全文获取类型
收费全文 | 57940篇 |
免费 | 4284篇 |
国内免费 | 6296篇 |
出版年
2023年 | 981篇 |
2022年 | 1283篇 |
2021年 | 1491篇 |
2020年 | 1556篇 |
2019年 | 2262篇 |
2018年 | 2066篇 |
2017年 | 1739篇 |
2016年 | 1586篇 |
2015年 | 1634篇 |
2014年 | 2902篇 |
2013年 | 3683篇 |
2012年 | 2281篇 |
2011年 | 2761篇 |
2010年 | 2111篇 |
2009年 | 2686篇 |
2008年 | 2828篇 |
2007年 | 3000篇 |
2006年 | 2749篇 |
2005年 | 2258篇 |
2004年 | 1939篇 |
2003年 | 1918篇 |
2002年 | 1676篇 |
2001年 | 1462篇 |
2000年 | 1241篇 |
1999年 | 1076篇 |
1998年 | 1044篇 |
1997年 | 944篇 |
1996年 | 916篇 |
1995年 | 898篇 |
1994年 | 876篇 |
1993年 | 765篇 |
1992年 | 749篇 |
1991年 | 748篇 |
1990年 | 558篇 |
1989年 | 557篇 |
1988年 | 505篇 |
1987年 | 503篇 |
1986年 | 451篇 |
1985年 | 727篇 |
1984年 | 972篇 |
1983年 | 784篇 |
1982年 | 839篇 |
1981年 | 664篇 |
1980年 | 669篇 |
1979年 | 600篇 |
1978年 | 482篇 |
1977年 | 439篇 |
1976年 | 393篇 |
1974年 | 272篇 |
1973年 | 272篇 |
排序方式: 共有10000条查询结果,搜索用时 22 毫秒
201.
H. Van Dijk 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(5):724-736
Summary A method has been developed which enables the estimation of the plant gene flow parameters p (pollen dispersal), s (seed dispersal) and t (outcrossing rate) from a selection-free continuously structured population in equilibrium. The method uses Wright's F-coefficients and introduces a new F-function which describes the genetic similarity as a function of the spatial distance. The method has been elaborated for wind pollinated plant species but can be modified for insect pollination and for animal species. In practice allozymes will provide for the necessary neutral genetic variation. The more loci used and the more intermediate the gene frequencies, the more reliable the results. For the estimation of p and t together (when the outcrossing rate is not known) at least two chromosomally unlinked loci are required. The method for estimating s depends on whether the plant species is annual or perennial. The mechanism of selfing has been analysed by the explanation of the value of t by three components: population density (d), pollen flow (p) and relative fertilization potential of own pollen (Z). The concepts of neighbourhood size and isolation by distance, developed by Wright, who used a single gene flow parameter , have been extended to the situation which is realistic for seed plants, using all three parameters p, s and t. When p is large with respect to s, s largely determines the value of the neighbourhood size, whereas p is the most dominating factor in isolation by distance. The use of local effective population size and mean gene transport per generation instead of neighbourhood size and neighbourhood area, respectively, is proposed to avoid confusion. Computer simulations have been carried out to check the validity and the reliability of the method. Populations of 200 plants, using two or three loci with intermediate allele frequencies, gave good results in the calculation of p with known value of t and of s and Ne. With unknown t, especially with lower values of t, larger populations of at least 1,000 plants are necessary to obtain reasonably accurate results for p and mean gene transport per generation M.Grassland Species Research Group Publication No. 81 相似文献
202.
Inbreeding under a cyclical mating system 总被引:1,自引:0,他引:1
A. Farid M. Makarechian C. Strobeck 《TAG. Theoretical and applied genetics. Theoretische und angewandte Genetik》1987,73(4):506-515
Summary General recursion formulae for the coefficient of inbreeding under a cyclical mating system were derived in which one male and one female are selected from each of the n families per generation (population size N = 2 n). Each male is given the family number of his sire in each generation, while his mate comes from another family, varying systematically in different generations. Males of the r-th family in generations 1, 2, 3,..., t = n–1 within each cycle mate with females from families r+1, r+2, r+3,..., r+t to produce generations 2, 3, 4,..., t+1=1, respectively. The change in heterozygosity shows a cyclical pattern of rises and falls, repeating in cycles of n–1 generations. The rate of inbreeding oscillates between <-3% to >6% in different generations within each cycle, irrespective of the population size. The average rate of inbreeding per generation is approximately 1/[4 N-(Log2N+1)], which is the rate for the maximum avoidance of inbreeding. The average inbreeding effective population size is approximately 2 N–2. 相似文献
203.
