全文获取类型
收费全文 | 6233篇 |
免费 | 677篇 |
国内免费 | 822篇 |
专业分类
7732篇 |
出版年
2024年 | 20篇 |
2023年 | 110篇 |
2022年 | 138篇 |
2021年 | 168篇 |
2020年 | 231篇 |
2019年 | 241篇 |
2018年 | 214篇 |
2017年 | 249篇 |
2016年 | 261篇 |
2015年 | 234篇 |
2014年 | 250篇 |
2013年 | 388篇 |
2012年 | 261篇 |
2011年 | 246篇 |
2010年 | 234篇 |
2009年 | 248篇 |
2008年 | 310篇 |
2007年 | 328篇 |
2006年 | 313篇 |
2005年 | 266篇 |
2004年 | 263篇 |
2003年 | 284篇 |
2002年 | 225篇 |
2001年 | 226篇 |
2000年 | 181篇 |
1999年 | 196篇 |
1998年 | 152篇 |
1997年 | 175篇 |
1996年 | 135篇 |
1995年 | 100篇 |
1994年 | 93篇 |
1993年 | 98篇 |
1992年 | 108篇 |
1991年 | 83篇 |
1990年 | 85篇 |
1989年 | 77篇 |
1988年 | 92篇 |
1987年 | 53篇 |
1986年 | 51篇 |
1985年 | 69篇 |
1984年 | 67篇 |
1983年 | 37篇 |
1982年 | 34篇 |
1981年 | 30篇 |
1980年 | 34篇 |
1979年 | 11篇 |
1978年 | 10篇 |
1977年 | 17篇 |
1975年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有7732条查询结果,搜索用时 15 毫秒
51.
Ginette Pauly Abdelhamid Yani Louis Piovetti Colette Bernard-Dagan 《Phytochemistry》1983,22(4):957-959
The essential oils of the leaves of Cupressus dupreziana and Cupressus sempervirens were compared. The composition of the hydrocarbon fraction showed a great similarity between the two species. 相似文献
52.
The spectral properties of the leaves of the herbaceous species Brassica oleracea L. var. botrytis L., Cerastum tomentosum L., Petunia hybrida Vilm., and Talinum paniculatum (Jacq.) Gaertn. were examined to see what effect the epidermis had on leaf absorptance, reflectance and transmittance. Removal of the epidermis from the side of the leaf surface being illuminated resulted in increases in leaf absorptance and transmittance, and a decrease in reflectance in the 400–800 nm waveband. Removal of the epidermis from the opposite side of an illuminated leaf (effect was similar in both abaxial and adaxial surfaces) resulted in small decreases in both absorptance and reflectance, and corresponding increases in transmittance. Removal of both the upper and lower epidermis resulted in a marked increase in transmittance, while both leaf reflectance and absorptance were decreased. The results suggest that the presence of the epidermis significantly increases leaf absorptance in the photosynthetic wavebands. 相似文献
53.
Specific photoperiodic stimulation of dry matter production in a high-latitude cultivar of Poa pratensis 总被引:2,自引:0,他引:2
Vegetative plants of Poa pratensis L. cv. Holt (origin 69°N) raised in short days gave large and significant increases in plant dry weight, plant height and leaf area upon exposure to continuous light, compared with 8-h short days, at essentially identical daily inputs of radiant energy (8-h summer daylight ± low intensity extension). For example, by the fourth harvest (after 26, 34 and 46 days at 21, 15 and 9°C, respectively), the dry weights of plants in long days were 81, 163 and 195% greater than those of the corresponding short-day controls at the respective temperatures. Plant leaf areas in long days were between two and four times as large as control values by the end of the experiment. This was mainly due to increased leaf length caused by long-day stimulation of cell extension and division. However, the photoperiod did not affect the partitioning of assimilates amongst leaves, culms and stolons. Most of these effects could also be brought about by exogenous gibberellin application to plants in short days. However, in contrast to the effect of long days, gibberellin treatment also induced stem internode elongation even in these vegetative plants. Examination by standard growth analysis procedures revealed that the observed increases in relative growth rate were due primarily to increased net assimilation rate followed, several days later, by increases in leaf area ratio when newly-emerged leaves began to constitute a significant proportion of the leaf area. It is concluded that these reactions are of great adaptive significance for growth at the marginal temperatures prevailing at high latitudes. 相似文献
54.
