全文获取类型
收费全文 | 2052篇 |
免费 | 23篇 |
国内免费 | 85篇 |
出版年
2023年 | 3篇 |
2022年 | 7篇 |
2021年 | 9篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 12篇 |
2017年 | 6篇 |
2016年 | 11篇 |
2015年 | 12篇 |
2014年 | 24篇 |
2013年 | 38篇 |
2012年 | 28篇 |
2011年 | 32篇 |
2010年 | 23篇 |
2009年 | 84篇 |
2008年 | 92篇 |
2007年 | 88篇 |
2006年 | 106篇 |
2005年 | 70篇 |
2004年 | 73篇 |
2003年 | 37篇 |
2002年 | 45篇 |
2001年 | 53篇 |
2000年 | 100篇 |
1999年 | 70篇 |
1998年 | 98篇 |
1997年 | 88篇 |
1996年 | 113篇 |
1995年 | 105篇 |
1994年 | 82篇 |
1993年 | 74篇 |
1992年 | 68篇 |
1991年 | 71篇 |
1990年 | 69篇 |
1989年 | 66篇 |
1988年 | 54篇 |
1987年 | 38篇 |
1986年 | 32篇 |
1985年 | 51篇 |
1984年 | 29篇 |
1983年 | 7篇 |
1982年 | 15篇 |
1981年 | 12篇 |
1980年 | 20篇 |
1979年 | 13篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1976年 | 6篇 |
排序方式: 共有2160条查询结果,搜索用时 15 毫秒
81.
82.
Summary For 25 Medicago sativa plants, measurements were made of in vitro pollen germination and growth and callus growth on mediums containing culture filtrate from two isolates of Fusarium oxysporum f. sp. medicaginis. In vivo resistance was determined by field inoculation with one isolate. There was no correlation between any in vitro measurement and in vivo resistance, and none of the in vitro measurements on control mediums were correlated. The only significant correlation for the same measurement between the two isolates was for pollen-tube length (r = 0.65). Percent of pollen germination was negatively correlated with callus growth for both isolates. Earlier work showed that callus growth in the presence of culture filtrate was linked to plant resistance, thus it appeared that percent pollen germination on culture filtrate had decreased for the more resistant plants. The haploid pollen may be used as a progeny test for identifying heterotic loci conferring resistance to Fusarium, but if the pollen of this plant species is used in direct selection by exposure to culture filtrate, the level of resistance to Fusarium may decrease. 相似文献
83.
M. G. Galli M. Bracale A. Falavigna F. Raffaldi C. Savini A. Vigo 《Sexual plant reproduction》1993,6(1):16-21
Summary In the dioecious plant Asparagus officinalis L. the female plants bear flowers that are all strictly of the same type, with well-developed pistils and collapsed and consistently sterile rudiments of anthers, while male plants, on the contrary, show a great variety of vestigial female organs, from small, rudimentary ovaries with no style and stigma, up to pistils provided with a rather long style that is often enlarged in a stigma. In our investigations, we used homozygous male and female doubled haploid plants obtained from in vitro anther culture, the all-male F1 progeny and male individuals from subsequent backcrosses. The results showed that: (1) the character length of the style is genetically inherited and involves at least two genes, the influence of the environment being quite negligible; (2) in male pistils provided with style and stigmatic papillae, the pollination and growth of the pollen tubes up to the ovules do actually occur as a rule, the only barrier to fertilization being the absence of normal embryo sacs inside the ovules; (3) the character length of the style is a very reliable marker of the trend towards hermaphroditism in Asparagus, since a correlation always exist between length of the style, size of the ovary, tendency to self-pollination, vascularization and rate of development of the ovules inside the male ovaries. On the whole, most of our observations, together with the high inbreeding depression observed when occasional andromonoecious plants are selfed, are consistent with the hypothesis of the origin of dioecy in Asparagus from hermaphroditism via the gynodioecy pathway. 相似文献
84.
The effects of grazing on the pollen production of grasses 总被引:3,自引:3,他引:0
W. Groenman-van Waateringe 《Vegetation History and Archaeobotany》1993,2(3):157-162
Samples from moss polsters taken over three consecutive years during a controlled grazing experiment in a nature reserve in the Netherlands were analysed for their pollen content, particularly grass pollen. Samples were also taken in the year following the experiment, with grazing pressure strongly diminished. This yielded data regarding the effect of grazing on the pollen production of grasses in open habitats, plantations, and unproductive and productive woods under different grazing pressures. 相似文献
85.