The mean dimensions of thecis N-methyl peptide unit have been arrived at by analysing the crystal structure data on compounds containing such units. These
dimensions can be used as standard in conformational studies on cyclic peptides. While the bonds meeting at C are almost coplanar,
those meeting at N show a slight pyramidal disposition. A comparison of the dimensions of the normal and N-methylatedcis peptide units show that there are perceptible differences in the parameters connected with N. In addition, the flexibility
of thecis peptide unit has been analysed by studying the distribution of the parameters in different classes of compounds such as cyclic
di, tri and higher peptides. The salient features are: (i) The angle CαCN in cyclic dipeptide and the angle CδNCα in higher peptides tend to be lower, when the peptide unit is associated with a prolyl residue; (ii) in cyclic tripeptides
the internal anglesviz., CαCN and CNCα are significantly larger thereby increasing the intra-annular space; (iii) the bond Cα-C is distinctly shorter when it occurs in cyclic dipeptides. The results lead to the conclusion that thecis peptide unit takes up aneed-based flexibility in its dimension. 相似文献
204.
Salil K. Niyogi Thomas S. Soper Robert S. Foote Frank W. Larimer Richard J. Mural Sankar Mitra Eva H. Lee Richard Machanoff Fred C. Hartman 《Journal of biosciences》1987,11(1-4):203-214
Both Lys-166 and His-291 of ribulosebisphosphate carboxylase/oxygenase fromRhodospirillum rubrum have been implicated as the active-site residue that initiates catalysis. To decide between these two candidates, we resorted
to site-directed mutagenesis to replace Lys-166 and His-291 with several amino acids. All 7 of the position-166 mutants tested
are severely deficient in carboxylase activity, whereas the alanine and serine mutants at position 291 are ∼40% and ∼18% as
active as the native carboxylase, essentially ruling out His-291 in theRhodospirillum rubrum carboxylase (and by inference His-298 in the spinach enzyme) as a catalytically essential residue. The ability of some of
the mutant proteins to undergo carbamate formation or to bind either ribulosebisphosphate or a transition-state analogue remains
largely unimpaired. This implies that Lys-166 is not required for substrate binding; rather, the results corroborate the earlier
postulate that Lys-166 functions as an acid-base group in catalysis or in stabilizing a transition state in the reaction pathway. 相似文献
205.
重楼属两种植物种子及其附属结构的发育 总被引:1,自引:0,他引:1
重楼属两种植物(五指莲Paris axialis和滇重楼Paris polyphylla var.yunnanensis)种子发育的过程基本一致。双受精发生于授粉后10—15天。胚乳为沼生目型。种子发育延续的时间约为150—170天。胚胎发育终止于球形或稍有分化的阶段。种子具二层种皮。 二种重楼种子成熟时的外部形态显著不同。五指莲Paris axialis的种子呈浅棕黄色,长椭圆形,部分为绿白色海绵质假种皮所包裹。假种皮由珠柄发育而来,呈楔形。滇重楼Parispolyphylla var.yunnanensis的种子鲜红色,不规则圆形,外种皮肉质多浆。无假种皮。珠柄橙黄色,短而纤细。 相似文献
206.
Presence and development of populations of the introduced brown alga Sargassum muticum in the southwest Netherlands 总被引:23,自引:10,他引:13
Of the three Brachionus species used in aquaculture, Brachionus rubens, B. calycilorus and B. plicatilis, the latter is most widely used in raising marine fish and shrimp larvae due to its tolerance to the marine environment. In freshwater aquaculture the use of B. rubens and B. calycilorus is limited, probably because inert food products are readily available as feed for freshwater larvae.The rotifer Brachionus plicatilis is used in large numbers as the first food organism in intensive cultures of marine fish and shrimp larvae. An adequate supply of these rotifers relies on mass cultures. The reproductive rate of rotifers in these cultures depends on food quality and quantity, salinity, temperature and pH of the medium. Removal of waste products from culture tanks leads to higher and more efficient production of rotifers over extended periods of time. Rotifers have to be enriched with polyunsaturated fatty acids, which are essential for proper development and survival of marine fish and shrimp larvae.The future use of preserved rotifers and their resting eggs may help to overcome unforeseen failures of live cultures and may lead to more efficient use of these organisms in raising freshwater and marine fish and shrimp larvae. 相似文献
207.
