全文获取类型
收费全文 | 5540篇 |
免费 | 704篇 |
国内免费 | 2篇 |
出版年
2021年 | 72篇 |
2020年 | 44篇 |
2019年 | 49篇 |
2018年 | 64篇 |
2017年 | 70篇 |
2016年 | 115篇 |
2015年 | 176篇 |
2014年 | 213篇 |
2013年 | 262篇 |
2012年 | 329篇 |
2011年 | 331篇 |
2010年 | 216篇 |
2009年 | 163篇 |
2008年 | 249篇 |
2007年 | 270篇 |
2006年 | 247篇 |
2005年 | 247篇 |
2004年 | 201篇 |
2003年 | 240篇 |
2002年 | 192篇 |
2001年 | 194篇 |
2000年 | 185篇 |
1999年 | 156篇 |
1998年 | 97篇 |
1997年 | 70篇 |
1996年 | 72篇 |
1995年 | 71篇 |
1994年 | 61篇 |
1993年 | 67篇 |
1992年 | 115篇 |
1991年 | 104篇 |
1990年 | 91篇 |
1989年 | 80篇 |
1988年 | 81篇 |
1987年 | 58篇 |
1986年 | 84篇 |
1985年 | 76篇 |
1984年 | 53篇 |
1983年 | 59篇 |
1982年 | 51篇 |
1981年 | 54篇 |
1980年 | 57篇 |
1979年 | 48篇 |
1978年 | 45篇 |
1977年 | 35篇 |
1976年 | 45篇 |
1974年 | 32篇 |
1973年 | 41篇 |
1972年 | 32篇 |
1968年 | 35篇 |
排序方式: 共有6246条查询结果,搜索用时 15 毫秒
1.
2.
Present and future needs for algae and algal products 总被引:1,自引:0,他引:1
A review of the present needs, mainly for production of phycocolloids and food condiments, is given. Supply and demand vary from balanced, in some, to disproportionate in other fields. World-wide shortage of agarophytes contrasts with huge, unexploited beds of brown seaweeds.In future, partly conflicting trends will decide the needs for algae and algal products. Growth in the human population, pollution, overexploitation of land and lack of freshwater will encourage use of seaweeds. Modern biotechnology will favour this development, but will also be a serious threat to industrial exploitation of seaweeds. Future uses of marine algae will be decisively influenced by the effort put into and the results coming out of seaweed research. 相似文献
3.
E. S. Jensen 《Plant and Soil》1991,133(1):83-92
Simultaneous determination of 15N and total N using an automated nitrogen analyser interfaced to a continuous-flow isotope ratio mass spectrometer (ANA-MS method) was evaluated. The coefficient of variation (CV) of repeated analyses of homogeneous standards and samples at natural abundance was lower than 0.1%. The CV of repeated analyses of 15N-labelled plant material and soil samples varied between 0.3% and 1.1%. The reproductibility of repeated total N analyses using the automated method was comparable to results obtained with a semi-micro Kjeldahl procedure. However, the automated method gave results which were 3% to 5% higher than those obtained with the Kjeldahl procedure. Since only small samples can be analysed, careful sample homogenization and fine grinding are very important. Evaluation of a diffusion method for preparing nitrate and ammonium in solution for automated 15N analysis showed that the recovery of inorganic N in the NH3 trap was lower when the N was diffused from water than from 2 M KCl. The results also indicated that different proportions of the NO3
- and the NH4
+ in aqueous solution were recovered in the trap after combined diffusion. The method is most suited for diffusing either NO3
- or NH4
+ alone, but can be used for combined diffusion of the two ions. 相似文献
4.
5.
6.
7.
8.
9.
Robert A. Kennedy Judith L. Eastburn Kenneth G. Jensen 《American journal of botany》1980,67(8):1207-1217
The genus Mollugo has been reported to have species which are C4 plants and a C3-C4 intermediate. In the present paper, we report on the anatomy and photosynthetic physiology of three additional species in the Molluginaceae, M. pentaphylla, M. nudicaulis, and M. lotoides, all of which possess some anatomical and physiological features of both C3 and C4 plants. Most notable among the variable C3-C4 features are C4-like bundle sheath cells, along with a C3-like arrangement of palisade and spongy parenchyma. M. nudicaulis appeared to have lower photorespiration based on its CO2 compensation point and lower oxygen sensitivity of photosynthesis. The occurrence of polyploidy within this group of plants and its relationship to lower photorespiration or evolution of these species is discussed. 相似文献
10.
N2O as a substrate and as a competitive inhibitor of nitrogenase 总被引:3,自引:0,他引:3
We have investigated the inhibitory effect of N2O on NH3 formation by purified component proteins from Klebsiella pneumoniae and have confirmed that the inhibition is competitive with respect to N2 and that N2O is reduced to N2, which in turn is further reduced to NH3. In addition, we have shown that N2O is unable to support HD formation from D2 and H2O. N2-supported HD formation from D2 and H2O was found to be inhibited by N2O. In contrast to N2, N2O was found to suppress nitrogenase-mediated H2 evolution completely at infinitely high pN2O. H2 was found to inhibit N2O-supported NH3 production but not N2O-supported N2 production. The steady-state kinetics of N2O reduction showed a good fit to Michaelis-Menten kinetics with a Km for N2O of 5 mM at 30 degrees C, corresponding to 24 kPa of N2O. A model is proposed that fits the observed results. 相似文献