全文获取类型
收费全文 | 9111篇 |
免费 | 955篇 |
国内免费 | 2篇 |
出版年
2022年 | 60篇 |
2021年 | 133篇 |
2020年 | 86篇 |
2019年 | 100篇 |
2018年 | 111篇 |
2017年 | 102篇 |
2016年 | 213篇 |
2015年 | 354篇 |
2014年 | 402篇 |
2013年 | 458篇 |
2012年 | 639篇 |
2011年 | 606篇 |
2010年 | 385篇 |
2009年 | 365篇 |
2008年 | 563篇 |
2007年 | 489篇 |
2006年 | 480篇 |
2005年 | 491篇 |
2004年 | 468篇 |
2003年 | 446篇 |
2002年 | 458篇 |
2001年 | 129篇 |
2000年 | 101篇 |
1999年 | 109篇 |
1998年 | 121篇 |
1997年 | 87篇 |
1996年 | 87篇 |
1995年 | 91篇 |
1994年 | 83篇 |
1993年 | 87篇 |
1992年 | 96篇 |
1991年 | 76篇 |
1990年 | 94篇 |
1989年 | 62篇 |
1988年 | 71篇 |
1987年 | 63篇 |
1986年 | 68篇 |
1985年 | 92篇 |
1984年 | 89篇 |
1983年 | 78篇 |
1982年 | 75篇 |
1981年 | 89篇 |
1980年 | 91篇 |
1979年 | 74篇 |
1978年 | 57篇 |
1977年 | 56篇 |
1976年 | 53篇 |
1974年 | 56篇 |
1973年 | 57篇 |
1972年 | 46篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
191.
Summary Difficulty was experienced in using direct silver nitrate impregnation techniques in the study of the chaetotaxy of the oncomiracidia of several freshwater ancyrocephalines. Eventually the Chatton-Lwoff method of silver impregnation, which was devised to study the cilia and basal bodies of ciliated protozoa, was utilized successfully following certain modifications. The improved technique is described in detail and photomicrographs showing the pattern of the sensillae are presented. ac]19800820 相似文献
192.
Summary -Glutamylamine cyclotransferase, an enzyme found in a number of animal tissues and cells, catalyzes the conversion of -(L--glutamyl)-L-lysine to free lysine and 5-oxo-L-proline as well as the release of free amines and the formation of 5-oxo-L-proline from a variety of other L--glutamylamines. Among its substrates are both the mono- and di--glutamyl derivatives of putrescine, spermidine and spermine, and a derivative of -(L--glutamyl)-L-lysine in which both the -amino group and the carboxyl group of the lysine moiety are blocked. The enzyme does not act on most -glutamyl--amino acids, nor is it active toward the -lysyl derivatives of L-aspartic acid or D-glutamic acid. Derivatives of -(L--glutamyl)-L-lysine in which the -amino or the -carboxyl function of the glutamyl moiety is blocked also do not serve as substrates. The specificity of -glutamylamine cyclotransferase is in accordance with the proposal that it functions biologically in the latter stages of the catabolism of products of the action of transglutaminases. Some suggestions as to the manner in which -glutamylamine cyclotransferase serves this function are made based on present knowledge of protein degradation. 相似文献
193.
Summary The effects of lanthanum on the activity of purified preparations of acetylcholinesterase (AChE) from the electric organ ofE. electricus and on the activity of AChE in intact electro-plaques from the same species were studied. 0.1mm LaCl3 produced an initial inhibition of purified AChE which was followed by a delayed activation of the enzyme. Upon pretreatment of purified enzyme with LaCl3, initial activity was markedly increased. LaCl3 exerted a marked, concentration-dependent inhibition of intact cell AChE.La3+ and Ca2+ appear to interact competitively. In the presence of both 10mm CaCl2 and 0.1mm LaCl3, the initial activity of purified AChE was increased at lower ACh concentrations and inhibited at ACh concentrations greater than 3 × 10–4
m. Inhibition of intact cell enzyme by 0.1mm LaCl3 was relieved by increasing the CaCl2 concentration to 10mm at ACh concentrations less than 2 × 10–4
m.The data were analyzed assuming Michaelis-Menten kinetics and interpreted with reference to the differential binding of divalent and trivalent cations to regulatory anionic sites which are separate and distinct from the anionic site of the active center of the enzyme. 相似文献
194.
