首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   59486篇
  免费   5696篇
  国内免费   22篇
  65204篇
  2023年   243篇
  2022年   504篇
  2021年   1005篇
  2020年   632篇
  2019年   807篇
  2018年   992篇
  2017年   864篇
  2016年   1476篇
  2015年   2442篇
  2014年   2705篇
  2013年   3158篇
  2012年   4210篇
  2011年   4073篇
  2010年   2581篇
  2009年   2345篇
  2008年   3383篇
  2007年   3433篇
  2006年   3259篇
  2005年   3092篇
  2004年   3029篇
  2003年   2772篇
  2002年   2704篇
  2001年   872篇
  2000年   713篇
  1999年   815篇
  1998年   854篇
  1997年   589篇
  1996年   513篇
  1995年   470篇
  1994年   498篇
  1993年   494篇
  1992年   623篇
  1991年   499篇
  1990年   486篇
  1989年   490篇
  1988年   440篇
  1987年   408篇
  1986年   403篇
  1985年   377篇
  1984年   434篇
  1983年   398篇
  1982年   419篇
  1981年   382篇
  1980年   395篇
  1979年   310篇
  1978年   299篇
  1977年   263篇
  1976年   257篇
  1974年   233篇
  1973年   208篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Summary Cells of the scutellum of immature embryos ofSorghum bicolor plated onto an agar medium containing 2,4-D give rise to shoots and embryo-like structures and to some callus. Some of the embryo-like structures later develop into typical sorghum embryos complete with scutellum, coleoptile and coleorhiza. The results of anatomical studies of the development of these secondary growth forms by light and scanning electron microscopy suggest that shoots and embryo-like structures can arise directly from cells of the primary scutellum without an intervening callus phase. In some cases it appears that the scutellum of the secondary embryos arises by folding of the scutellum of the sexual embryo and does not arisede novo. In other cases the structures arise from single cells. No evidence was found to indicate that organized structures arose from proliferating callus cells. The unorganized callus which arises initially is not capable of growth through continuous subculture; it produces a purple-black pigment and rapidly becomes necrotic. The significance of these observations is discussed in relation to present views on morphogenesis in cereal cell cultures and their implications forin vitro cell genetics.Abbreviations used in the text 2,4-D 2,4-dichlorophenoxyacetic acid - MS Murashige and Skoog  相似文献   
962.
963.
This paper analyzes allogrooming (social grooming) data collected from two large, fully integrated and long established social groups of macaques (one of pigtail macaques and one of bonnet macaques). The data demonstrates a species and sex difference for total allogrooming given with females of both species giving more grooming than the males and with pigtails as a species giving more grooming than bonnets. Also, pigtail females gave more allogrooming to clan members than to nonclan members, but this was not true for pigtail males, bonnet males, and bonnet females. Allogrooming given and allogrooming received by age class for both species showed development of a sexual dichotomy at three to four years. The analysis characterized some of the social structures extant in two closely related species of macaque, particularly the somewhat different use of allogrooming in pigtail females as opposed to the other three categories of animals. Social implications with relation to macaque societies are discussed.  相似文献   
964.
In 1,506 hours of field observations on free-ranging rhesus monkeys in Kathmandu Valley, Nepal, in 1974 and 1975 18 cases of favorable social interactions between adult males and infants were observed. Eleven of these were brief encounters of play or grooming; seven were more extended cases of male care. One of the latter was a complete adoption of a neonatal orphan by a dominant male. This adoption was possessive and restrictive and it resulted in the death of the infant by starvation within three days. A similar adoption involving the same male occurred in 1976 and it also resulted in the death of the infant. Most of the favorable male-infant interactions occurred during the winter and spring when the infants were 6 to 12 months of age. These favorable social interactions involved eight males in six different troops, out of a total of about 48 males in 12 troops in our study population of approximately 600 monkeys. These observations are discussed in light of current sociobiological theories.  相似文献   
965.
Fertile and sterile frond segments of the lyginopterid seed fern Feraxotheca culcitaus are described. In both the fertile and sterile specimen three orders of frond axes are borne alternately and in the same plane. Antepenultimate and penultimate frond axes are characterized by a C-shaped vascular bundle and numerous canals containing a golden-colored material in the fossilized condition. Synangia with only four sporangia demonstrate radial symmetry; bilateral symmetry is present in larger synangia. Pinnules possess dichotomous venation, with a single vein entering each lobe of the deeply incised lamina. A single canal occurs below each vein, and vein ends have an amplified vascular bundle. Morphologically Feraxotheca compares most closely with the compression genus Crossotheca. The morphology of the vascular bundle in the largest pinna is similar to a Lower Carboniferous specimen of Rhodea.  相似文献   
966.
A 10-month-old infant was found to have an isolated congenital arteriovenous fistula between the right subclavian artery and vein. Ligation of the fistula resulted in complete anatomic correction and rapid resolution of cardiomegaly.  相似文献   
967.
The effect of injections of arginine vasotocin (AVT) on plasma free fatty acid (FFA) levels was studied in anadromous sea lampreys collected in the St. John River, New Brunswick, during their upstream spawning migration. Plasma FFA was significantly higher in lampreys injected with a single dose of 1 000 mU vasotocin/kg body weight than in those receiving only the vehicle solution, the difference being the greater at 90 than at 30 min post-injection. The significance of AVT in migration is discussed.  相似文献   
968.
The dependence of the water proton magnetic resonance spin-lattice relaxation rate (T1??1) in the rotating frame on the strength of the spin-locking (H1) field has been investigated for packed oxy and deoxy normal and sickle erythrocytes at temperatures from 9 to 40 °C. The T1??1 of oxy or deoxy normal erythrocytes shows no dependence on H1 up to ~7 G at any temperature studied. On the other hand, T1??1 decreases from about 40 s?1 to 15 s?1 (H1 from 0 to ~7 G) for deoxygenated packed sickle cells at 40 °C. The magnitude of this variation of T1??1 with H1 decreases with decreasing temperature. Oxy packed sickle cells also show a dependence of T1??1 on H1 but the magnitude is <10% of that of the deoxygenated samples. These results suggest that water proton T1??1 measurements are a sensitive probe of hemoglobin S polymerization and provide a novel technique for the study of slow water motions in these systems. The T1??1 results are compared with low frequency T1?1 results of other investigators on hemoglobin S solutions. Analysis of the data suggests that water proton motions with correlation times of the order of 10?5 s are present in the deoxygenated sickle cell samples at temperatures above 10 °C.  相似文献   
969.
970.
A method is described for the quantitative analysis of (3-methoxy-4-sulphoxyphenyl)-ethylene glycol (MHPG sulphate) in human urine, based on selected ion monitoring gas chromatography—mass spectrometry and using a specifically deuterium-labelled analogue of MHPG sulphate as internal standard. The procedure involves extraction of the urine sample on Amberlite XAD-2, followed by isolation of MHPG sulphate by column chromatography on Sephadex LH-20. Cleavage of the sulphate conjugate and formation of the MHPG tris(trifluoroscetate) derivative are carried out in a one-step reaction, without recourse to enzymic hydrolysis.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号