Pollen, phytolith, and plant macrofossil analyses were carried out on the cultural layers of Helsinki Old Town excavated by the Helsinki City Museum and dated to the 15–17th centuries. Cultural mineral soil layers, a former ditch, a well, one waste pit, one waste heap and three house areas were investigated. The results show that-within certain limits-pollen in the mineral soil reflects the cultural changes of the local settlement from rural to urban. Among the main anthropogenic indicators, the high NAP (non-arboreal pollen) frequencies, the low pollen concentration values, and especially the high frequencies of Cichoriaceae (Liguliflorae) pollen should be emphasized. The macrofossil data were used to determine the species list of the herb pollen and to make a more detailed reconstruction of the local vegetation at Helsinki Old Town. 相似文献
86.
乌云伞的解剖学及花粉形态学研究 总被引:3,自引:0,他引:3
八角莲属(Dysosma)是小檗科(Berberidaceae)一个重要属,全国共有7个种,大约分布于北纬23—32°,东经94—122°之间。湖北八角莲属有4个种,其中乌云伞(Dysosma lichuanensis Z. Cheng, sp. nev., ined.)为内定种,是多年生草本,它的形态特征与八角莲(Dysosma 相似文献
87.
两种药用黄芪比较生物学研究 总被引:10,自引:1,他引:9
通过栽培实验,本文对两种药用黄芪的个体发育节律,根,茎,叶,花及种子的生物学特性进行了较系统的比较研究,为确定蒙古黄芪(AstragalusmongholicusBunge)为独立的种,提供了更为全面的依据。 相似文献
88.
空间条件对几种粮食作物的同工酶和细胞学特性的影响 总被引:47,自引:0,他引:47
搭载1987年发射的两颗返地卫星(8月5—10日和9月9—17日)的几种小麦、大麦种子,经回收后初步研究表明:(1)对发芽率的影响:空间处理的种子,其发芽率与对照无差别;(2)对茎及生育期的影响:空间处理的种子长成的幼苗比对照强壮,植株高度有些品种明显的矮于对照。生育期有不同程度延长;(3)对染色体结构的影响:花粉母细胞减数分裂观察表明搭载卫星的种子当代可诱导出比地面更多的染色体桥、落后染色体及染色体数目异于正常体细胞数目的现象。小麦品种Cocorit—71经空间处理后花粉母细胞的染色体数目异常可高达20.51%,而地面对照组其花粉母细胞基本正常(2n=28);(4)对同工酶的影响:经空间处理的小麦种子酯酶、过氧化物酶的同工酶谱带空间处理的比地面对照减少。由此可见,空间处理可诱导纯合植物种子发生变异,为农作物育种创造一个新途径。 相似文献
89.
E. Pacini 《Sexual plant reproduction》1996,9(6):362-366
During pollen development, soluble carbohydrates of sporophytic origin may be consumed immediately, polymerized to form starch
reserves or intine, or transformed into other molecules. Disregarding intine, in mature pollen there are three different types
of carbohydrates: (1) polysaccharides such as starch in amyloplasts or polysaccharides in cytoplasmic vesicles, (2) disaccharides
such as sucrose and (3) monosaccharides such as glucose and fructose. At dispersal, pollen may be partly or slightly dehydrated,
or not dehydrated at all. Partly dehydrated pollen has the capacity to lose or acquire water within limits without detriment
to its viability. Slightly and non-dehydrated pollen is vulnerable to water loss and quickly becomes inviable. In partly dehydrated
of pollen the carbohydrates consist of cytoplasmic polysacharides and sucrose; in slightly and non-dehydrated pollen these
are absent or in low concentrations but there may be reserves of cytoplasmic callose. Starch, glucose and fructose are found
in both types. It is postulated that cytoplasmic carbohydrates and sucrose are involved in protecting pollen viability during
exposure and dispersal. 相似文献
90.
Pollen allergens interact with the human immune system and the resulting IgE antibodies provide specific probes for their
identification and characterisation. In one case, grass allergenic proteins are expressed late in pollen development coincident
with the laying down of reserves. Sequence similarity of allergens has indicated possible functions for some allergens. The
major birch pollen allergen shows sequence similarity with pathogenesis-related proteins, which form a secondary response
in plant host-pathogen interactions and show anti-microbial activity. Some allergens of unknown function are cysteine-rich
proteins, while some others have cysteine-rich regions; for example, the major allergen from rye-grass pollen, Lol p 1, has
a cysteine-rich N-terminal region, while at the C-terminal region four tryptophan residues together with tyrosine and phenylalanine
residues resemble those of cellulose- or sugar-binding domains of other proteins. Several pollen allergens show sequence similarity
to cell wall-associated enzymes, while others show hydrolytic enzyme activity often associated with cell walls. 相似文献