全文获取类型
收费全文 | 6687篇 |
免费 | 543篇 |
国内免费 | 520篇 |
出版年
2023年 | 90篇 |
2022年 | 89篇 |
2021年 | 130篇 |
2020年 | 163篇 |
2019年 | 233篇 |
2018年 | 234篇 |
2017年 | 195篇 |
2016年 | 182篇 |
2015年 | 236篇 |
2014年 | 321篇 |
2013年 | 481篇 |
2012年 | 231篇 |
2011年 | 309篇 |
2010年 | 247篇 |
2009年 | 288篇 |
2008年 | 281篇 |
2007年 | 277篇 |
2006年 | 265篇 |
2005年 | 258篇 |
2004年 | 219篇 |
2003年 | 194篇 |
2002年 | 190篇 |
2001年 | 169篇 |
2000年 | 181篇 |
1999年 | 156篇 |
1998年 | 162篇 |
1997年 | 133篇 |
1996年 | 123篇 |
1995年 | 133篇 |
1994年 | 117篇 |
1993年 | 119篇 |
1992年 | 110篇 |
1991年 | 101篇 |
1990年 | 107篇 |
1989年 | 78篇 |
1988年 | 97篇 |
1987年 | 82篇 |
1986年 | 64篇 |
1985年 | 98篇 |
1984年 | 103篇 |
1983年 | 74篇 |
1982年 | 73篇 |
1981年 | 67篇 |
1980年 | 48篇 |
1979年 | 35篇 |
1978年 | 35篇 |
1977年 | 38篇 |
1975年 | 27篇 |
1974年 | 27篇 |
1973年 | 30篇 |
排序方式: 共有7750条查询结果,搜索用时 31 毫秒
41.
C. Loret B. Chaufer B. Sebille
S. Candau
C. Strazielle
《International journal of biological macromolecules》1988,10(6):361-365Commercial samples of gelatin modified by succinylation and currently used as plasma substitutes and fractionated samples obtained by diafiltration have been studied by viscometry, light scattering and osmometry. Viscometric results show that the aqueous medium containing potassium phosphate (0.1
) and NaCl (0.12
) at pH 3.3 behaves nearly like a theta solvent (a=0.48) for these modified gelatins. The Stockmayer-Fixman diagram reveals a negative slope attributed to a swelling of the macromolecules which decreases as the molecular weight
w increases. The Stokes radius RH determined by quasielastic light scattering is independent of the pH of the medium in a range 7-3.3. The conformation of gelatins in solution has been characterized through the ratio
G·
H−1, the radius of gyration
G being determined by viscometry. This ratio decreases as the molecular weight increases. The low molecular weight fractions have a more compact structure than the Gaussian chains in theta conditions. For high molecuar weight fractions, the values of
G·
H−1 tend to those of an hard sphere. 相似文献
42.
The light-induced increase in chloroplast DNA was not inhibited by two inhibitors of protein synthesis on 70S polysomes, chloramphenicol and lincomycin, in greening pea leaves. The changes in chloroplast DNA were observed by fluorescence microscopy and measured by hybridization to specific cloned probes. The results suggest that the light-induced increase in chloroplast DNA proceeds without de novo protein synthesis in the chloroplast, in agreement with those with mutants and cultured leaf tissue. 相似文献
43.
Transfer efficiencies between phycobilisomes and photosystem II antenna chlorophylls were determined on membrane fragments isolated from low and high light adapted Anabaena cells. The observed increase in energy transfer in high light adapted cells is a consequence of shorter interchromophore distances and a decrease in the number of jumps of the exciting photons. Calculation of the rates of energy transfer and the coupling energies indicate that the weak interaction inferred for energy transfer between phycobilisome and photosystem II in low light adapted cells is replaced by an intermediate interaction in high light adapted cells.Abbreviations LLA
low light adapted
- HLA
high light adapted
- PBS
phycobilisome
- PS
photosystem 相似文献
44.
Summary Time-course experiments of phosphate uptake by size-fractionated phytoplankton were conducted in oligotrophic Kennedy and Sproat Lakes. The objective was to determine if large phytoplankton obtained more phosphate than smaller cells, when the nutrient was present at higher concentrations. Studies at Kennedy Lake revealed that uptake rates in the 0.2–3.0 m fraction were very sensitive to the time they were exposed to elevated concentrations; rates determined over the 60–120 min interval were less than 30% of those recorded over the 0–60 min interval. In contrast, there was little difference in uptake rates over these intervals for cells>3.0 m. At Sproat Lake phosphate incorporation into the two size fractions was followed after the aerial fertilization of the lake with inorganic nutrients. Following nutrient addition the proportion of phosphate entering the>3.0 m size fraction increased from ca. 35% to ca. 85%. Despite these observations, it is doubtful that larger cells are able to sequester enough phosphate from pulses to realize the same specific growth rates as their smaller counterparts. 相似文献
45.
Summary Plants of white clover Trifolium repens were grown under the canopies of three grass species, Lolium perenne, Agrostis tenuis and Holcus lanatus, and under simulated canopies of black polythene and controls were exposed to unfiltered natural radiation. The canopies were adjusted so that they transmitted equal intensities of Photosynthetically Active Radiation (P.A.R.). The ratio of red to far red radiation () was unchanged under the black polythene canopies but was reduced under canopies of Lolium and Agrostis and even more so under Holcus. The effect of canopy filtered radiation on the growth of clover was greatly to reduced internode length, mean number of nodes, the number of branched nodes and the number of rooted nodes and greatly to increase petiole length. The effect of canopies of Holcus was greater than that of the other grass species both in its effect on and on the responses of the clover plant to its shade. 相似文献
46.
