全文获取类型
收费全文 | 3603篇 |
免费 | 283篇 |
国内免费 | 489篇 |
专业分类
4375篇 |
出版年
2024年 | 5篇 |
2023年 | 35篇 |
2022年 | 44篇 |
2021年 | 45篇 |
2020年 | 77篇 |
2019年 | 83篇 |
2018年 | 94篇 |
2017年 | 106篇 |
2016年 | 94篇 |
2015年 | 126篇 |
2014年 | 123篇 |
2013年 | 183篇 |
2012年 | 102篇 |
2011年 | 123篇 |
2010年 | 120篇 |
2009年 | 184篇 |
2008年 | 204篇 |
2007年 | 206篇 |
2006年 | 205篇 |
2005年 | 170篇 |
2004年 | 157篇 |
2003年 | 144篇 |
2002年 | 134篇 |
2001年 | 98篇 |
2000年 | 110篇 |
1999年 | 74篇 |
1998年 | 100篇 |
1997年 | 89篇 |
1996年 | 89篇 |
1995年 | 109篇 |
1994年 | 91篇 |
1993年 | 82篇 |
1992年 | 65篇 |
1991年 | 61篇 |
1990年 | 58篇 |
1989年 | 74篇 |
1988年 | 72篇 |
1987年 | 61篇 |
1986年 | 46篇 |
1985年 | 66篇 |
1984年 | 57篇 |
1983年 | 27篇 |
1982年 | 41篇 |
1981年 | 44篇 |
1980年 | 23篇 |
1979年 | 25篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 9篇 |
1973年 | 7篇 |
排序方式: 共有4375条查询结果,搜索用时 0 毫秒
1.
2.
Iain J. Mcgaw 《Marine and Freshwater Behaviour and Physiology》2006,39(2):131-141
Cardiovascular and respiratory variables were recorded in the blue crab, Callinectes sapidus, during injury and subsequent autotomy of a chela. Cardiac function and haemolymph flow rates were measured using a pulsed-Doppler flowmeter. Oxygen uptake was recorded using an intermittent flow respirometry system. Crabs reacted to the loss of a chela with a rapid increase in heart rate, which was sustained for 2 h. Stroke volume of the heart also increased after the chela was autotomized. A combined increase in heart rate and stroke volume led to an increase in cardiac output, which was maintained for an hour after the loss of a chela. There was also differential haemolymph perfusion of various structures. There was no change in perfusion of the anterolateral arteries or posterior and anterior aortae, during injury of the chela or subsequent autotomy. Haemolymph flow rates did increase significantly through the sternal artery during injury and immediately following autotomy of the chela. This was at the expense of blood flow to the digestive gland: a sustained decrease in haemolymph flow through the hepatic arteries occurred for 3 h following autotomy. Fine-scale cardiac changes associated with the act of autotomy included a bradycardia and/or associated cardiac pausing before the chela was shed, followed by a subsequent increase in cardiac parameters. Changes in the cardiovascular physiology were paralleled by an increase in oxygen uptake, which was driven by an increased ventilation of the branchial chambers. Although limb loss is a major event, it appears that only acute changes in physiology occur. These may benefit the individual, allowing rapid escape following autotomy with a subsequent return to normal activity. 相似文献
3.
E. É. Kolesnikova 《Neurophysiology》2008,40(1):48-52
Results of a comparative study of the sensitivity of the system of respiratory control to increases in the CO2 concentration and the intensity of free-radical processes in young and elderly subjects are described. It is shown that normal
(natural) aging is accompanied by a decrease in the sensitivity of the respiratory system to hypercapnic stimulation and a
parallel significant decrease in the activity of catalase in the blood of examined subjects. Mechanisms responsible for the
modifications of the sensitivity of the system of respiratory control to hypercapnia are discussed; these shifts can be at
least partly related to changes in the intensity of production of free radicals observed in elderly subjects.
Neirofiziologiya/Neurophysiology, Vol. 40, No. 1, pp. 53–57, January–February, 2008. 相似文献
4.
ABSTRACT. A non-dispersive infrared gas analyser equipped with a Luft-type sonic detector and flow-through reference cell was automated to monitor the total volume of carbon dioxide (CO2) respired by single insects or groups of insects. The infrared analyser was interfaced with an integrator for quantification, a microprocessor to control intermittent air flow through the insect respiration chambers, and a microcomputer for data storage and reduction. This technique has been used to monitor the CO2 Output of diapausing and non-diapausing mature fifth instar larvae and of developing pupae of the codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae). The resulting data were accurate, quantitative and reproducible. 相似文献
5.
