全文获取类型
收费全文 | 2760篇 |
免费 | 306篇 |
国内免费 | 262篇 |
出版年
2024年 | 6篇 |
2023年 | 78篇 |
2022年 | 45篇 |
2021年 | 123篇 |
2020年 | 119篇 |
2019年 | 145篇 |
2018年 | 116篇 |
2017年 | 120篇 |
2016年 | 127篇 |
2015年 | 120篇 |
2014年 | 165篇 |
2013年 | 188篇 |
2012年 | 123篇 |
2011年 | 111篇 |
2010年 | 99篇 |
2009年 | 132篇 |
2008年 | 139篇 |
2007年 | 147篇 |
2006年 | 153篇 |
2005年 | 139篇 |
2004年 | 137篇 |
2003年 | 76篇 |
2002年 | 74篇 |
2001年 | 62篇 |
2000年 | 57篇 |
1999年 | 80篇 |
1998年 | 45篇 |
1997年 | 30篇 |
1996年 | 29篇 |
1995年 | 32篇 |
1994年 | 40篇 |
1993年 | 32篇 |
1992年 | 30篇 |
1991年 | 35篇 |
1990年 | 21篇 |
1989年 | 21篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 21篇 |
1984年 | 18篇 |
1983年 | 4篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1973年 | 2篇 |
1971年 | 1篇 |
排序方式: 共有3328条查询结果,搜索用时 31 毫秒
61.
Studies were conducted to determine the effects of continuous constant amounts of artificial defoliation through the preheading and heading growth stages on head weight of cabbage. High levels of continuous artificial defoliation caused a reduction in head weight in all three years of the study, but the highest yield was always attained at some low level of preheading or heading defoliation. These results demonstrate that cabbage is tolerant to some levels of continuous defoliation before and after head formation. Results from this study are incorporated into a cost-benefit analysis to estimate an economic threshold.
Résumé L'étude a porté sur le poids des pommes de choux (Brassica oleracea) soumis à différentes intensités de défoliations répétées avant et pendant la formation des pommes. Pendant les trois années de l'étude, huit intensités de défoliation continue ont réduit le poids des pommes, mais le poids le plus élevé a toujours été obtenu avec une faible défoliation avant et pendant la formation des pommes. Ceci montre que le chou tolère une certaine défoliation avant et pendant la formation des pommes. Les résultats de cette étude ont été utilisés dans une analyse coût-bénéfice pour estimer le seuil économique de défoliation.相似文献
62.
The rate of decrease of mean metacarpal cortical width was assessed by sequential radiographic morphometry for each hand separately in a series of 46 right-handed postmenopausal women observed for 222 patient-years. For the left hand, the mean rate was 0.0331 ± 0.0053 mm/year and for the right, 0.0310 ± 0.0060 mm/year. The difference between the hands in the rate of decrease was not significant. It is concluded that the higher work load of the dominant hand has no effect on the rate of bone loss from the metacarpals of postmenopausal women. 相似文献
63.
Oxygen consumption and ammonia excretion of herbivorous chironomid larvae in Lake Balaton 总被引:1,自引:1,他引:0
István Tátrai 《Hydrobiologia》1982,96(2):129-135
The oxygen consumption and ammonia excretion of a herbivorous midge larva,Chironomus sp., inhabiting Lake Balaton was measured at two different temperatures. The loss of energy through respiration and that through ammonia excretion were calculated. The daily respiratory energy loss amounted to 655.5 ± 123.8 J g–1 at 17 °C and to 1 160.0 ± 168.4 J g–1 (dry weight) at 25 °C. Mean energy loss through ammonia excretion was about 40% less than through respiration. 相似文献
64.
Makoto Kimura Masaki Funakoshi Shinpei Sudo Takehiro Masuzawa Toshie Nakamura Kozue Matsuda 《Journal of plant research》1982,95(4):359-373
Annual net production was estimated in the secondary coppice forest near Tokyo, which was dominated by a deciduous oak,Quercus serrata Thunb. Lateral growth of stems and old branches was directly estimated by examining the annual rings for 35 shoots in a clear-cut
quadrat of 10m×10m. Phytomasses of current organs were also weighed in the quadrat. Preharvest losses of current organs were
determined by twelve 0.5 m2 litter traps for fine litter and twelve 6 m2 quadrats for woody litter. Branch production was also assessed indirectly by use of the stem-branch allometry and death of
branches. The results of the indirect method were in sufficient agreement with the result of the direct one. Grazing loss
of leaves from the canopy was estimated directly from the loss in leaf area and indirectly from the animal faeces caught by
the litter traps.
