全文获取类型
收费全文 | 1099篇 |
免费 | 52篇 |
国内免费 | 72篇 |
出版年
2023年 | 14篇 |
2022年 | 13篇 |
2021年 | 21篇 |
2020年 | 28篇 |
2019年 | 44篇 |
2018年 | 27篇 |
2017年 | 20篇 |
2016年 | 26篇 |
2015年 | 19篇 |
2014年 | 52篇 |
2013年 | 54篇 |
2012年 | 32篇 |
2011年 | 33篇 |
2010年 | 31篇 |
2009年 | 43篇 |
2008年 | 46篇 |
2007年 | 61篇 |
2006年 | 65篇 |
2005年 | 47篇 |
2004年 | 48篇 |
2003年 | 37篇 |
2002年 | 35篇 |
2001年 | 30篇 |
2000年 | 22篇 |
1999年 | 31篇 |
1998年 | 26篇 |
1997年 | 28篇 |
1996年 | 25篇 |
1995年 | 22篇 |
1994年 | 21篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 20篇 |
1988年 | 14篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 13篇 |
1984年 | 21篇 |
1983年 | 4篇 |
1982年 | 7篇 |
1981年 | 8篇 |
1980年 | 11篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 8篇 |
1973年 | 9篇 |
1972年 | 5篇 |
1970年 | 5篇 |
排序方式: 共有1223条查询结果,搜索用时 328 毫秒
171.
Arundo donax L. is a rhizomatous perennial, asexually reproducing species that has invaded riparian habitats throughout Mediterranean
climate zones. This research evaluated ramet demography of A. donax in two California riparian communities that differed in nitrogen availability. Quadrats were established along 100 m transects
at each site and oriented across the advancing fronts of established populations. Morphology and phenology were assessed monthly
over 1 year for calculation of demographic parameters and rhizomes were excavated and mapped at the end of the experiment.
A. donax exhibited seasonal patterns of recruitment but no dormancy at the high nitrogen site, while at the low nitrogen site no recruitment
occurred in the winter and maximum recruitment was delayed by a month relative to the high nitrogen site. Spread of A. donax was delayed until spring and lower overall in the low nitrogen site compared to the high nitrogen site, where lower initial
density, greater production of shoots, and higher linear and areal addition indicated that this population was spreading more
rapidly. Temperature and precipitation influenced seasonality and amount of recruitment of A. donax in this study. Several recently established, immature clumps were found in gaps at the low nitrogen site, likely due to flood-mediated
dispersal of propagules. Recruitment in these clumps occurred from shoot buds, in contrast to the mature populations that
reproduced from rhizome buds. Ecologically based management strategies for A. donax and other exotic species should account for differences such as those described here and be tailored to local conditions
where the species occurs. 相似文献
172.
173.
K. E. MOCK C. A. ROWE M. B. HOOTEN J. DEWOODY V. D. HIPKINS 《Molecular ecology》2008,17(22):4827-4844
Clonality is a common phenomenon in plants, allowing genets to persist asexually for much longer periods of time than ramets. The relative frequency of sexual vs. asexual reproduction determines long‐term dominance and persistence of clonal plants at the landscape scale. One of the most familiar and valued clonal plants in North America is aspen (Populus tremuloides). Previous researchers have suggested that aspen in xeric landscapes of the intermountain west represent genets of great chronological age, maintained via clonal expansion in the near absence of sexual reproduction. We synthesized microsatellite data from 1371 ramets in two large sampling grids in Utah. We found a surprisingly large number of distinct genets, some covering large spatial areas, but most represented by only one to a few individual ramets at a sampling scale of 50 m. In general, multi‐ramet genets were spatially cohesive, although some genets appear to be fragmented remnants of much larger clones. We conclude that recent sexual reproduction in these landscapes is a stronger contributor to standing genetic variation at the population level than the accumulation of somatic mutations, and that even some of the spatially large clones may not be as ancient as previously supposed. Further, a striking majority of the largest genets in both study areas had three alleles at one or more loci, suggesting triploidy or aneuploidy. These genets tended to be spatially clustered but not closely related. Together, these findings substantially advance our understanding of clonal dynamics in western North American aspen, and set the stage for a broad range of future studies. 相似文献
174.
