全文获取类型
收费全文 | 1355970篇 |
免费 | 122027篇 |
国内免费 | 1517篇 |
专业分类
1479514篇 |
出版年
2021年 | 17788篇 |
2019年 | 16160篇 |
2018年 | 17380篇 |
2017年 | 16155篇 |
2016年 | 27883篇 |
2015年 | 42379篇 |
2014年 | 50546篇 |
2013年 | 76777篇 |
2012年 | 35634篇 |
2011年 | 23506篇 |
2010年 | 42882篇 |
2009年 | 44447篇 |
2008年 | 22582篇 |
2007年 | 20569篇 |
2006年 | 26270篇 |
2005年 | 27228篇 |
2004年 | 26593篇 |
2003年 | 24207篇 |
2002年 | 22540篇 |
2001年 | 35091篇 |
2000年 | 32689篇 |
1999年 | 32377篇 |
1998年 | 25511篇 |
1997年 | 25310篇 |
1996年 | 24794篇 |
1995年 | 23004篇 |
1994年 | 22816篇 |
1993年 | 21899篇 |
1992年 | 27827篇 |
1991年 | 25952篇 |
1990年 | 24573篇 |
1989年 | 25366篇 |
1988年 | 23328篇 |
1987年 | 22078篇 |
1986年 | 21220篇 |
1985年 | 23131篇 |
1984年 | 22789篇 |
1983年 | 20025篇 |
1982年 | 20751篇 |
1981年 | 19970篇 |
1980年 | 18567篇 |
1979年 | 19012篇 |
1978年 | 18016篇 |
1977年 | 17278篇 |
1976年 | 16449篇 |
1975年 | 16078篇 |
1974年 | 16773篇 |
1973年 | 17098篇 |
1972年 | 14660篇 |
1971年 | 13403篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
G. F. Katekak 《Biologia Plantarum》1985,27(2-3):92-99
The chemical and stereochemical properties of phytotropins are described in terms of a receptor model, based on their ability
to bind to a receptor isolated from maize coleoptiles. Their root gravitropic properties are correlated with their ability
to bind to the receptor. It is suggested that their ability to inhibit auxin transport, without more, may not be enough to
explain their physiological effects. 相似文献
952.
Old (N = 9) and middle-aged (N = 9) rhesus males (Macaca mulatta) that were housed in the same type of home cage and the same room for several years were moved to a new location in an adjacent building. Their sexual behavior and hormone levels in the weeks preceding the move were compared to those after the move. The hypothesis that the nonspecific sensory stimulation provided by the move would increase levels of sexual behavior was not supported by the data. To the contrary, the percentages of tests with contacts, mounts, and penile erections, and the rate of contacting the female, decreased significantly for both old and middle-aged males. The intromission rate of middle-aged, but not of old, males also decreased. In consonance with previous findings, middle-aged males displayed significantly higher levels of sexual activity than did old males. The differences were observed both before and after the move. Serum testosterone levels did not differ between middle-aged and old males, and the move did not produce significant changes in testosterone levels. Serum cortisol levels were significantly higher immediately after the move (1 hr) but returned to premove levels the following day. 相似文献
953.
954.
Infections of one and two Hymenolepis diminuta established in newly weaned rats continued to grow for the duration of the experiment (238 days), whereas infections of 5 worms per rat became asymptotic around Day 55 postinfection and remained at or below this level thereafter as shown by biomass and mean weight per worm measurements. Infections of 50 worms established in newly weaned rats became asymptotic around Day 28 postinfection and thereafter worms were lost from the rats. Initially the biomass fell with the loss of worms, but by Day 56 a new lower biomass persisted for the remainder of the infection period. This level was maintained, despite diminishing numbers of worms, due to the growth of surviving individuals to a weight exceeding the original weight at maturity by a factor of more than 2. Experiments using rats that were mature at the time of infection demonstrated that the same response occurred, but approximately 3 weeks earlier. 相似文献
955.
956.
Ice crystal formation temperature was determined in the region of the crown in one group of 7-day-old intact unhardened high-salt plants of winter wheat (Triticum aestivum L. cv. Weibulls Starke II) with TA (Thermal Analysis) and DTA (Differential Thermal Analysis) methods. After exposure of another group of plants, grown for the first 7 days in the same way as the first group, to various sub-zero temperatures (-1 to 5°C), influx in roots of Rb+(86Rb+) and Ca2+(45Ca2+) and contents of K+ and Ca2+ were determined at intervals during 7 days of recovery. Ice crystal formation in the crown tissue was probably extracellular and took place at about -4°C. There was a large loss of K+ from the roots after treatment at sub-zero temperatures. This loss increased as the temperature of the sub-zero treatment decreased. During recovery, roots of plants exposed to -1, -2 and -3°C gradually reabsorbed K+. Reabsorption of K+ in roots of plants exposed to -4°C was greatly impaired. Rb+ influx decreased and Ca2+ influx increased after sub-zero temperature treatments of the plants. Active Rb+ influx mechanisms and active extrusion of Ca2+ were impaired or irreversibly damaged by the exposure. While Rb+ influx mechanisms were apparently repaired during recovery in plants exposed to temperatures down to -3°C, Ca2+ extrusion mechanisms were not. The temperature for ice crystal formation in the region of the crown tissue coincides with the temperature at which the plants lost the ability to reabsorb K+ and to repair Rb+ influx mechanisms during the recovery period. Plants were lethally damaged at temperatures below ?4°C. 相似文献
957.
958.
M. P. De Leon T. Yanagi M. Kikuchi J. Mu O. Ayau V. Matta M. Paz S. Juarez H. Kanbara I. Tada K. Hirayama 《International journal for parasitology》1998,28(12):1867-1874
Fifty fresh isolates of Trypanosoma cruzi from Triatoma dimidiata vectors and 31 from patients with Chagas disease were analysed for DNA polymorphisms within the 432-bp core region of the cruzipain gene which encodes the active site of cathepsin L-like cystein proteinase. The cruzipain gene showed signs of polymorphism consisting of four different DNA sequences in Central and South American isolates of T. cruzi. The PCR fragments of Guatemalan isolates could be divided into three groups, Groups 1, 2 and 3, based on different patterns of single-stranded DNA conformation polymorphism. All of the strains isolated from Brazil, Chile, and Paraguay, except for the CL strain, showed a Group 4 pattern. Two to four isolates from each group were analysed by cloning and sequencing. A silent mutation occurred between Groups 1 and 2, and five nucleotides and two aa substitutions were detected between Groups 1 and 3. The DNA sequence of Group 4 contained five nucleotides and one aa substitution from Group 1. All of the DNA sequences corresponded well with the single-stranded DNA conformation polymorphism. The Group 1 isolates, the majority in the Guatemalan population (70/81, 86.4%), were isolated from both triatomines and humans, but Group 3 were isolated only from humans. Moreover, the Group 2 isolates were detected only in triatomine vectors (9/50; 18%), but never in humans (0/32, P<0.05) suggesting that this group has an independent life-cycle in sylvatic animals and is maintained by reservoir hosts other than humans. 相似文献
959.
960.