. MULLINS COLLETTE R. THOMAS TIMOTHY S. ANDERSON 《Austral ecology》1995,20(2):239-247
Abstract Fire frequency is the number of fires experienced by a particular community within a given time period. This concept can potentially be resolved into a number of interacting variables, including: time since the most recent fire, the length of the inter-fire intervals, and the variability of the length of the inter-fire intervals. We estimated the effects of these three variables on the floristic composition of 65 samples from dry sclerophyll vegetation with different fire histories in Brisbane Water, Ku-ring-gai Chase and Royal National Parks near Sydney. Our analyses suggest that fire frequency may account for about 60% of the floristic variation among our samples. They confirm the hypothesis that the recent (<30 years) fire frequency produces effects on floristic composition of fire-prone communities that can recognizably be attributed both to the time since the most recent fire and to the length of the intervals between fires. These effects are equal in magnitude but are different in the nature of the floristic variation they are associated with. Increasing time-since-fire is associated with a decline in the evenness of fire-tolerant species, indicating that fewer of these species come to dominate the community in the prolonged absence of fire. Herbs and small shrubs decrease in abundance, while larger shrubs increase in abundance. Inter-fire intervals of decreasing length are associated with a decrease in the evenness of the fire-sensitive species, particularly those large Proteaceae shrubs that often dominate the community biomass in dry sclerophyll shrublands of southeastern Australia. Furthermore, the variation associated with inter-fire intervals is not necessarily solely related to the shortest inter-fire interval, but is related to combinations of inter-fire intervals through time. Thus, increasing variability of the length of the inter-fire intervals is associated with an increase in the species richness of both fire-sensitive and fire-tolerant species, implying that it may be variation of the inter-fire intervals through time that is primarily responsible for maintaining the presence of a wide variety of plant species in a particular community. Our results also suggest that the floristic variation associated with different inter-fire intervals decreases with increasing time-since-fire. 相似文献
8.
9.
10.
DAVID L. BRAFF TIFFANY A. GREENWOOD NEAL R. SWERDLOW GREGORY A. LIGHT NICHOLAS J. SCHORK The Investigators of the Consortium on the Genetics of Schizophrenia 《World psychiatry》2008,7(1):11-18
The search for the genetic architecture of schizophrenia has employed multiple, often converging strategies. One such strategy entails the use of tracing the heritability and neurobiology of endophenotypes. Endophenotypes are quantifiable traits not visible to the eye, which are thought to reflect an intermediate place on the path from genes to disorder. Endophenotype abnormalities in domains such as neurophysiology or neurocognition occur in schizophrenia patients as well as their clinically “unaffected” relatives, and reflect polymorphisms in the DNA of schizophrenia spectrum subjects which create vulnerability to developing schizophrenia. By identifying the single nucleotide polymorphisms (SNPs) associated with endophenotypes in schizophrenia, psychiatric neuroscientists can select new strong inference based molecular targets for the treatment of schizophrenia. 相似文献