Summary Soil tests, plant performance, and plant tissue analyses were used to study the availability of sulfur to wetland rice in
30 Philippine soils.
The critical concentrations of available sulfur by the calcium phosphate, lithium chloride, ammonium acetate, and hydrochloric
acid extractions were 9, 25, 30, and 5 mg/kg, respectively.
The critical total sulfur limits were 0.11% in the shoot at maximum tillering 0.055% in the straw at maturity, and 0.065%
in the grain. The critical N:S ratio was 15 in the shoot at maximum tillering, 14 in the straw at maturity, and 26 in the
grain. The critical sulfate-sulfur limit was 150 mg/kg in the shoot at maximum tillering and 100 mg/kg in the straw at maturity.
The critical sulfate-sulfur/total sulfur percentage ratio was 15% in the shoot at maximum tillering and the straw at maturity.
Plant performance, judged by appearance and yield of dry matter, straw, and grain, was generally poorer in the sulfur deficient
soils than in the other soils. Although the calcium phosphate and ammonium acetate methods gave a better correlation between
plant performance and available sulfur than the others, all four methods separated sulfur-deficient soils from non-deficient
ones. The hydrochloric acid method merits further study because it is simple and versatile. 相似文献
6.
Scaling of metabolic rate with body mass and temperature in teleost fish 总被引:27,自引:0,他引:27
7.
Translocation - A key factor limiting the efficiency of nitrogen fixation in legume nodules 总被引:2,自引:0,他引:2
John G. Streeter 《Physiologia plantarum》1993,87(4):616-623
A hypothesis is presented that the availability of water for export of nitrogenous products from legume nodules is a major factor limiting the efficiency of symbiotic nitrogen fixation. Water for export of solutes in the xylem probably depends largely on the import of water and reduced carbon in the phloeum, and one function of respiration may be to dispose of reduced carbon in order to increase the supply of water. A second hypothesis presented is that control of gas diffusion in soybean nodules is largely restricted to the cortex nearby the vascular bundles, thus making possible the linkage of solute balances in xylem and phloem with resistance to diffusion. These concepts are used in a re-examination of literature on manipulations of nodules and nodulated plants such as lowering of light levels, water stress, defoliation, stem girdling, and alteration of oxygen supply. The concept of translocation as a major factor limiting efficiency of symbiotic fixation is consistent with the failure of superior rhizobial isolates to improve N input significantly, and this limitation could also prevent exploitation of superior bacterial symbionts in the future 相似文献
8.
Cucumber ( Cucumis sativus L. cv. Poinsett 76) seeds were chilled at 2.5°C in a study of the chilling sensitivity and recovery of radicle tissue. The effect of chilling on radicle growth and the production of carbon dioxide and ethylene was measured. Chilling sensitivity of radicles increased as they grew from 1 to 7 mm in length. The length, not the age of the radicles, determined the level of chilling sensitivity. Apical tissue was most sensitive to chilling and slowest to recover from chilling, followed by subapical and basal tissue. Our data demonstrate that the chilling sensitivity of young seedling radicles differs along their length and that the rapid chilling-induced inhibition of elongation is probably due to an inability of meristematic cells to remain viable and active when chilled. 相似文献
9.
10.
The relationship between respiration and temperature in leaves of the arctic plant Saxifraga cernua 总被引:4,自引:4,他引:0
Abstract Saxifraga cernua, a perennial herb distributed throughout the arctic and subarctic regions, shows high levels of dark respiration. The amount of respiration exhibited by leaves and whole plants at any temperature is influenced by the pretreatment temperature. Plants grown at 10°C typically show higher dark respiration rates than plants grown at 20°C. The levels of alternative-pathway respiration (or cyanide-insensitive respiration) in leaves of S. cernua grown at high and low temperatures were assessed by treating leaf discs with 0.25 mol m?3 salicylhydroxamic acid during measurements of oxygen consumption. Alternative pathway respiration accounted for up to 75% of the total respiration. Tissues from 20°C-grown plants yielded a Q10 of 3.37 for normal respiration, and of 0.97 for alternative-pathway respiration. Tissues from 10°C-grown plants yielded a Q10 of 2.55 for normal respiration, and of 0.79 for alternative-pathway respiration. The alternative pathway does not appear to be as temperature sensitive as the normal cytochrome pathway. A simple energy model was used to predict the temperature gain expected from these high rates of alternative-pathway respiration. The model shows that less than 0.02°C can be gained by leaves experiencing these high respiration rates. 相似文献