Sid Staubach Peter Schützner Roger P. Croll Annette Klussmann-Kolb 《Zoomorphology》2008,127(4):203-212
This study describes the innervation patterns for the cerebral nerves which project to the cephalic sensory organs (CSOs)
in the opisthobranch Haminoea hydatis (Linnaeus 1758) and uses axonal tracing techniques (backfilling) to reveal the central cellular origins for these cerebral
nerves. Cell clusters projecting into the cerebral nerves can be defined by their positions in the ganglion relative to other
clusters, nerve roots and lobes. The number of cell clusters and the relative sizes of somata are constant in a given cluster,
whereas the absolute number of somata and absolute sizes of single somata in a given cluster increase with the size of the
animal. Additionally, the invariable morphological characteristics of the cell clusters are used to define criteria for the
assessment of possible homology for the clusters innervating the CSOs in Opisthobranchia. The data suggest two different strategies
to accommodate the increasing body size; first, the additions of nerve cells and second, the growth of nerve cells.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
175.
Tropical hardwood hammocks, among the rarest and most threatened vegetative communities in the United States, occur throughout the 225-km Florida Keys archipelago as it extends toward the Caribbean from the southeast tip of peninsular Florida. Compounding their critical conservation status, tropical hardwood hammocks and the dynamics that support their peculiar species diversity in the region are poorly understood. The goal of this study was to explore the dynamics of the species compositional gradient of the hammocks along the Florida Keys, and to identify significant ecological units within the gradient. The primary data for this research were assembled from the Institute for Regional Conservation's floristic database of South Florida. We were able to extract presence/absence data for 295 species from comprehensive surveys of 23 study sites. Nonmetric multidimensional scaling was used to deconstruct the compositional trends into a reduced ordination space. Cluster analysis was subsequently used to identify discrete ecological units. Additionally, we used vector fitting to interpret the significant correlated ancillary variables. Our main results were three well-fitted nonmetric multidimensional scaling axes with three nonoverlapping ecological units. Of the ancillary variables, latitude, longitude, percent composition from biogeographical regions, richness, and area were correlated to the nonmetric multidimensional scaling results. These results increase our understanding of the community structure of the hammocks along the Florida Keys, and can contribute to increasing our ability to adequately protect and restore tropical hardwood hammocks and other similar tropical dry forest communities throughout the Caribbean. 相似文献
176.
The gene Brassica campestris male fertility 13 (BcMF13, GenBank accession number EF158459) was isolated as a reproductive organ-specific gene from Chinese cabbage (Brassica campestris L. ssp. chinensis Makino, syn. B. rapa ssp. chinensis). It is exclusively expressed in stage four and five flower buds of fertile lines and is most strongly expressed in stamens.
Here, we report a functional characterization of this BcMF13 gene in the antisense-silenced plants. The inflorescence of the BcMF13 mutant was compacted with anthers curved outside. The fertility of this mutant was greatly reduced with less than 5 seeds
per silique. Under scanning electron microscopy, the mutant demonstrated numerous shriveled pollen grains with deep invaginations.
The frequency of normal pollen grains was just 45.34%. The pollen mother cell, the tetrad, and the mature pollen of the BcMF13 mutant were abnormal resulting in the poor pollen vitality. Germination test in vivo suggested BcMF13 delayed the pollen tubes’ extension in the style. All these indicated BcMF13 had a vital role in pollen development of Chinese cabbage. 相似文献
177.
月平均采集并解剖滇蛙14—16只(雌雄约各半),测量体长、体重、脂肪体重、精巢重、卵巢重、精巢体积和输卵管直径等。以体长为协变量做协方差分析,结果显示,精巢重、卵巢重、精巢体积、输卵管直径在年周期内差异显著,呈周年规律性变化。滇蛙的繁殖期为4—6月,5月下旬至6月上旬为繁殖高峰。异速生长分析和回归分析结果显示,性腺发育与体重和体长多呈正异速生长(b>1),个体增长对性腺的发育有显著影响。个体较大的滇蛙年生殖频次至少为2次,这是提高繁殖成功率的繁殖策略之一。性腺发育与脂肪体重呈负异速生长(b<1)。脂肪体为繁殖和冬眠提供能量储备,脂肪体重在繁殖高峰期达最小值。少数脂肪储存不足的个体通过延迟冬眠时间积累能量,以便安全越冬。相关性分析和交叉相关分析结果显示,两性生殖腺的发育具有显著的相关性,但生殖腺的发育并非完全同步。雄蛙精巢的发育具有一定的滞后性,因为卵的成熟比精子的成熟需要更长的时间。但精巢体积和输卵管直径发育具同步性,这恰是为授精做好了准备。 相似文献
178.
