全文获取类型
收费全文 | 2572篇 |
免费 | 221篇 |
国内免费 | 86篇 |
出版年
2024年 | 6篇 |
2023年 | 41篇 |
2022年 | 43篇 |
2021年 | 96篇 |
2020年 | 87篇 |
2019年 | 92篇 |
2018年 | 62篇 |
2017年 | 85篇 |
2016年 | 99篇 |
2015年 | 93篇 |
2014年 | 104篇 |
2013年 | 153篇 |
2012年 | 89篇 |
2011年 | 106篇 |
2010年 | 83篇 |
2009年 | 107篇 |
2008年 | 127篇 |
2007年 | 110篇 |
2006年 | 119篇 |
2005年 | 102篇 |
2004年 | 94篇 |
2003年 | 89篇 |
2002年 | 90篇 |
2001年 | 77篇 |
2000年 | 66篇 |
1999年 | 73篇 |
1998年 | 62篇 |
1997年 | 51篇 |
1996年 | 60篇 |
1995年 | 39篇 |
1994年 | 51篇 |
1993年 | 35篇 |
1992年 | 35篇 |
1991年 | 35篇 |
1990年 | 31篇 |
1989年 | 23篇 |
1988年 | 17篇 |
1987年 | 20篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 14篇 |
1983年 | 9篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 12篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1967年 | 2篇 |
排序方式: 共有2879条查询结果,搜索用时 399 毫秒
211.
南瓜茎、叶、花的营养成分分析 总被引:5,自引:0,他引:5
对南瓜茎、叶、花等部位进行了粗脂肪、粗蛋白、粗纤维、总糖、Vc等主要营养成分和K、Na、Ca、Mg、Zn等矿质元素的测定。结果表明,南瓜茎、叶、花营养丰富,各种营养成分较为全面。南瓜叶中粗脂肪、粗蛋白、Vc、K、Ca、Mg、Mn、Fe、Zn等成分含量均高于南瓜茎和南瓜花。与其他8种常见蔬菜相比,主要营养成分和矿质元素含量也远远高于这8种蔬菜。为南瓜茎、叶、花的菜用价值提供了科学依据,也为增加人们的膳食营养、改变餐桌食品花样、开发蔬菜资源及其利用价值提供了参考。 相似文献
212.
1. Although leaf morphology and chemistry show profound changes as trees age, the consequences of such changes to herbivory have received little attention, particularly late in the ontogeny of canopy trees. 2. Using a mobile aerial lift for canopy access, patterns of leaf damage were evaluated in canopy‐dominant mature sugar maple (Acer saccharum Marsh) trees ranging from ~20 to 70 cm in diameter, corresponding to an age range of ~40–180 years. 3. Herbivore damage patterns varied in relation to both vertical canopy position (among upper‐, mid‐, and lower‐canopy positions) and with tree size. Damage types attributable to herbivores active on leaf surfaces, including leaf skeletonizers and leaf cutters (both principally Lepidoptera), and leaf stippling inducers (Hemiptera) showed decreases with tree size, and with increasing height in the canopy. In contrast, leaf damage from the most abundant gall‐forming arthropod in the system, the eriophyid mite Vasates aceriscrumena, increased markedly with tree size. 4. The results indicate that herbivory patterns vary with both canopy stratum and with tree size in sugar maple, and that the relative strength of vertical stratification and tree ontogeny effects are similar in magnitude. The predominant patterns are of a decrease in herbivory with increasing height in the canopy and with tree size, but certain galling arthropods exhibit the reverse trends. 相似文献
213.
214.
Alberto Pérez-Huerta 《Palaeontology》2004,47(6):1519-1538
Terebratuloid brachiopods from two localities in the eastern Great Basin, Nevada, USA, include the new taxa Cryptacanthia savagei sp. nov., Fletcherithyris infrequens sp. nov., Cryptonella simplex sp. nov., and Albelenina alvarezi gen. et sp. nov. The faunas are considered to be mid Desmoinesian (late Moscovian) in age. The brachiopods were found associated in clusters, and many of the specimens are well preserved. Records of Pennsylvanian terebratuloid brachiopods are uncommon, and these faunas have enabled an understanding of internal features previously unknown or poorly understood. Systematic analyses of the faunas have generated some new concepts on the ontogeny and evolution of the loop in Late Palaeozoic terebratuloids. From ontogenetic analyses, it is inferred that the stage of loop development may be the most important aspect in taxonomic classification. Analyses of loop evolution suggest the possibility of a transition from a teloform stage to long-flanged deltiform and deltiform stages. These transitions may be possible through reduction in length of the descending lamellae of the crura by resorption of calcium carbonate or posteroventral folding of flanges. Subsequently, these changes in loop morphology are retained by paedomorphosis within long-term evolutionary processes. 相似文献
215.
