全文获取类型
收费全文 | 32420篇 |
免费 | 2745篇 |
国内免费 | 130篇 |
出版年
2021年 | 464篇 |
2020年 | 321篇 |
2019年 | 449篇 |
2018年 | 556篇 |
2017年 | 457篇 |
2016年 | 694篇 |
2015年 | 1087篇 |
2014年 | 1183篇 |
2013年 | 1665篇 |
2012年 | 1848篇 |
2011年 | 1702篇 |
2010年 | 1240篇 |
2009年 | 975篇 |
2008年 | 1445篇 |
2007年 | 1371篇 |
2006年 | 1320篇 |
2005年 | 1174篇 |
2004年 | 1236篇 |
2003年 | 1115篇 |
2002年 | 1176篇 |
2001年 | 1059篇 |
2000年 | 970篇 |
1999年 | 862篇 |
1998年 | 397篇 |
1997年 | 402篇 |
1996年 | 315篇 |
1995年 | 320篇 |
1994年 | 254篇 |
1993年 | 295篇 |
1992年 | 571篇 |
1991年 | 549篇 |
1990年 | 511篇 |
1989年 | 470篇 |
1988年 | 385篇 |
1987年 | 372篇 |
1986年 | 367篇 |
1985年 | 403篇 |
1984年 | 383篇 |
1983年 | 333篇 |
1982年 | 228篇 |
1981年 | 241篇 |
1980年 | 220篇 |
1979年 | 294篇 |
1978年 | 256篇 |
1977年 | 283篇 |
1976年 | 268篇 |
1975年 | 255篇 |
1974年 | 251篇 |
1973年 | 244篇 |
1968年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Summary Pollen mother cells at early meiotic prophase fromFritillaria lanceolata, F. mutica, Tulbaghia violacea, the lily “Formobel”,Triticum aegilopoides, T. dicoccoides, T. aestivum and synaptic and asynaptic forms ofT. durum were studied in thin sections with the electron microscope (a) in relation to distribution of nuclear pores (b) in respect
of fine structure of the pore complex in those of the first four. The pores were distributed in random clusters during leptotene
to pachytene in all plants, except in the two forms ofT. durum where there were either no pores or so few that they were not detectable. Probably correlated with this, the two membranes
of the nuclear envelope were often widely separated and frequently sacculated. No pores were seen at leptotene in the part
of the envelope to which, in theFritillarias and lily, the nucleolus was adpressed at this time. Evidence supporting a recent model which proposes that annuli are composed
of three rings of eight granular subunits was obtained. These subunits as well as a dense central element, observed in most
pores, were composed of filaments about 3 nm in diameter and evidently protein in character. There was evidence of a continuity
between filaments in the central element and those in the rings of subunits which encircle the pore aperture at both the nuclear
and cytoplasmic sides of the pore. In profiles of pores knobbed filaments were sometimes seen extending laterally from the
pore wall into the perinuclear space at two sides. Questions concerning the role of the annulus are discussed.
The author wish to thank Mr. R. F. Scott for construction to the model. 相似文献
62.
63.
This research examines factors associated with the timing of first birth in Canada, focusing primarily on the role of women's educational attainment. Proportional hazards modelling techniques are applied to data from the 1984 Canadian Fertility Survey (CFS) in order to determine how educational attainment, estimated as close as possible to the date of first birth, influences the timing of first birth and whether the importance of this variable varies according to age cohorts. The results suggest that among a number of variables useful for distinguishing different levels of risk, educational attainment proves to be the most important predictor in the model. Education exerts a substantial positive influence on birth timing for women of all age groups. As expected, moreover, significant cohort differences are also evident, with the greatest to the smallest impact on the risk from the youngest to the oldest cohorts. These clear-cut cohort differences indicate a fundamental change in the effects of education over time, a trend most likely resulting from substantial changes in both the content and social significance of formal schooling during the past few decades. 相似文献
64.
65.
66.
67.
Alkalinity of the medium was shown to be the chief factor involved in the accumulation of oxalate by T. cinnabarina. Glutamate and aspartate are shown to lead to oxalate with this organism and with L. lepideus by dehydrogenation to α-ketoglutarate and oxaloacetate, respectively. Malate was also shown to be dehydrogenated. It is proposed that oxaloacetate may either undergo β-decarboxylation to yield CO2 and pyruvate, or splitting by coenzyme A to yield oxalate and acetylated coenzyme A. The reversal of this latter reaction is suggested as the explanation of the disappearance of oxalate from culture media. The reduction of resazurin by the dehydrogenase systems of the molds is inhibited by cyanide, indicating the participation of metal systems, such as the cytochromes. 相似文献
68.
J Daigneault G Aubin F Simard M De Braekeleer 《Human biology; an international record of research》1992,64(1):115-119
The incidence of cystic fibrosis (CF) in Saguenay-Lac-St.-Jean, a geographically isolated region of Quebec, was estimated to be 1 in 902 during the period 1975-1988. The carrier rate was calculated to be 1 in 15 inhabitants. The high incidence of CF in Saguenay-Lac-St.-Jean is probably the result of a founder effect and genetic drift for one or more mutations. Historical, demographic, and social factors also may have contributed to the high incidence. 相似文献
69.
70.
S D Kroll J Chen M De Vivo D J Carty A Buku R T Premont R Iyengar 《The Journal of biological chemistry》1992,267(32):23183-23188