B. L. Haines 《Oecologia》1977,26(4):295-303
Summary Nitrogen uptake from applied nutrient solutions was evaluated in two old fields, in a pine plantation, and in a hardwood stand, to test the idea that plant communities become more efficient trappers and retainers of plant nutrients during succession. Uptake was estimated as the difference between nutrient concentrations in water collected from beneath soil profiles with and without roots by lysimeters within each successional stage. Results suggest that nitrate uptake decreased while ammonia uptake increased with succession. This apparent shift from a nitrate to an ammonia nitrogen economy during succession has been reported by other workers and is evolutionarily significant as an energy, nitrogen, and cation saving mechanism. 相似文献
195.
196.
Previous studies had shown that both migratory flight and ovarian maturation in Oncopeltus fasciatus were stimulated by juvenile hormone (JH), yet the two behaviors were mutually exclusive. To understand the relationship of this hormone to these behaviors, haemolymph juvenile hormone titers were determined in both sexes of Oncopeltus throughout the adult stage by the Manduca pigmentation bioassay. Animals reared under long day conditions (17L:7D, 24°C) showed an immediate rise in haemolymph titers of JH after adult emergence whereas those reared in a short day photoperiod (12L:12D, 24°C) had a more gradual increase in hormone titers. Migratory flight behavior occurred during periods of intermediate hormone titers while oviposition did not begin until JH titers had reached their peak. It was concluded that lower JH titers normally stimulate flight in the prereproductive adult whereas higher titers are required for complete ovarian development. The corpus allatum in Oncopeltus thus coordinates migration and reproduction in response to the environmental cues of photoperiod, temperature, and food quality. 相似文献
197.
NH4+ and NO3? uptake were measured by continuous sampling with an autoanalyzer. For Hypnea musciformis (Wulfen) Lamouroux, NO3?up take followed saturable kinetics (K2=4.9 μg-at N t?1, Vmax= 2.85 μg- at N, g(wet)?1. h?1. The ammonium uptake data fit a trucatd hyperbola, i.e., saturation was not reach at the concentrations used. NO3? uptake was reduced one-half in the presence of NH4+, but presence of NO3? had no effect on NH4+ uptake. Darkness reduced both NO3? and NH4+ uptake by one-third to one-half. For Macrocystis pyrufera (L) C. Agardh, NO3? uptake followed saturable kinetices: K2=13.1 μg-at N. l?1. Vmax=3.05 μg-at N. g(wet)?1. h?1.NH4+ uptake showed saturable kinetics at concentration below 22 μg-at N l -1 (K2=5.3 μg-at N.1–1, Vmax= 2.38 μg-at N G (wet)?1.h?1: at higher concentration uptake increased lincarly with concentrations. NO3?and NH4+ were taken up simulataneously: presence of one form did not affect uptake of the other. 相似文献
198.
Lagenospermum imparirameum Arnold, originally described from a few specimens of cupulate seeds borne on two or three times dichotomous branches, is now shown to be borne on more complex branching systems. Details of the cupule and seed morphology are added and an emended diagnosis of the taxon is given. A new species,Gnetopsis hispida, is described as the third occurrence of this genus and the first occurrence in beds of Lower Mississippian Age in North America. The classification, evolutionary implications, and dispersal biology are discussed for each of the seeds 相似文献
199.
200.
Measurement of carboxypeptidase A, one of the pancreatic proteolytic enzymes, in human serum is made possible by a combination of affinity chromatography to isolate and concentrate the enzyme followed by monitoring activity spectrophotometrically with a high-turnover peptide substrate. Concentrations of enzyme in the nanogram-per-milliliter range can be determined with high precision and reliability. Initial clinical application of this method demonstrates no detectable activity in serum from normal individuals, but the enzyme is present in the sera of individuals with pancreatitis. 相似文献