Einarsson S Andersson K Wallgren M Lundström K Rodriguez-Martinez H 《Theriogenology》2009,71(2):302-310
The objective of this study was to determine the short and long term effects of a gonadotropin-releasing hormone (GnRH) vaccine (Improvac™ Pfizer Ltd.), on sexual maturity, development of the reproductive organs, and the morphology of caudal epididymal spermatozoa in non-castrated male pigs. The pigs were slaughtered 4, 16 or 22 weeks after the second Improvac™ vaccination. A total of 80 crossbred non-castrated male pigs were included in this study comprising two experiments, a short-effect (Experiment 1) and a long-effect (Experiment 2). The first experiment included 56 pigs, 24 of them were maintained as controls and 32 were vaccinated twice, and slaughtered 4 weeks after the second vaccination. The second experiment included 24 pigs, 12 controls and 12 vaccinated twice, and slaughtered either 16 weeks (n = 6) or 22 weeks (n = 6) after the second vaccination. None of the immunized pigs was sexually mature at slaughter, i.e. 4, 16 or 22 weeks after second vaccination. Corresponding results of the control pigs showed that 50% had reached sexual maturity at the age corresponding to 4 weeks after the second vaccination, and 100% at slaughter 16, respectively, 22 weeks after vaccination. At 4, 16 and 22 weeks after second vaccination both testes weight and bulbourethral length were significantly reduced (p < 0.001). The percentages of proximal droplets and abnormal heads were significantly lower in the control pigs than in the immunized pigs at slaughter 4 weeks after vaccination, whereas distal droplets were higher. For the other morphological parameters no significant differences were seen, but all mean values except for acrosome defects were numerically lower in the control pigs compared with the immunized pigs. For pigs slaughtered 16 or 22 weeks after vaccination, the vaccination effect was significant for percentages of proximal droplets, distal droplets, acrosome defects, acrosome abnormality and abnormal heads (p = 0.017-0.001). The immunization clearly disrupted the number and morphology of the interstitial Leydig cells, lasting throughout the study period (4-22 weeks after vaccination). Spermatogenesis was also clearly affected in the immunized pigs, to various degrees, from mild disruption (spermatocyte loss, decrease of the normal number of layers of germ cells) to severe loss of germ cells including tubuli with Sertoli cells-only (complete disappearance of germ cells), also covering the entire study period. The results indicated that the effect of immunization persisted for at least 22 weeks after the second vaccination. 相似文献
179.
柄天牛属八种(亚种)雌性生殖器的比较研究(鞘翅目,天牛科,天牛亚科) 总被引:1,自引:1,他引:0
记述了柄天牛属8个种(亚种)的雌性生殖器,比较研究了该属雌性生殖器的结构特点.结果表明:柄天牛属雌性生殖器在骨杆形状、受精囊管长短等方面存在不同程度的差异;通过雌性生殖器特征对比,黄颈柄天牛、暗胸柄天牛、红腹柄天牛三者虽然存在一定差异,是否应作为同一种依据不足,故仍保留亚种地位. 相似文献
180.
The effect of expression of the enzyme producing nitric oxide on Drosophila melanogaster morphogenesis was studied using ectopic expression of a truncated splice form of the enzyme that has a dominant negative effect on the full-length enzyme form. The suppressed enzyme expression was shown to affect the cell polarity and movement as well as the structural pattern of organs during Drosophila development. The effect of nitric oxide production on the intercellular distribution of significant factors of cell polarity, protein products of the Strabismus and Prospero genes, has been revealed. 相似文献