全文获取类型
收费全文 | 3659篇 |
免费 | 291篇 |
国内免费 | 324篇 |
出版年
2023年 | 56篇 |
2022年 | 50篇 |
2021年 | 99篇 |
2020年 | 102篇 |
2019年 | 134篇 |
2018年 | 112篇 |
2017年 | 108篇 |
2016年 | 130篇 |
2015年 | 110篇 |
2014年 | 219篇 |
2013年 | 255篇 |
2012年 | 186篇 |
2011年 | 176篇 |
2010年 | 150篇 |
2009年 | 178篇 |
2008年 | 198篇 |
2007年 | 187篇 |
2006年 | 158篇 |
2005年 | 165篇 |
2004年 | 129篇 |
2003年 | 122篇 |
2002年 | 103篇 |
2001年 | 95篇 |
2000年 | 73篇 |
1999年 | 81篇 |
1998年 | 72篇 |
1997年 | 63篇 |
1996年 | 51篇 |
1995年 | 63篇 |
1994年 | 48篇 |
1993年 | 46篇 |
1992年 | 43篇 |
1991年 | 36篇 |
1990年 | 31篇 |
1989年 | 33篇 |
1988年 | 44篇 |
1987年 | 39篇 |
1986年 | 30篇 |
1985年 | 41篇 |
1984年 | 41篇 |
1983年 | 30篇 |
1982年 | 25篇 |
1981年 | 34篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 20篇 |
1977年 | 11篇 |
1975年 | 9篇 |
1974年 | 10篇 |
1973年 | 9篇 |
排序方式: 共有4274条查询结果,搜索用时 31 毫秒
71.
Douglas P. Swain 《Evolution; international journal of organic evolution》1992,46(4):998-1013
Variation in the number of vertebrae is widespread in fishes, and is partly genetic in origin. The adaptive significance of this variation was tested by exposing larvae of the threespine stickleback (Gasterosteus aculeatus) to predation by sunfish (Lepomis gibbosus). Two vertebral characters were considered: the total number (VN) and the ratio of abdominal to caudal vertebrae (VR). Predation was selective for both characters, but selection was more directly related to VR than to VN. The direction of selection depended on larval length: as length increased, optimal VR decreased. Total selection for VR was a combination of direct selection and an indirect effect of selection acting on a correlated trait, the ratio of precaudal to caudal length. Direct and indirect selection were in opposing directions at a given larval length. Variation in vertebral number may be maintained in populations partly because the strength of selection is reduced by opposing directions between direct and indirect selection, and between total selection at different larval lengths. 相似文献
72.
The application of molecular markers to the study and conservation of fish populations, with special reference to Salmo 总被引:4,自引:0,他引:4
A. Ferguson J. B. Taggart † P. A. Prodöhl ‡ O. McMeel C. Thompson § C. Stone P. McGinnity R. A. Hynes 《Journal of fish biology》1995,47(SA):103-126
The main molecular techniques which can be used to generate genetic markers, and the applications of these markers to studies of fish populations are outlined. Published and ongoing studies, in the authors' laboratories, on brown trout and Atlantic salmon are used to compare the resolution and applicability of allozyme, mitochondrial DNA and minisatellite (variable number of tandem repeats) markers for studies on population structuring, genetic variation within populations, and the impact of the accidental and deliberate introduction of non-native salmonids on the genetic make-up of natural populations. 相似文献
73.
Martin Burd 《Evolution; international journal of organic evolution》1995,49(1):100-109
The modular morphology of plants has important consequences for reproductive strategies. Ovules are packaged in discrete structures (flowers) that usually vary stochastically in pollen capture and ovule fertilization, because of the vagaries of pollen transfer by external agents. Different ovule packaging schemes may use limited reproductive resources more or less effectively, so that some number of ovules per flower may be optimal, given the prevailing probabilities of ovule fertilization. I derive a phenotypic model for ovule number per flower that maximizes the expected total ovule fertilizations on a plant when pollination and fertilization vary randomly among individual flowers. This model predicts that, except for small or inexpensive flowers, ovules should be “oversupplied” relative to the mean receipt of pollen tubes, so that pollen limitation of seed set should be common. Published data are congruent with this prediction. Additional hypotheses on the relation of ovule packaging to floral cost, plant size, and variance in pollen receipt are suggested by the model, but few data exist to evaluate these hypotheses. 相似文献
74.
宋光江 李桂信 李启清 孙安堂 宋兴军 杜克明 刘京升SONG Guang-Jiang LI Gui-Xin LI Qi-Qing SUN An-Tang SONG Xing-Jun DU Ke-Ming LIU Jiang-Sheng 《遗传》1995,17(4):1-3
CT所见肝脏肿大占据左右上腹,纠正了既往把肝左叶当做脾大的结论;B超发现前所没有报道的二尖瓣赘生物将给患者终生致残;标准品DS行双向电泳,首次发现其分离为DS1与GS2两个斑点;杂合子、患者尿GAG均出现DS1斑点,而正常人则不出现。实验显示,DS1的出现在杂合子检出和患者确诊上有重要意义。 相似文献
75.