47.
Relation of light and nitrogen source to growth, nitrate reductase and glutamine synthetase activity of jack pine seedlings 总被引:5,自引:0,他引:5
Two-month-old jack pine ( Pinus banksiana Lamb.) seedlings were placed in a greenhouse where both nitrogen source and light level were varied. After 4 months, whole seedling biomass, leaf biomass and relative growth rate were greatest in seedlings grown with NH+ 4 /NO/NO− 3 -N and full light (FL) and least in seedlings grown with NO − 3 -N and low light (LL). NO − 3 -seedlings grown under full light and NH+ 4 /NO− 3 -seedlings grown under low light were approximately equal. This indicates that the extra carbon costs of assimilating only NO− 3 -N were similar to the reduction of carbon fixation resulting from a 50% decrease in photon flux density. Percentage and total nitrogen content of needles were greater in seedlings grown under low light independent of nitrogen fertilization. Percentage and total nitrogen content of roots were higher under low light and lower when fertilized with NO− 3 .
Nitrate reductase (NR) activity was higher in roots than in needles, while glutamine synthetase (GS) activity was higher in needles than in roots. Low light resulted in decreased NR activity (mg N)−1 in needles, but not in roots. However, no nitrate was detected in the needles in any treatment. GS activity, on the other hand, was greater under low light in both needles and roots. GS activity in needles is most likely involved with the reassimilation rather than the initial assimilation of ammonium. Some implications of these shifts in enzymatic activity for ecological phenomena in forests are discussed. 相似文献
Nitrate reductase (NR) activity was higher in roots than in needles, while glutamine synthetase (GS) activity was higher in needles than in roots. Low light resulted in decreased NR activity (mg N)
48.
Phytochrome is not involved in the red-light-enhancement of the stomatal blue-light-response in wheat seedlings 总被引:2,自引:0,他引:2
Per Erik Karlsson 《Physiologia plantarum》1988,74(3):544-548
The stomatal response to blue light (BL) in wheat seedlings ( Triticum aestivum L. cv. Starke II, Weibull) was enhanced by background red light (R). This enhancement was only slightly affected by the addition of background far-red light (FR). Under similar light treatments, the addition of FR induced a 43% transformation from the far-red-absorbing form towards the red-absorbing form of phytochrome from etiolated oat ( Avena sativa L. cv. Sol II), immobilized on phenyl-sepharose. Furthermore, the enhancement of the stomatal BL-response by 15 min R was not reversed by a subsequent irradiation with 5 min FR. It is concluded that the red-light-enhancement of the stomatal blue-light-response in wheat seedlings does not involve a change in the photostationary state of phytochrome. 相似文献
49.
Vasilij A. Novak Natalija G. Ivankina Elena A. Morokova Antonina I. Miclashevich 《Physiologia plantarum》1988,73(1):165-169
A dependence of the plasmalemma redox activity, determined by the reduction of external electron acceptors (ferricyanide, nitro-blue tetrazolium), on the energy state of the cell, which was modified by light conditions or introduction of glucose into the media, was shown on leaves of Elodea canadensis Rich. Glucose (10 m M ) and light (40 W m-2 ) caused hyperpolarization of the membrane potential and stimulated the redox activity of the plasmalemma. 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) completely inhibited the light activation of electrogenic and redox functions of the plasmalemma. The light saturation intensity for membrane potential and ferricyanide reductase activity was 10–30% of the light saturation of photosynthesis. Membrane potential, K+ transport and plasmalemma redox activity changed in parallel in response to light and darkness and when DCMU was added. Ferricyanide reductase activity is suggested to be a simple parameter for characterizing the energy state of the cell. The functional significance of the light-induced hyperpolarization of the membrane potential is discussed. 相似文献
50.
Thomas C. Vogelmann Alan K. Knapp Therese M. McClean William K. Smith 《Physiologia plantarum》1988,72(3):623-630
Vogelmann, T. C., Knapp, A. K., McClean, T. M. and Smith, W. K. 1988. Measurement of light within thin plant tissues with fiber optic microprobes. - Physiol. Plant. 72: 623–630.
The measurement of light with fiber optic microprobes has been extended to thin (200–300 μm) plant tissue samples. To test the method, light measurements were made in thin aqueous films and paradermal sections from 10-day-old etiolated Cucurbita pepo L. cv. Fordhook cotyledons. The measurements obtained were highly reproducible. Paradermal sections of spongy mesophyll that were irradiated with collimated light scattered light more effectively than the palisade layer of intact cotyledons. These results demonstrate that different plant tissues have different light scattering characteristics. The successful extension of the fiber optic microprobe technique to thin systems makes it possible to examine the optical properties of different cell layers within leaves and other plant organs. 相似文献
The measurement of light with fiber optic microprobes has been extended to thin (200–300 μm) plant tissue samples. To test the method, light measurements were made in thin aqueous films and paradermal sections from 10-day-old etiolated Cucurbita pepo L. cv. Fordhook cotyledons. The measurements obtained were highly reproducible. Paradermal sections of spongy mesophyll that were irradiated with collimated light scattered light more effectively than the palisade layer of intact cotyledons. These results demonstrate that different plant tissues have different light scattering characteristics. The successful extension of the fiber optic microprobe technique to thin systems makes it possible to examine the optical properties of different cell layers within leaves and other plant organs. 相似文献