全文获取类型
收费全文 | 7657篇 |
免费 | 689篇 |
国内免费 | 448篇 |
出版年
2024年 | 4篇 |
2023年 | 477篇 |
2022年 | 191篇 |
2021年 | 324篇 |
2020年 | 386篇 |
2019年 | 305篇 |
2018年 | 340篇 |
2017年 | 287篇 |
2016年 | 356篇 |
2015年 | 403篇 |
2014年 | 399篇 |
2013年 | 573篇 |
2012年 | 395篇 |
2011年 | 374篇 |
2010年 | 333篇 |
2009年 | 431篇 |
2008年 | 295篇 |
2007年 | 314篇 |
2006年 | 315篇 |
2005年 | 256篇 |
2004年 | 233篇 |
2003年 | 187篇 |
2002年 | 191篇 |
2001年 | 154篇 |
2000年 | 148篇 |
1999年 | 157篇 |
1998年 | 125篇 |
1997年 | 107篇 |
1996年 | 101篇 |
1995年 | 79篇 |
1994年 | 60篇 |
1993年 | 70篇 |
1992年 | 58篇 |
1991年 | 60篇 |
1990年 | 32篇 |
1989年 | 41篇 |
1988年 | 33篇 |
1987年 | 31篇 |
1986年 | 21篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 10篇 |
1982年 | 18篇 |
1981年 | 15篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1973年 | 2篇 |
排序方式: 共有8794条查询结果,搜索用时 15 毫秒
21.
K. Kortmulder 《Acta biotheoretica》1987,36(4):275-280
The potential and realized impact of Bohm's views on biology are briefly discussed. 相似文献
22.
23.
24.
D. EISIKOWITCH Z. LAZAR 《Botanical journal of the Linnean Society. Linnean Society of London》1987,95(2):101-111
Flower change in Oenothera drummondii Hooker as a response to pollinators' visits. The colour and shape of flowers of Oenothera drummondii change owing to senescence, pollination and/or nectar withdrawal. This phenomenon is interpreted as a signal emitted by the plant to 'inform' the potential pollinator of its new status as a rewardless flower. This method of signalling might be important for the pollinators in order to save energy in seeking the correct flowers and for the plant to ensure visits to those flowers that have not yet been visited. 相似文献
25.
Regional environments, life-history patterns, and habitat use of spirostreptid millipedes in arid regions 总被引:1,自引:0,他引:1
C. S. CRAWFORD K. BERCOVITZ M. R. WARBURG 《Zoological Journal of the Linnean Society》1987,89(1):63-88
Among diplopods with desert populations, only three species of Spirostreptida have been studied in an ecological context. The present review compares regional environments, life-history patterns, and uses of habitat by Orlhoporus ornatus (Girard) from southwestern North America, Archispirostreptus tumuliporus judaicus (Attems) from the eastern Mediterranean seaboard, and Harpagophora nigra (Attems) from southwestern Africa. Published and unpublished studies are used to explore evidence for convergence among these species, as opposed to traits adapting them to physical aspects of given regions or habitats. Unlike A. t. judaicus, O. ornatus and H. nigra are relatively restricted to arid habitats, although populations of all three species experience a variety of rainfall regimes and regional topographies. Where studied, O. ornatus and H. nigra hibernate during the long, often cool or cold dry season; they forage following warm-season rains. A. t. judaicus , in contrast, forages during its long, warm dry season and hibernates in the cool, wet winter. Populations from the Judaean and Negev deserts differ from those inhabiting a mesic habitat (Megiddo) closer to the coast in regard to rates of development, seasonal activity and seasonal water balance. Convergence in the form of well-developed desiccation resistance characterizes the two strictly desert species. All three species, together with other subtropical millipedes exposed to long dry seasons, are convergent with respect to patterns of diel surface activity and use of shelter. However species- and habitat-specific life-history features such as the seasonal timing of dormancy and emergence tend to mask convergence at the habitat level. Hence, the independent evolution of the three species with desert populations has resulted in life histories and habitat use that combine a moderate amount of convergence with considerable opportunistic adaptation to regional and local conditions. 相似文献
26.
