全文获取类型
收费全文 | 22226篇 |
免费 | 2332篇 |
国内免费 | 9篇 |
专业分类
24567篇 |
出版年
2022年 | 236篇 |
2021年 | 429篇 |
2020年 | 252篇 |
2019年 | 326篇 |
2018年 | 395篇 |
2017年 | 333篇 |
2016年 | 557篇 |
2015年 | 1004篇 |
2014年 | 1021篇 |
2013年 | 1156篇 |
2012年 | 1577篇 |
2011年 | 1553篇 |
2010年 | 934篇 |
2009年 | 821篇 |
2008年 | 1202篇 |
2007年 | 1234篇 |
2006年 | 1163篇 |
2005年 | 1062篇 |
2004年 | 1014篇 |
2003年 | 970篇 |
2002年 | 914篇 |
2001年 | 309篇 |
2000年 | 264篇 |
1999年 | 296篇 |
1998年 | 303篇 |
1997年 | 209篇 |
1996年 | 203篇 |
1995年 | 194篇 |
1994年 | 173篇 |
1993年 | 196篇 |
1992年 | 202篇 |
1991年 | 205篇 |
1990年 | 187篇 |
1989年 | 180篇 |
1988年 | 203篇 |
1987年 | 148篇 |
1986年 | 142篇 |
1985年 | 155篇 |
1984年 | 202篇 |
1983年 | 165篇 |
1982年 | 162篇 |
1981年 | 149篇 |
1980年 | 132篇 |
1979年 | 130篇 |
1978年 | 123篇 |
1977年 | 111篇 |
1976年 | 112篇 |
1975年 | 97篇 |
1974年 | 142篇 |
1973年 | 121篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
52.
The excretory systems of terrestrial prosobranch snails of the family Cyclophoridae, collected in Jamaica, Costa Rica and South Africa, have been examined physiologically and as regards their gross and fine structure. The process of urine formation commences in the heart, where fluid is filtered across the wall of the ventricle. Filtration through the auricular wall is believed to be negligible. The kidney, which contains three types of cell, modifies the composition of the filtrate. One type of resorptive cell, characterized by basal infoldings associated with mitochondria, takes up salts. Another type, with basal subcellular spaces, may be responsible for taking up water. The third type of cell is secretory, producing concretions of uric acid and phospholipid which are liberated into the kidney lumen when the cell degenerates.
The rate and mechanism of urine production have been investigated using injections of inulin. The filtration rate at 25°C is 0.5 μl/g/min, and in 100% R.H. the average rate of urine production is 0–39 μl/g/min.
An accessory excretory organ has been developed from the hypobranchial gland of aquatic forms. It is composed of groups of subepithelial tubular glands opening into the mantle cavity by one or a series of pores, and secreting purines, phospholipids and mucus. There is evidence that this organ becomes progressively more complex in forms occupying drier habitats.
The systems of excretion and osmoregulation in the Cyclophoridae are considered to be very similar to those in their aquatic relatives, the Viviparidae and Ampullariidae. Certainly the cyclophorids are not as well adapted to a terrestrial life as are the Pulmonata, and in many respects they may be considered "aquatic" snails living on land. 相似文献
The rate and mechanism of urine production have been investigated using injections of inulin. The filtration rate at 25°C is 0.5 μl/g/min, and in 100% R.H. the average rate of urine production is 0–39 μl/g/min.
An accessory excretory organ has been developed from the hypobranchial gland of aquatic forms. It is composed of groups of subepithelial tubular glands opening into the mantle cavity by one or a series of pores, and secreting purines, phospholipids and mucus. There is evidence that this organ becomes progressively more complex in forms occupying drier habitats.
The systems of excretion and osmoregulation in the Cyclophoridae are considered to be very similar to those in their aquatic relatives, the Viviparidae and Ampullariidae. Certainly the cyclophorids are not as well adapted to a terrestrial life as are the Pulmonata, and in many respects they may be considered "aquatic" snails living on land. 相似文献
53.
A group of filamentous fungi were assayed for their ability to utilize a series of n-alkanes and 1-alkenes as the sole source of carbon. Although strains of Cunninghamella exhibited profuse growth on most of the hydrocarbons tested, the majority of fungi tested were able to produce definite growth on one or more of the compounds. The hydrocarbons with a 14-carbon chain length appeared to be more consistently utilized than any other. Strains of Aspergillus appeared to differ in their capacity to utilize individual members of the hydrocarbon series. Thin-layer chromatographic analyses of ether extracts from C. blakesleeana grown on n-tetradecane and 1-tetradecene were similar and revealed the presence of a monocarboxylic acid, a primary alcohol, and a secondary alcohol. 相似文献
54.
D W Allen 《Biochimica et biophysica acta》1967,133(1):180-183
55.
C. D. Cook F. Geller G. B. Hutchison P. Gerald F. H. Allen 《American journal of human genetics》1962,14(3):290-294
56.
57.
F. M. B. Allen 《BMJ (Clinical research ed.)》1964,2(5425):1645-1649
58.
The assimilation of H14CN by a variety of fungi 总被引:1,自引:0,他引:1
59.
60.
Studies with cyanidium caldarium,an anomalously pigmented chlorophyte 总被引:12,自引:0,他引:12
Mary Belle Allen 《Archives of microbiology》1959,32(3):270-277
Summary
Cyanidium caldarium, an alga found in acid hot springs troughout the world, has a morphology and developmental history resembling those of Chlorella, but contains C-phycocyanin and no chlorophyll other than chlorophyll a. The reasons for considering it to be a member of the Chlorophyta are reviewed.
Cyanidium is also remarkable for its thermal and acid tolerance. It grows readily in the dark on sugar media. However, light is required for the formation of chlorophyll and phycocyanin except in occasional variant cells which can form limited amounts of these pigments in the dark. Light-grown Cyanidium carries out normal green plant photosynthesis but resembles the red and some of the blue-green algae in that chlorophyll-absorbed light is used with lower efficiency than that absorbed by phycocyanin.The possible significance of the unusual pigmentation of Cyanidium is discussed.Contribution no.23 from the Laboratory of Comparative Physiology and Morphology of The Kaiser Foundation. 相似文献