全文获取类型
收费全文 | 7464篇 |
免费 | 548篇 |
国内免费 | 409篇 |
专业分类
8421篇 |
出版年
2023年 | 133篇 |
2022年 | 146篇 |
2021年 | 189篇 |
2020年 | 159篇 |
2019年 | 193篇 |
2018年 | 190篇 |
2017年 | 176篇 |
2016年 | 190篇 |
2015年 | 179篇 |
2014年 | 370篇 |
2013年 | 482篇 |
2012年 | 306篇 |
2011年 | 303篇 |
2010年 | 204篇 |
2009年 | 331篇 |
2008年 | 363篇 |
2007年 | 376篇 |
2006年 | 353篇 |
2005年 | 332篇 |
2004年 | 264篇 |
2003年 | 265篇 |
2002年 | 211篇 |
2001年 | 207篇 |
2000年 | 215篇 |
1999年 | 199篇 |
1998年 | 157篇 |
1997年 | 124篇 |
1996年 | 149篇 |
1995年 | 128篇 |
1994年 | 112篇 |
1993年 | 151篇 |
1992年 | 115篇 |
1991年 | 93篇 |
1990年 | 89篇 |
1989年 | 86篇 |
1988年 | 85篇 |
1987年 | 71篇 |
1986年 | 67篇 |
1985年 | 77篇 |
1984年 | 74篇 |
1983年 | 51篇 |
1982年 | 71篇 |
1981年 | 49篇 |
1980年 | 58篇 |
1979年 | 54篇 |
1978年 | 45篇 |
1977年 | 27篇 |
1976年 | 33篇 |
1975年 | 23篇 |
1973年 | 28篇 |
排序方式: 共有8421条查询结果,搜索用时 8 毫秒
51.
Summary A re-examination of earlier NPK fertilization experiments in Douglas fir stands on sandy soils shows the effects of high nitrogen
input by air pollution during the last 10–15 years on plant nutrition at these sites. In 1960, experimental plots showed a
positive growth reaction to nitrogen, phosphorus, and potassium fertilization. All suffered from severe phosphorus deficiency
in 1984, low phosphorus in the needles was invariably accompanied by a high nitrogen content, with all N/P ratios between
20 and 30. The same conclusion emerges from an independent investigation of nutrient status of a selection of Douglas fir
stands. Hence, if stand productivity and a balanced nutrient status of the trees is to be maintained, the increase in atmospheric
input of nitrogen calls for supplementary fertilization. Given the current N/P ratios in the needles, a positive growth response
to phosphorus fertilization is to be expected. 相似文献
52.
Summary Non-symbiotic heterotrophic N2 fixation in coniferous bark litter was investigated with the acetylene reduction assay under aerobic and anaerobic conditions. The litter studied was composed essentially of bark, of pH 5 and a C/N ratio of 101; the ratio of available C to available N, which governs N2 fixation, was considerably higher. The rate of N2 fixation was estimated as 2.5–4.4 g N. g–1 dry wt. day–1. Nitrogenase activity was still evident after seven months of incubation under aerobic conditions. The N2-ase activity was O2 dependent: under anaerobic conditions no N2-ase activity was found unless a fermentable C source was added. The importance of N2 fixation in N-poor litter for the maintenance of soil fertility is emphasized. 相似文献
53.
54.
55.
56.
Substrate and inhibitor specificity of anion exchangers on the brush border membrane of rabbit ileum
Roy G. Knickelbein Peter S. Aronson John W. Dobbins 《The Journal of membrane biology》1985,88(2):199-204
Summary In previous studies we have found that several anions can be transported by an exchange process in rabbit ileal brush border membranes. We demonstrated exchanges of Cl for OH or HCO3, SO4 for OH, oxalate for OH, and oxalate for Cl. The purpose of these studies was to determine the number of distinct carriers mediating these exchanges. We utilized substrate and inhibitor specificity studies to distinguish between different anion exchange transporters. We conclude that SO4OH and oxalate: OH exchange occur on the same carrier because: (i) pH-gradient stimulated transport of both14C-oxalate and35SO4 were equally sensitive tocis-inhibition by unlabeled SO4 or oxalate; and (ii) both were inhibited equally by K. We conclude that oxalate: OH and oxalate: Cl exchanges occur on different carriers because: (i) Cl or SO4 caused unequalcis-inhibition of these two exchanges; and (ii) as compared to oxalate: Cl exchange, oxalate: OH exchange was more sensitive to inhibition by probenecid and K and less sensitive to inhibition by bumetanide. Finally, we conclude that oxalate: Cl exchange and ClHCO3 exchange occur on different carriers because: (i) ClHCO3 exchange was almost completely insensitive tocis-inhibition by oxalate; and (ii) oxalate: Cl exchange was more sensitive to inhibition by DIDS and bumetanide than ClHCO3 exchange. Thus we have found that there are at least three separate anion exchangers on rabbit ileal brush border: (i) a ClHCO3 exchanger; (ii) a SO4OH exchanger, which also transports oxalate; and (iii) an oxalate: Cl exchanger. 相似文献
57.
