全文获取类型
收费全文 | 12267篇 |
免费 | 1093篇 |
国内免费 | 6篇 |
专业分类
13366篇 |
出版年
2022年 | 76篇 |
2021年 | 152篇 |
2020年 | 91篇 |
2019年 | 118篇 |
2018年 | 164篇 |
2017年 | 125篇 |
2016年 | 236篇 |
2015年 | 407篇 |
2014年 | 447篇 |
2013年 | 555篇 |
2012年 | 737篇 |
2011年 | 721篇 |
2010年 | 503篇 |
2009年 | 445篇 |
2008年 | 707篇 |
2007年 | 663篇 |
2006年 | 639篇 |
2005年 | 697篇 |
2004年 | 670篇 |
2003年 | 650篇 |
2002年 | 654篇 |
2001年 | 114篇 |
2000年 | 93篇 |
1999年 | 156篇 |
1998年 | 194篇 |
1997年 | 124篇 |
1996年 | 130篇 |
1995年 | 127篇 |
1994年 | 120篇 |
1993年 | 112篇 |
1992年 | 103篇 |
1991年 | 89篇 |
1990年 | 101篇 |
1989年 | 88篇 |
1988年 | 87篇 |
1987年 | 86篇 |
1986年 | 87篇 |
1985年 | 118篇 |
1984年 | 143篇 |
1983年 | 113篇 |
1982年 | 137篇 |
1981年 | 143篇 |
1980年 | 151篇 |
1979年 | 82篇 |
1978年 | 94篇 |
1977年 | 83篇 |
1976年 | 69篇 |
1975年 | 77篇 |
1974年 | 92篇 |
1973年 | 84篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
Catherine E. Rogers Karl R. Mattox Kenneth D. Stewart 《American journal of botany》1980,67(5):774-783
Chlorokybus atmophyticus has a sarcinoid growth habit and produces scale-covered zoospores. Flagella are laterally inserted and attached internally to a multilayered structure characteristic of the Charophyceae. There are two kinds of pyrenoid in each cell, a feature previously observed in only one scaly green flagellate. C. atmophyticus demonstrates that the sarcinoid growth habit arose independently at least twice in the green algae and cannot be used to define taxonomic groups unless combined with other criteria. It is further concluded that C. atmophyticus should be classified in a separate family Chlorokybaceae and a separate order Chloroky bales. 相似文献
12.
13.
14.
15.
Spherical, membrane-bound inclusions occur in the proplastids and vacuoles of cells of Bryophyllum and Kalanchoë shoot apices. Evidence is presented indicating that the inclusions arise by the accumulation of material within the cisternae of certain tubular proplastid membranes and are then transferred to vacuoles. The results obtained from electron microscopy and from histochemical studies indicate that the contents of the inclusions are predominantly lipid. 相似文献
16.
17.
18.
19.
20.
The Yanomama Indians of Southern Vanezuela and Northern Brazil are one of the largest, relatively unacculturated tribes of the tropical rain forest. Over a period of eight years data have been collected from a considerable portion of their territory on estimated age, sex ratio, fertility rates (as determined by physical examination and urine tests), and infant death rates. Although it has been impossible to collect direct data on infanticide, this subject can be approached indirectly through distortions of the sex ratio and anecdotal information. Some historical data are also available as a basis for estimating tribal expansion in the past 100 years. With this material it has been possible to construct Life Tables for the Yanomama, and to explore the results of various perturbations of the input parameters. Data are also presented on patterns of mating and reproduction: number of spouses, mean and variance in number of surviving children, frequency of “extra-marital conceptions” based on the results of extensive blood group typings, and consanguinity rates as determined by observation and computer simulation. Although we do not present the Yanomama as typical, these data are seen as providing a basis for more realistic population models than have existed in the past. In addition, the data provide a basis for relatively precise estimates of such demographic measures as Fisher's Reproductive Value, Crow's Index of Total Selection, and Weiss' Index of Growth Regulation. 相似文献