Effect of rainstorms on heterotrophic bacterial activity in a hypertrophic African lake 总被引:1,自引:1,他引:0
Richard D. Robarts 《Hydrobiologia》1987,148(3):281-286
Hartbeespoort Dam is a hypertrophic man-made lake which is located in the Transvaal Province of South Africa. This region has recently experienced its most severe drought of the century. However, on three occasions in the summer rainy seasons of 1984 and 1985, major rainfalls (> 50 mm) occurred which caused large inflows to the lake. Inflowing river water entered as a density current causing marked silting of the water. Within the epilimnion (0–10 m) prior to these rainfalls there was usually no variation of bacterial numbers with depth, but heterotrophic bacterial activity (glucose uptake) decreased with depth concomitant with primary production. With the increased river inflow bacterial numbers did not increase but bacterial activity at the bottom of the epilimnion (10 m) increased to as high as 2.7 µg C l–1 h–1 in January 1985, reversing the depth profile of bacterial activity within the epilimnion. This resulted in decreased glucose concentrations (Kt + Sn) and turnover times. Heterotrophic activity per cell increased by between 2.5 and 5 times. These data demonstrate that storm events are important phenomena causing short-term changes in the metabolic activity of planktonic heterotrophic bacteria in lakes. 相似文献
208.
Summary Ca2+-activated K+ channels were studied in cultured medullary thick ascending limb (MTAL) cells using the patch-clamp technique in the inside-out configuration. The Ca2+ activation site was modified using N-bromoacetamide (NBA). 1mm NBA in the bath solution, at 2.5 m Ca2+ reduces the open probability,P
o
, of the channel to <0.01, without an effect on single-channel conductance. NBA-modified channels are still Ca2+-sensitive, requiring 25mm Ca2+ to raiseP
o
to 0.2. Both before and after NBA modification channel openings display at least two distributions, indicative of more than one open state. High Ca2+ (1mm) protects the channels from modification. Also presented is a second class of Ca2+-activated K+ channels which are normally present in MTAL cells which open infrequently at 10 m Ca2+ (P
o
=0.01) but have aP
o
of 0.08 at 1mm Ca2+. We can conclude (i) that NBA modifies the channel by shifting Ca2+-sensitivity to very high Ca2+, (ii) that NBA acts on a site involved in Ca2+ gating, and (iii) that a low affinity channel is present in the apical cell membrane with characteristics similar to those of normal channels modified with NBA. 相似文献
209.
Amplification of chirality based upon the association of nucleic acid strands of opposite handedness
C. A. A. van Boeckel G. M. Visser R. A. Hegstrom J. H. van Boom 《Journal of molecular evolution》1987,25(2):100-105
Summary It is proposed that nucleotide strands of opposite handedness may strongly associate and thereby provide the key step of a mechanism for the amplification of a small enantiomeric excess in an initially near-racemic mixture of poly- or oligonucleotides. This hypothesis, if confirmed by experimentation, may have important implications for the question of the origin of biomolecular chirality. The results of preliminary NMR experiments are given, which do show evidence of a strong association between pentanucleotide RNA strands whose monomers have opposite chirality. Simple kinetic equations are solved to demonstrate the conditions under which such association can produce amplification of chirality. 相似文献
210.
Arthur L. Weber 《Journal of molecular evolution》1987,25(1):7-11
Summary Reaction ofDl-serine and adenosine-5-phosphorimidazolide in the presence of adenosine-5-(O-methylphosphate) and imidazole resulted in the stereoselective synthesis of the aminoacyl nucleotide ester 2(3)-O-seryl-adenosine-5-(O-methylphosphate). The enantiomeric excess ofd-serine incorporated into 2(3)-O-seryl-adenosine-5-(O-methylphosphate) was about 9%. Adenylyl-(5N)-serine and an unknown product also incorporated an excess ofd-serine; however, serylserine showed an excess ofl-serine. The relationship of these results to the origin of the biological pairing ofl-amino acids and nucleotides containingd-ribose is discussed. 相似文献