Figs (Ficus spp.) and fig wasps (Chalcidoidea, Agaonidae): hypotheses for an ancient symbiosis 总被引:1,自引:0,他引:1
MARTYN G. MURRAY 《Biological journal of the Linnean Society. Linnean Society of London》1985,26(1):69-81
Figs and their pollinating fig wasps are dependent on one another for propagation of their own kinds. The wasps reproduce by ovipositing through the styles of female flowers within the closed fig receptacles (syconia). About half of the female flowers within the syconia of monoecious figs have styles which are longer than the ovipositors of the wasp, and they will therefore produce seeds rather than wasp larvae. Since a longer ovipositor would enable a wasp to reach more ovules and deposit more eggs, the question arises at to why longer ovipositors have not evolved.
In an attempt to answer this question, four seemingly plausible hypotheses are examined but each is shown to be problematical in some way. Consideration is then given to a fifth hypothesis which proposes that ovipositor length is constrained by abortion of syconia with relatively few seed embryos and many agaonid larvae. It is argued first that this pattern of abortion will be selected during a sustained period of heavy wasp infestation because seeds will then become scarce relative to pollen-carrying wasps. Increased expenditure by the fig on seed production would therefore be favoured by natural selection. A greater expenditure on seeds would occur if young syconia with exceptionally heavy wasp infestations were dropped and the saved nutrients invested in syconia of a subsequent crop containing average levels of wasp larvae and seeds. Provided that the energy and nutrient cost of dropping young syconia is small, the selective advantage to the wasp of longer ovipositors is eliminated in this way. A stable coexistence of figs and wasps is therefore possible. The paper concludes by discussing two general predictions of the abortion hypothesis, and how these may be tested. 相似文献
In an attempt to answer this question, four seemingly plausible hypotheses are examined but each is shown to be problematical in some way. Consideration is then given to a fifth hypothesis which proposes that ovipositor length is constrained by abortion of syconia with relatively few seed embryos and many agaonid larvae. It is argued first that this pattern of abortion will be selected during a sustained period of heavy wasp infestation because seeds will then become scarce relative to pollen-carrying wasps. Increased expenditure by the fig on seed production would therefore be favoured by natural selection. A greater expenditure on seeds would occur if young syconia with exceptionally heavy wasp infestations were dropped and the saved nutrients invested in syconia of a subsequent crop containing average levels of wasp larvae and seeds. Provided that the energy and nutrient cost of dropping young syconia is small, the selective advantage to the wasp of longer ovipositors is eliminated in this way. A stable coexistence of figs and wasps is therefore possible. The paper concludes by discussing two general predictions of the abortion hypothesis, and how these may be tested. 相似文献
58.
59.
D. Chourrout B. Chevassus F. Krieg A. Happe G. Burger P. Renard 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1986,72(2):193-206
Summary First generation tetraploids were produced by hydrostatic pressure treatment before the first cleavage and raised until the adult stage. Their survival and growth were severely depressed when compared to the diploid control: after two years, no ovulated females were found although males produced sperm at 1 and 2 years of age and were mated individually with diploid females. The progenies were consistently normal with high survival rates. They were found to be almost all triploids by karyology, which failed to detect a significant rate of aneuploidies. However, the fertilizing ability of tetraploid males was always low (0 to 97% of the control; average 40%). Several arguments presented here support the hypothesis that diploid spermatozoas, which are wider than haploid ones, would be frequently blocked during their penetration through the micropyle canal. Second generation tetraploids were produced after such matings by heat shocks, causing the retention of the second polar body. Their survival and growth were much more satisfactory than in the first generation, although still lower than in diploid and triploid controls issuing from diploid parents. Performances of second generation triploids were comparable to those of diploids, and slightly better than those of conventional triploids issuing from diploid parents. 94.5% of the second generation tetraploids were male. 相似文献
60.
Blood was obtained from three species of macaques for a study of platelets. A rapidly mobilizable platelet pool was demonstrated in rhesus macaques given intravenous epinephrine. The number of platelets/ml of blood increased about 30% 3 to 5 min after epinephrine was given. The size distributions of the platelets were similar before and after injection. Platelet aggregability was increased after injection. Basal platelet aggregabilities, as measured by platelet aggregate ratios, were similar in rhesus macaques. Celebes black macaques, and human subjects, but were significantly greater in cynomolgus macaques than in the other three species. 相似文献