全文获取类型
收费全文 | 22374篇 |
免费 | 1814篇 |
国内免费 | 1938篇 |
出版年
2024年 | 81篇 |
2023年 | 541篇 |
2022年 | 563篇 |
2021年 | 931篇 |
2020年 | 954篇 |
2019年 | 1128篇 |
2018年 | 971篇 |
2017年 | 883篇 |
2016年 | 873篇 |
2015年 | 1125篇 |
2014年 | 1362篇 |
2013年 | 2090篇 |
2012年 | 938篇 |
2011年 | 1084篇 |
2010年 | 758篇 |
2009年 | 1228篇 |
2008年 | 1268篇 |
2007年 | 1230篇 |
2006年 | 1128篇 |
2005年 | 902篇 |
2004年 | 848篇 |
2003年 | 702篇 |
2002年 | 571篇 |
2001年 | 475篇 |
2000年 | 410篇 |
1999年 | 370篇 |
1998年 | 349篇 |
1997年 | 355篇 |
1996年 | 269篇 |
1995年 | 222篇 |
1994年 | 201篇 |
1993年 | 196篇 |
1992年 | 165篇 |
1991年 | 145篇 |
1990年 | 123篇 |
1989年 | 102篇 |
1988年 | 93篇 |
1987年 | 78篇 |
1986年 | 67篇 |
1985年 | 75篇 |
1984年 | 52篇 |
1983年 | 33篇 |
1982年 | 64篇 |
1981年 | 41篇 |
1980年 | 30篇 |
1979年 | 19篇 |
1978年 | 11篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1975年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils
Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy
loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to
60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter
penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance
to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting
the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots
grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores. 相似文献
92.
Five strains of Bradyrhizobium japonicum (USDA 6, 110, 122, 138, and 143) were screened in cell culture for tolerance to acidity (pH 4.2, 4.4, and 4.6) and Al (0, 3, 4, 5, and 6 mg L–1) under low P conditions. Each strain was later grown in association with seven soybean [Glycine max. (L) Merr.] cultivars which were also screened for tolerance to the same stresses in nutrient culture to determine which soybean-Bradyrhizobium combinations would establish the most effective symbiotic N2 fixing relationships. Results indicated that strains USDA 110 and 6 were more tolerant than USDA 122, 138 and 143 with USDA 110 being the most tolerant. Acidity appeared to be the more severe stress; but even when strains showed tolerance to the stresses, cell numbers were significantly reduced. This suggests that colonization of soils and soybean roots can be adversely affected under similar conditions in the field which may result in reduced nodulation. The strains found to be more tolerant to the stresses were more effective N2 fixers in symbiosis with all soybean cultivars, with USDA 110 being definitely superior. The association between the more tolerant strains and cultivars had the largest nitrogenase activity. Further studies on the inclusion of tolerant Bradyrhizobium strains in inoculum used on tolerant soybean cultivars in the field are warranted. 相似文献
93.
94.
用相关和回归处理方法,研究了8条正常狗咽喉部高频喷射通气时,调节驱动压、呼吸比和频率对喷气量、吸入气氧浓度、动脉血气及气道内压的作用。结果显示,驱动压和呼吸此对各观察指标几乎有同等重要的作用,频率的影响很小,喷气量与吸入气氧浓度、动脉血气、气道内压间存在显著的正相关关系。说明调节参数的意义主要在于改变了喷气量。 相似文献
95.
96.
Taurine Levels in Discrete Brain Nuclei of Rats 总被引:7,自引:1,他引:6
Miklós Palkovits István Elekes Tibor Láng rás Patthy 《Journal of neurochemistry》1986,47(5):1333-1335
Concentrations of taurine have been measured in 44 microdissected rat brain nuclei or areas. Taurine is ubiquitously present and distributed unevenly in the rat brain: the ratio of the highest (pyriform cortex) to lowest (midbrain reticular formation) concentrations is 4.7:1. High taurine levels were found in cerebral cortical areas, caudate-putamen, cerebellum, median eminence, and supraoptic nucleus. Acute pain stress reduced taurine levels in the hypothalamus and the lower brainstem nuclei but not in cortical areas. Increased locomotor and behavioral activities following a high dose of amphetamine elevated taurine concentrations significantly in the substantia nigra and locus ceruleus. 相似文献
97.
98.
R. H. van der Eijk 《Oecologia》1986,69(1):41-46
Summary Survival chances have been estimated for populations of the waterbeetle Gyrinus marinus Gyll. from capture-recapture sequences in 1974–1978. The average weekly survival chance is estimated as 0.9247 in spring, decreasing to 0.7701 in autumn; males survive longer than females. Survival shows little variation in time or space (variation coefficient V<0.1). Survival chance did not significantly decrease with age. Freshly emerged tenerals apparently have a lower survival chance than older beetles. The survival chance during hibernation (October–April) is about 0.33, with a greater variation between years (V=0.7) than between populations (V=0.4).Communication N0 301 of the Biological Station, Wijster 相似文献
99.
Summary Soil columns were exposed to balanced (low Na+) or unbalanced (high Na+) high-salt solutions for a period of 7 days followed by 7 days of stress reflief. Total numbers of bacteria released into
the perfusates rose under both types of stress, but the proportion of displaced bacteria that were viable fell significantly.
Relief from both types of stress stimulated rapid increases in the number of viable micro-organisms released from soil. Examination
of the soils at the end of the relief periods revealed that soils exposed to stress contained more viable bacteria than the
non-stressed controls. However, high levels of balanced stress led to a significant decrease in species diversity within the
microbial population, but a similar effect was not observed in soils exposed to unbalanced, high Na+ stress. These results suggest that, while salt stress may cause a significant reduction in the number of microorganisms in
a soil, a large portion of the microbial population can rapidly adapt to marked changes in salinity. 相似文献
100.
Relationship between growth,nitrogen fixation and assimilation in a legume (Medicago sativa L.) 总被引:1,自引:0,他引:1
Summary Symbiotic N2 fixation, NO
3
−
assimilation and protein accumulation in the shoots were measured simultaneously in alfalfa (Medicago sativa L.) grown in the field or in pots, in order to study how the balance between the two modes of nitrogen nutrition could be
influenced by agronomic factors, such as harvest, mineral nitrogen supply and drought stress.
During periods of rapid growth, fixation and assimilation may function simultaneously; they are antagonistic at the beginning
and at the end of the growth cycle, when the nitrogen requirement of the plant is lower. When nitrogen nutrition does not
limit growth, mineral nitrogen supply favours assimilation at the expense of fixation, but does not modify the amount of nitrogen
accumulated, which is adjusted to the growth capacity of the plant.
After cutting, nitrate assimilation compensated for the decrease in fixation and supplied the plant with the nitrogen required
by the regrowth, the proliferation of which determined the fixation recovery.
Drought stress decreased N2 fixation much more than NO
3
−
assimilation. The latter made growth recovery possible when water supply conditions became normal again.
These results suggested the existence of an optimum level of nitrate assimilation, which differed depending on the age of
the plants and allowed both maximum growth and fixing activity. 相似文献