LYNN B. GRAVES RICHARD N. TRELEASE ALBIN GRILL WAYNE M. BECKER 《The Journal of eukaryotic microbiology》1972,19(3):527-532
SYNOPSIS. We demonstrated previously microbodies in Euglena gracilis grown in the dark on 2-carbon substrates. We have now established in Euglena the particulate nature of enzymes known in other organisms to be localized in microbodies (glyoxysomes and leaf peroxisomes). On a linear sucrose gradient the glyoxylate cycle enzymes band together at a nigner equilibrium density (1.20 g/cm3) than mitochondrial marker enzymes (1.17 g/cm3), establishing the existence in Euglena of glyoxysomes similar to those of higher plants. Glyoxylate (hydroxypyruvate) reductase and, under certain conditions, also glycolate dehydrogenase co-band with the glyoxylate cycle enzymes, suggesting that Euglena glyoxysomes, like those of higher plants, may contain peroxisomal-type enzymes. Catalase, an enzyme characteristic of microbodies from a variety of sources, was not detected in Euglena. 相似文献
55.
蚕豆叶片发育与衰老过程中超氧物歧化酶活性与丙二醛含量变化 总被引:2,自引:0,他引:2
蚕豆植株叶片随茎节自上而下表现出明显的发育与衰老顺序,可作为衰老特征的是叶绿素和蛋白质含量明显下降。蚕豆叶中SOD活性主要定位于12 000× g离心后所得的上清液和叶绿体组分。衰老叶片的SOD总活性和叶绿体组分的相对活性都有所下降,SOD同工酶谱也发生了改变。O_2~ 产生速率随叶龄增大而稍上升;而MDA含量在叶片外观表现枯黄衰老征兆前就急剧上升。可能因为衰老叶片过氧化氢酶活性大幅度下降与SOD之间的不平衡,致使O_2~ 代谢中间产物累积而引起膜的损伤. 相似文献
56.
Plant resistance, plant traits, and host plant choice of the leaf-folding sawfly on the arroyo willow 总被引:1,自引:0,他引:1
Abstract. 1. The hypotheses that genetic variation in host plant resistance of the arroyo willow affected leaf folder ( Phyllocolpa sp.) (Hymenoptera: Tenthredinidae) density and that genetic variation in shoot length and leaf length was correlated with resistance were tested.
2. Willows grown in pots and exposed to ovipojsition by the leaf folding sawfly in cages had significantly different densities among clones, indicating variation in resistance caused by genetic differences among conspecific host plants.
3. There was a general correspondence between leaf folder density on potted cuttings and on the plants in the field that were the sources of cuttings.
4. In behavioural choice experiments, susceptible clones (with highest leaf fold densities) had the highest oviposition activity of female leaf folders compared to clones that were resistant to the leaf folder.
5. Clones differed significantly in shoot length and leaf length among clones grown in pots, among clones in the field, and between shoots with galls and shoots without galls on clones in the field.
6. Leaf folder density was significantly positively correlated with mean shoot length on field clones in 1985 and 1986, but was not correlated with leaf length, although leaf length and shoot length were correlated.
7. Leaf length variation among willow clones accounted for a significant portion of the variation in resistance of potted willows, but shoot length was unimportant. 相似文献
2. Willows grown in pots and exposed to ovipojsition by the leaf folding sawfly in cages had significantly different densities among clones, indicating variation in resistance caused by genetic differences among conspecific host plants.
3. There was a general correspondence between leaf folder density on potted cuttings and on the plants in the field that were the sources of cuttings.
