全文获取类型
收费全文 | 49817篇 |
免费 | 3437篇 |
国内免费 | 4938篇 |
专业分类
58192篇 |
出版年
2023年 | 819篇 |
2022年 | 1091篇 |
2021年 | 1257篇 |
2020年 | 1244篇 |
2019年 | 1933篇 |
2018年 | 1706篇 |
2017年 | 1381篇 |
2016年 | 1280篇 |
2015年 | 1263篇 |
2014年 | 2447篇 |
2013年 | 3091篇 |
2012年 | 1886篇 |
2011年 | 2268篇 |
2010年 | 1750篇 |
2009年 | 2257篇 |
2008年 | 2358篇 |
2007年 | 2574篇 |
2006年 | 2307篇 |
2005年 | 1901篇 |
2004年 | 1608篇 |
2003年 | 1600篇 |
2002年 | 1432篇 |
2001年 | 1221篇 |
2000年 | 1019篇 |
1999年 | 898篇 |
1998年 | 860篇 |
1997年 | 810篇 |
1996年 | 786篇 |
1995年 | 776篇 |
1994年 | 761篇 |
1993年 | 669篇 |
1992年 | 662篇 |
1991年 | 663篇 |
1990年 | 504篇 |
1989年 | 513篇 |
1988年 | 467篇 |
1987年 | 452篇 |
1986年 | 427篇 |
1985年 | 687篇 |
1984年 | 909篇 |
1983年 | 726篇 |
1982年 | 764篇 |
1981年 | 616篇 |
1980年 | 622篇 |
1979年 | 553篇 |
1978年 | 442篇 |
1977年 | 411篇 |
1976年 | 376篇 |
1974年 | 244篇 |
1973年 | 258篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
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
162.
雄性水狼蛛生殖球顶部的扫描电镜研究 总被引:1,自引:0,他引:1
本文对我国已知的十一种水狼蛛(蜘蛛目:狼蛛科)生殖球顶部用扫描电镜进行了研究。结果表明,水狼蛛属蜘蛛插入器细长,其后端位于顶突上一突起的凹槽内,顶突作为功能引导器保护着插入器,而无其他狼蛛属具有的次生真正引导器;顶突的形状和其上的突起、顶突与插入器的相互关系对于种的分类与鉴定,特别是对于近似种的区分有较大的参考价值,但仅根据插入器从顶部升起的位置来作为分新属的主要标准,笔者认为还值得商榷和进一步探讨。 相似文献
163.
10 new Turkish taxa are described:Arenaria eliasiana, A. sivasica, A. monscragus, A. angustifolioides; Campanula lycica; Scutellaria orientalis subsp.tortumensis; Stachys choruhensis, S. tundjeliensis; Calamintha caroli-henricana; Aristolochia rechingeriana, the latter two species named in honour ofKarl Heinz Rechinger;Allium vuralii.
Dedicated to Prof. DrKarl Heinz Rechinger on the occasion of his 80th birthday. For part I see Pl. Syst. Evol.154, 111–128. 相似文献
164.
Burns-Balogh Pamela Szlachetko Dariusz L. Dafni Amots 《Plant Systematics and Evolution》1987,156(1-2):91-115
The various classifications of the orchid tribeNeottieae are reviewed and a new classification is proposed that divides the tribe into three subtribes,Neottiinae, Limodorinae, andCephalantherinae, based primarily on characters of the column (gynostemium). A cladistic analysis illustrates that these three subtribes are more closely related to one another than either is to any other group in subfam.Neottioideae, although there are very few apomorphic characters for the tribe. Pollination biology is also discussed showing links between breeding systems and distribution. There is also a possible role between column and labellum morphology and the emergence of a deceptive pollination syndrome from one of reward. 相似文献
165.
Anna Chrtková 《Plant Systematics and Evolution》1987,155(1-4):307-309
A new species,Lotus digii, has been found in Morocco growing on the coastal sandy soils. Further localities are from Algeria and Egypt. It should be expected also in Libya. 相似文献
166.
