全文获取类型
收费全文 | 3991篇 |
免费 | 291篇 |
国内免费 | 12篇 |
专业分类
4294篇 |
出版年
2022年 | 28篇 |
2021年 | 50篇 |
2019年 | 37篇 |
2018年 | 75篇 |
2017年 | 55篇 |
2016年 | 79篇 |
2015年 | 125篇 |
2014年 | 129篇 |
2013年 | 196篇 |
2012年 | 260篇 |
2011年 | 252篇 |
2010年 | 173篇 |
2009年 | 152篇 |
2008年 | 186篇 |
2007年 | 187篇 |
2006年 | 155篇 |
2005年 | 147篇 |
2004年 | 152篇 |
2003年 | 128篇 |
2002年 | 119篇 |
2001年 | 119篇 |
2000年 | 85篇 |
1999年 | 102篇 |
1998年 | 48篇 |
1997年 | 48篇 |
1996年 | 36篇 |
1995年 | 46篇 |
1994年 | 36篇 |
1993年 | 27篇 |
1992年 | 59篇 |
1991年 | 72篇 |
1990年 | 64篇 |
1989年 | 51篇 |
1988年 | 40篇 |
1987年 | 42篇 |
1986年 | 43篇 |
1985年 | 28篇 |
1984年 | 35篇 |
1983年 | 41篇 |
1982年 | 28篇 |
1979年 | 27篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 23篇 |
1975年 | 27篇 |
1958年 | 24篇 |
1957年 | 26篇 |
1956年 | 26篇 |
1955年 | 22篇 |
1954年 | 22篇 |
排序方式: 共有4294条查询结果,搜索用时 0 毫秒
41.
42.
43.
44.
45.
46.
47.
48.
The climate of the native tropical forest habitats of Hylocereus undatus, a hemiepiphytic cactus cultivated in 20 countries for its fruit, can help explain the response of its net CO2 uptake to environmental factors. Under wet conditions, about 85% of the total daily net CO2 uptake occurs at night via Crassulacean acid metabolism, leading to a high water‐use efficiency. Total daily net CO2 uptake is reduced 57% by only 10 days of drought, possibly involving stomatal closure induced by abscisic acid produced in the roots, which typically occupy a small substrate volume. Total daily net CO2 uptake for H. undatus is maximal at day/night air temperatures of 30/20°C, optimal temperatures that are higher than those for desert cacti but representative of ambient temperatures in the tropics; its total daily net CO2 uptake becomes zero at day/night air temperatures of 42/32°C. Stem damage occurs at 45°C for H. undatus, whose photosynthetic cells show little acclimation to high temperatures compared with other cacti and are also sensitive to low temperatures, ‐1.5°C killing half of these cells. Consistent with its shaded habitat, total daily net CO2 uptake is appreciable at a total daily PPF of only 2 mol m2 day' and is maximal at 20 mol m?2 day?1, above which photoinhibition reduces net CO2 uptake. Net CO2 uptake ability, which is highly correlated with stem nitrogen and chlorophyll contents, changes only gradually (halftimes of 2–3 months) as the concentration of applied N is changed. Doubling the atmospheric CO2 concentration raises the total daily net CO2 uptake of H. undatus by 34% under optimal conditions and by even larger percentages under adverse environmental conditions. 相似文献
49.
G. Wayne Brodland Michael J. Scott Andrew F. MacLean M. Globus S. Vethamany-Globus R. Gordon Jim H. Veldhuis R. Del Maestro 《Development genes and evolution》1996,205(5-6):311-318
During neurulation in vertebrate embryos, epithelial cells of the neural plate undergo complex morphogenetic movements that culminate in rolling of the plate into a tube. Resolution of the determinants of this process requires an understanding of the precise movements of cells within the epithelial sheet. A computer algorithm that allows automated tracking of epithelial cells visible in digitized video images is presented. It is used to quantify the displacement field associated with morphogenetic movements in the axolotl (Ambystoma mexicanum) neural plate during normal neural tube formation. Movements from lateral to medial, axial elongations and area changes are calculated from the displacement field data and plotted as functions of time. Regional and temporal differences are identified. The approach presented is suitable for analyzing a wide variety of morphogenetic movements. 相似文献
50.
Bombesin and the two mammalian bombesin-related peptides, gastrin-releasing peptide (GRP) and neuromedin C, at physiological concentrations have been previously shown to stimulate significantly in vitro the antibody-dependent cellular cytotoxicity (ADCC) and natural killer (NK) activities in BALB/c mouse leukocytes from axillary nodes, spleen and thymus. In the present work we have shown that adherent cells are required in leukocyte samples for stimulation of cytotoxicity by the neuropeptides, which suggests that this effect may be mediated by those cells. Here we demonstrate the specificity of the effects by reversing them in the presence of the bombesin-antagonist (Leu13-ΨCH2NH-Leu14)-BN, and by detecting specific receptors for GRP on macrophages of high and low affinity. Using the same binding technics, no receptors for this neuropeptide were found in non-adherent leukocytes. 相似文献