The cost of reproduction in the glaucous-winged gull 总被引:3,自引:1,他引:2
W. V. Reid 《Oecologia》1987,74(3):458-467
Summary Experimental enlargement of brood size in the glaucous-winged gull (Larus glaucescens) resulted in increased adult foraging time, decreased adult body weight at the end of the breeding season, and decreased over-winter adult survival. The decreased survival of breeding adults was associated with reduced body condition at the end of breeding (resulting from physiological costs of reproduction). Decreased survival was not due to an increased risk of injury or predation during the breeding season. Brood size did not directly affect the fecundity of surviving birds in the subsequent year. However, brood size may have an indirect effect on subsequent fecundity because the probability of mate loss increased among birds with large broods and the reproductive performance of birds with new mates was reduced. Based on estimates of life-time fitness calculated from fecundity and survivorship, birds with two- or three-chick broods (the normal brood size) have higher fitness than birds with one- or four-chick broods. However, the decreased fitness of birds with four-chick broods was slight, and probably not a sufficient explanation for the absence of natural four-chick broods in the glaucouswinged gull. 相似文献
27.
Advances in salt tolerance 总被引:6,自引:0,他引:6
Summary Advances in and prospects for the development of salt tolerant crops are discussed. The genetic approach to the salinity problem
is fairly new, but research has become quite active in a short span of time. Difficulties and opportunities are outlined.
Salinity varies spatially, temporally, qualitatively, and quantitatively. In addition, the responses of plants to salt stress
vary during their life cycle. Selection and breeding, including the use of wide crosses, are considered the best short-term
approaches to the development of salt tolerant crops, but the new biotechnological and molecular biological techniques will
make increasingly important contributions. Cooperation is called for among soil and water scientists, agronomists, plant physiologists
and biochemists, cytologists, and plant geneticists, breeders, and biotechnologists. Given such cooperation and adequate support
for these endeavors, the potential for increasing productivity in salt-affected areas can be realized. 相似文献
28.
Preliminary data on the skeletal biology of 78 Taino skeletons belonging to Juan Dolio, an archaeological site of the Maguana
province, 80 Km. east of S. Domingo, are presented. The minimum number of individuals, sex, age, stature, and morphologic
and morphometric characters were determined. Dental wear and pathology of cranial and post-cranial bones were also recorded. 相似文献
29.
Takakazu Yumoto 《Ecological Research》1987,2(2):133-145
Animal pollination in a warm temperate evergreen broad-leaved forest was observed on Yaku-shima Island, south of Kyushu, Japan.
Three groups of plants were categorized: canopy-flowering tree species, understory-flowering tree species, and climber and
epiphyte species. Each of these formed different pollination systems. The canopy-flowering tree species had shallow, dish-shaped
flowers and utilized various types of opportunistic pollinators. Most of the climber and epiphyte species had deep, tube-shaped
flowers and specialized pollinators, although some climber species which bloomed in the canopy especially in winter, had opportunistic
pollinators. The understory-flowering tree species had large dish- or funnel-shaped flowers and endothermic pollinators able
to tolerate the dark and cold conditions under the canopy. The individual trees of canopy-flowering tree species produced
large numbers of flowers simultaneously (mass-flowering) and had a well synchronized flowering period. Each canopy-flowering
tree species segregated its flowering time from those of the anothers. Climber and epiphyte species and most of the understory-flowering
tree species produced small numbers of flowers sequentially (extended flowering) and showed a long flowering period. 相似文献
30.
J. B. T. M. Roerdink 《Journal of mathematical biology》1989,27(3):309-319
In a previous paper (J. Math. Biol. 26, 199–215 (1988)) we calculated the mean and variance of the long-run geometric growth rate of a discrete-time population model with two age classes in a random environment. The formula which was used in that paper as the starting point for the computation of the variance represents only the contribution of the one-period variances. Here we supplement these results by a calculation of the exact variance. All qualitative conclusions reached before are unaffected. 相似文献