Manfred A. Fischer 《Plant Systematics and Evolution》1987,155(1-4):105-132
A character analysis reveals a clearly intermediate position of the tetraploidV. persica (2n = 28) between the two diploid speciesV. polita andV. ceratocarpa (both 2n = 14) which are morphologically rather different and have been placed by several authors in different sections of the genus.V. ceratocarpa is native to subhumid deciduous forests of the Caucasus and of the Elburz mountains (N. Iran);V. polita has its centre of variation in the Elburz range where it grows in therophyte habitats. Three other closely related species,V. bungei, V. siaretensis, andV. francispetae, are endemic to the Elburz range which is the main centre of diversity and variability of theV. agrestis group. This comprises all the above mentioned species and also two more European weeds:V. agrestis andV. opaca. Veronica polita, was probably originally native to open places in deciduous mountain forests, before becoming a weed in neolithic times and migrating to Europe; nowadays it has an almost world-wide distribution. The allotetraploidV. persica combines the ecological characters of its parents, the slightly xerophyticV. polita and the more mesophyticV. ceratocarpa, thus being preadapted to become a highly successful weed with a large ecological range. It has spread rapidly almost all over the world since the early 19th century.Dedicated to Hofrat Univ.-Prof. DrKarl Heinz Rechinger on the occasion of the 80th anniversary of his birthday. 相似文献
167.
The phytogeographical territories of Israel and Sinai are revised using a large mass of species distribution data. With the help of chorotype frequencies for 25 km2 individual squares in Israel, territories were defined on the basis of the first two most frequent chorotypes. The similarity of boundaries in the resulting phytogeographical map with a physiognomic map suggests that the latter may be used in preliminary stages of the preparation of plant geographical maps. 相似文献
168.
V. Melzheimer 《Plant Systematics and Evolution》1987,155(1-4):251-256
Silene thebana
Orph. exBoiss. is transferred as a subspecies toS. fabaria (L.)Sm., resulting in the new combinationS. fabaria (L.)Sm. subsp.thebana (Orph. exBoiss.)Melzh. The chromosome count of 2n = 24 is recorded for the first time for this taxon.Dedicated to Prof. DrK. H. Rechinger on the occasion of his 80th birthday, 16th October 1986. 相似文献
169.
M. Iranshahr 《Plant Systematics and Evolution》1987,155(1-4):55-57
Two species ofConsolida are described as new:C. lorestanica is distributed in W. Iran (Lorestan), andC. kandaharica is endemic to S. Afghanistan.Dedicated to Hofrat Prof. DrK. H. Rechinger on the occasion of this 80th birthday. 相似文献
170.
A comparison of karyotypes ofBrachyscome breviscapis (2n = 8),B. lineariloba cytodemes E (2n = 10), B (2n = 12) and C (2n = 16) suggests that these species have a homoelogous basic set of four chromosome pairs, two large pairs and two small, and that theB. lineariloba cytodemes E, B and C are related toB. breviscapis by successive additions of small chromosomes. A pronounced asynchrony of chromosome condensation between these large and small chromosomes has been observed. In the artificial hybrids betweenB. dichromosomatica (2n = 4) ×B. breviscapis, and theB. lineariloba cytodemes, theB. dichromosomatica chromosomes are similar in size and condensation behaviour to the small chromosomes ofB. breviscapis and ofB. lineariloba cytodemes E, B and C. Meiotic pairing in these hybrids also demonstrates the strong affinities between these chromosomes. It is suggested thatB. breviscapis may be of amphidiploid origin between a species with two large early condensing chromosome pairs and another,B. dichromosomatica-like species with two small late condensing pairs. It seems most likely that the additional small and late condensing chromosomes inB. lineariloba cytodemes E, B and C are derived from theB. dichromosomatica-like parent, and that each addition increases vigour, fecundity and drought tolerance, allowing these cytodemes to colonize more open and arid environments. Transmission of the univalents in the quasidiploidB. lineariloba cytodeme E was verified as being via the pollen, and not via the embryo sacs.The cytology ofBrachyscome lineariloba (Compositae, Asteroidae), 10. 相似文献