4. In behavioural choice experiments, susceptible clones (with highest leaf fold densities) had the highest oviposition activity of female leaf folders compared to clones that were resistant to the leaf folder.
5. Clones differed significantly in shoot length and leaf length among clones grown in pots, among clones in the field, and between shoots with galls and shoots without galls on clones in the field.
6. Leaf folder density was significantly positively correlated with mean shoot length on field clones in 1985 and 1986, but was not correlated with leaf length, although leaf length and shoot length were correlated.
7. Leaf length variation among willow clones accounted for a significant portion of the variation in resistance of potted willows, but shoot length was unimportant. 相似文献
57.
SYED Z. HUSAIN F.L.S. PETAR D. MARIN EDOMIL ILI M. QAISER BRANIMIR PETCOVIC 《Botanical journal of the Linnean Society. Linnean Society of London》1990,103(1):59-80
HUSAIN, S. Z., MARIN, P. D., ŠILIĆ, Č., QAISER, M. & PETCOVIĆ, B., 1990. A micromorphological study of some representative genera in the tribe Saturejeae (Lamiaceae). The Old World genera in the tribe Saturejeae are usually distributed either in Europe and North Africa or in the temperate parts of Asia. The centres of distribution of investigated genera are mainly in the Mediterranean region. In taxonomic revisions very little reference is made to micromorphological characters, in particular, to nutlets and leaf indumentum, in spite of the stability of these characters. Scanning electron microscopy of nutlet surface and patterns of leaf indumentum show a wide range of variation, not only among genera, but also at lower levels of classification. In view of this, nutlet surface and leaf indumentum structure, as seen with the SEM, of representative species of eight genera in the tribe Saturejeae provides useful additional character combinations in delimiting these closely related genera. This study also supports Boissier's delimitation of sections Micromeria and Pseudomelissa. 相似文献
58.
Growth of Pyrola rotundifolia ssp. maritima in relation to shade 总被引:2,自引:2,他引:0
59.
The kinetics of NADH oxidation by the outer membrane electron transport system of intact beetroot (Beta vulgaris L.) mitochondria were investigated. Very different values for Vmax and the Km for NADH were obtained when either antimycin A-insensitive NADH-cytochrome c activity (Vmax= 31 ± 2.5 nmol cytochrome c (mg protein)?1 min?1; Km= 3.1 ± 0.8 μM) or antimycin A-insensitive NADH-ferricyanide activity (Vmax= 1.7 ± 0.7 μmol ferricyanide (mg protein)?1 min?1; Km= 83 ± 20 μM) were measured. As ferricyanide is believed to accept electrons closer to the NADH binding site than cytochrome c, it was concluded that 83 ± 20 μM NADH represented a more accurate estimate of the binding affinity of the outer membrane dehydrogenase for NADH. The low Km determined with NADH-cytochrome c activity may be due to a limitation in electron flow through the components of the outer membrane electron transport chain. The Km for NADH of the externally-facing inner membrane NADH dehydrogenase of pea leaf (Pisum sativum L. cv. Massey Gem) mitochondria was 26.7 ± 4.3 μM when oxygen was the electron acceptor. At an NADH concentration at which the inner membrane dehydrogenase should predominate, the Ca2+ chelator, ethyleneglycol-(β-aminoethylether)-N,N,-tetraacetic acid (EGTA), inhibited the oxidation of NADH through to oxygen and to the ubiquinone-10 analogues, duroquinone and ubiquinone-1, but had no effect on the antimycin A-insensitive ferricyanide reduction. It is concluded that the site of action of Ca2+ involves the interaction of the enzyme with ubiquinone and not with NADH. 相似文献
60.
Buoyancy effect on forced convection in the leaf boundary layer 总被引:1,自引:0,他引:1
Abstract. Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences ( T L - T A ) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T L - T A . An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance ( G A ) for mixed convection, which became higher with higher T L - T A as compared with the conductance for pure forced convection without buoyancy. This increase in G A appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'. 相似文献