Net production of the canopy trees was 149 kg a−1 year−1, in which leaf production was 36.9 kg. Animals grazed about 14% of the leaf area by the end of the growing season. True consumption
of leaves by animals was 7.6% of leaf production or 10% of leaf mass. Production of undergrowth, mainly a dwarf bamboo,Pleioblastus chino Makino, was 28 kg a−1 year−1, being 15% of the total stand production.
Productivity of this forest was significantly higher than that of cool-temperate deciduous broadleaf forests. 相似文献
65.
Summary The effect of sulfur (S) placement and S rate on the efficiency of urea (U) relative to ammonium sulfate (AS) and ammonium
nitrate (AN) for cotton were examined in a pot experiment using sandy clay loam soil (pH 7.9). The results showed that AS
and AN application in the absence of S increased the yield than U partly because U-induced damage to plants. The combined
application of the N sources with S increased the yields and that, the placement of S in the seed horizon in contact with
N was more effective than mixing throughout the soil especially with U. These effects were observed with three cotton cultivars.
The addition of S to a maximum of 1.5 g/pot gave further increases in yields or the N content of leaves for U, AS or AN. Using
the least squares method, it was found that the presence of S significantly increased the efficiency of U than AS or AN. Incubation
of S and the N sources with S in the soil was carried out to understand the growth conditions of cotton fertilized by U in
alkalin soil. In the case of U-soil system, the pH increased. NO2−N accumulated and considerable loss of N took place. The pH, NO2−N accumulation and the loss of N decreased with S increments. 相似文献
66.
Robert O. Hussa Roland A. Pattillo 《In vitro cellular & developmental biology. Plant》1980,16(7):585-590
Summary The BeWo line of trophoblastic cells, maintained in continuous culture since 1966, was employed to investigate the phenomenon
of gonadotropin α-subunit predominance that exists in several cell lines. The secretion of complete human chorionic gonadotropin
(hCG) relative to α-subunit was compared in several different BeWo sublines, all of which were derived from BeWo stock roller
tube colonies. In all of the BeWo sublines, secretion of hCG originally exceeded secretion of α-subunit. With time in culture,
however, there was a marked decline in production of hCG/hCGβ, but not in α-subunit. Thus it appears that the production of
hCGβ by BeWo choriocarcinoma cells is more labile than the production of the α-subunit. 相似文献
67.
The Little Bustard is suffering a widespread population decline mainly due to agricultural intensification. This study evaluates the effects of intensification level, habitat availability and rainfall on the population dynamics of this species. The population density of males was monitored for 7 years (2002–2008) at 184 points located within three sites with contrasting levels of agricultural intensification in southern Portugal. Densities decreased along the intensification gradient from 8.2 to 2.3 males / km2. Overall, there was an approximately 50% population decline during the period 2002–2008, driven by a decline observed in one of the less intensive sites, whereas in the other two sites densities remained fairly constant. Yearly variations in male densities were influenced by intensification level, amount of grassland habitat and rainfall patterns. Thus, agricultural intensification is having a negative effect on population densities of this threatened species, particularly through the loss of grasslands (fallow fields and pastures) suitable for displaying males. The results also suggest a positive impact of rainfall on male densities, although this is more likely in grasslands within less intensive agricultural regions of poorer soil quality, where higher breeding male densities occur. Grassland habitat quality, driven by both climate and human management, probably plays a major role in the population dynamics of this threatened steppe bird in its strongholds. 相似文献
68.
69.
Functional redundancy of multiple forest taxa along an elevational gradient: predicting the consequences of non‐random species loss 下载免费PDF全文
Akira S. Mori Takayuki Shiono Takashi F. Haraguchi Aino T. Ota Dai Koide Takayuki Ohgue Ryo Kitagawa Ryo Maeshiro Toe Toe Aung Taizo Nakamori Yusuke Hagiwara Shunsuke Matsuoka Anzu Ikeda Takuo Hishi Satoru Hobara Eri Mizumachi Andreas Frisch Göran Thor Saori Fujii Takashi Osono Lena Gustafsson 《Journal of Biogeography》2015,42(8):1383-1396
70.
The influence of red deer (Cervus elaphus) grazing on grassland production for forage conservation at the forest border during the vegetation period was studied on
three locations (Mala gora, Cvišlerji, and Mačkovec) in the Kočevje region (SE Slovenia). The experiment lasted from the 25th
of March until the 8th of October 2002. Portable cages were used to exclude red deer from grazing the herbage. At four sampling
dates in the season, herbage air dry matter (DM) yield was measured at three different observations (cage-protected plot,
cage-protected plot only two to three weeks before sampling date, otherwise freely grazed (removed), and unprotected plot).
The results from the experiment showed that red deer grazed on grassland through the entire season and that the regeneration
capability of sward was the highest in summer, middle in spring and smallest in autumn. On unprotected plots, an average reduction
of 50% of herbage DM yield was found with the most damaged sites also up to 80% reduction. 相似文献