Urs M. Weber 《Trees - Structure and Function》1997,11(5):277-290
Outbreaks of the larch budmoth (LBM) (Zeiraphera diniana) recur cyclically approximately every 7 to 10 years in subalpine larch-cembran pine and montane to subalpine larch-Norway
spruce forests of the relatively dry valleys of the European Alps. By dendroecologically analyzing increment cores from 570
host (European larch –Larix decidua) and non-host trees (cembran pine –Pinus cembra, Norway spruce –Picea abies) through the use of skeleton plots, at least 57 (59) outbreaks could be reconstructed in the optimum Upper Engadine Valley
(suboptimum Goms Valley), Switzerland, during the time period 1503 (1472) to 1990. The average interval between initial years
of successive outbreaks was 8.58 (8.95) years, SD 1.66 (2.13) years. Over the centuries spatial shifts of LBM activity between
the two study areas occurred, probably due to climatic changes. Clear, site-specific differences in LBM attack could only
be found in the suboptimum area where high-lying (>1800 m) and/or south-facing stands were infested most. LBM-afflicted trees
proved to be unsuitable for climate reconstructions because the impact of the persistently recurring outbreaks on tree growth
is dominant. In order to provide sufficient information for a detailed ecological interpretation of the course of an outbreak,
latewood widths and/or densities have to be analyzed in addition to the ring-widths.
Received: 11 February 1995 / Accepted: 19 July 1996 相似文献
76.
Christian Biémont Cristina Vieira Christine Hoogland Géraldine Cizeron Catherine Lœvenbruck Claude Arnault Jean-Pierre Carante 《Genetica》1997,100(1-3):161-166
To investigate the main forces controlling the containment of transposable elements (TE) in natural populations, we analyzed
the copia, mdg1, and 412 elements in various populations of Drosophila melanogaster and D. simulans. A lower proportion of
insertion sites on the X chromosome in comparison with the autosomes suggests that selection against the detrimental effects
of TE insertions is the major force containing TE copies in populations of Drosophila. This selection effect hypothesis is
strengthened by the absence of the negative correlation between recombination rate and TE copy number along the chromosomes,
which was expected under the alternative ectopic exchange model (selection against the deleterious rearrangements promoted
by recombination between TE insertions). A cline in 412 copy number in relation to latitude was observed among the natural
populations of D. simulans, with very high numbers existing in some local populations (around 60 copies in a sample from Canberra,
Australia). An apparent absence of selection effects in this Canberra sample and a value of transposition rate equal to 1–2
× 10-3 whatever the population and its copy number agree with the idea of recent but temporarily drastic TE movements in local populations.
The high values of transposition rate in D. simulans clearly disfavor the hypothesis that the low amount of transposable elements
in this species could result from a low transposition rate.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
77.
78.
蓟属两种植物的染色体研究 总被引:2,自引:0,他引:2
本文对蓟属(Cirsium)的两个形态相似的近缘种大刺儿菜和小刺儿菜进行了染色体研究,其中后者为首次报道。观察结果表明:两个种的染色体数目均为2n=2x=34:它们的核型是:大刺儿菜.2n=2x=34=20m+12sm+2st:小刺儿菜.2n=2x=34=22m+10km(2SAT)+2st。通过核型比较,认为它们是两个独立的种.而且后者比前者进化。 相似文献
79.
80.
The chick oviduct in tissue culture. I. Initial characterization of growing primary oviduct tissue cultures 总被引:1,自引:0,他引:1
A simple three-enzyme treatment of collagenase, dispase and hyaluronidase on finely minced chick oviduct yields clumps of 50-150 cells. These cells attach to collagen-treated dishes and survive in culture for at least 2 weeks without subculturing. Oviduct cell cultures can also be induced to grow. Estradiol or epidermal growth factor (EGF) induce a 40% increase in cells in 4 days when cultures are grown in serum levels that do not support growth. Serum from estrogen-stimulated chicks promotes rapid cellular proliferation (doubling times of 1-2 days). Sera from estrogen withdrawn chicks, laying hen or horse do not support as rapid proliferation. The oviduct growth-promoting factors in serum from estrogen-stimulated chicks are not steroids or fibroblast growth factors (FGF). Removal of steroids from these sera by charcoal treatment or delipidization does not decrease the rate of growth. The addition of 1-100 nM estradiol does not increase a serum's ability to promote growth. Purified FGF or platelet-derived growth factor (PDGF) do not induce oviduct proliferation. These results were reproduced in oviduct cell cultures started from estrogen-stimulated and withdrawn chicks as well as laying hens. Thus the factors in serum from estrogen-stimulated chicks that promote rapid oviduct growth are induced by estrogen treatments in vivo, but do not seem to be only steroids. 相似文献