H. T. Lumbsch 《Plant biology (Stuttgart, Germany)》1999,1(3):321-326
Abstract: The structure and development of the ascomata in Mycoporum elabens were studied. No pseudoparaphyses or para-physoids are developed but the asci grow into a network formed by stretched ascostromatic tissue which appears similar to pseudoparaphyses at the mature stage. The ascomata do not open regularly but burst open at several points, thus imitating plurilo-cularous stromata. This development agrees with that observed for the Dothideales sensu stricto and so it is proposed to place the Mycoporaceae here. Although algal cells were found in the surroundings of the ascomata, they did not have close contact with the fungal hyphae. Thus, Mycoporum elabens is interpreted as being non-lichenized. 相似文献
216.
Inflorescence and floral development of two tropical legume trees, Dahlstedtia pinnata and Dahlstedtia pentaphylla, occurring in the Atlantic Forest of south-eastern and southern Brazil, were investigated and compared with other papilionoids. Few studies have been made of floral development in tribe Millettieae, and this paper is intended to fill that gap in our knowledge. Dahlstedtia species have an unusual inflorescence type among legumes, the pseudoraceme, which comprises axillary units of three or more flowers, each with a subtending bract. Each flower exhibits a pair of opposite bracteoles. The order of flower initiation is acropetal; inception of the floral organs is as follows: sepals (5), petals (5), carpel (1) plus outer stamens (5) and finally inner stamens (5). Organ initiation in sepal, petal and inner stamen whorls is unidirectional; the carpel cleft is adaxial. The vexillum originates from a tubular-shaped primordium in mid-development and is larger than other petals at maturity, covering the keels. The filament tube develops later after initiation of inner-stamen primordia. Floral development in Dahlstedtia is almost always similar to other papilionoids, especially species of Phaseoleae and Sophoreae. But one important difference is the precocious ovule initiation (open carpel with ovules) in Dahlstedtia, the third citation of this phenomenon for papilionoids. No suppression, organ loss or anomalies occur in the order of primordia initiation or structure. Infra-generic differences in the first stages of ontogeny are rare; however, different species of Dahlstedtia are distinguished by the differing distribution pattern of secretory cavities in the flower. 相似文献
217.
MARTIN RODRIGUEZ-PONTES 《Botanical journal of the Linnean Society. Linnean Society of London》2007,153(2):169-179
The main aspects of seed ontogeny in Senna corymbosa were studied by standard anatomical microtechniques for light microscope observations. The results revealed an ana-campylotropous, bitegmic, and crassinucelate mature ovule. A single archesporocyte developed by an archesporial cell enlargement from the subhypodermal multicellular archesporium. Meiosis originated linear or T-shaped megasporic tetrads. The functional megaspore was the chalazal one. Megagametophytic development conformed to the Polygonum type. Fertilization was porogamic. Endosperm development was free nuclear and conformed to a chalazal haustorium. Cellular endosperm was initiated from the micropylar end during the globular embryo stage. Embryogeny derived from a linear proembryonal tetrad. The mature embryo showed an oblique axis. The testa derived from the outer ovular integument. Nucellar and endosperm remnants, and the micropylar region of the inner ovular integument, persisted at embryo maturity. The absence of a pleurogram would be adaptative to wetland habitats. The taxonomic use of the mature embryo axis in the Cassieae and the phylogenetic employment of megasporic arrangements in Leguminosae needs some reinterpretation. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 153 , 169–179. 相似文献
218.
The triacyl anthocyanins, Leschenaultia blue anthocyanins 1 and 2 (LBAs 1 and 2) were isolated from the blue flowers of Leschenaultia R. Br. cv. Violet Lena (Goodeniaceae), in which LBA 1 was present as a dominant pigment. The structure of LBA 1 was elucidated to be delphinidin 3-O-[6-O-(malonyl)-beta-D-glucopyranoside]-7-O-[6-O-(4-O-(6-O-(4-O-(beta-D-glucopyranosyl)-trans-caffeoyl)-beta-D-glucopyranosyl)-trans-caffeoyl)-beta-D-glucopyranoside] by application of chemical and spectroscopic methods. Since LAB 2 was isolated in small amount, its structure was tentatively assigned as either delphinidin 3-(malonylglucoside)-7-[(glucosyl-p-coumaroyl)-(glucosylcaffeoyl)-glucoside] or delphinidin 3-(malonyl-glucoside)-7-[(glucosyl-caffeoyl)(glucosyl-p-coumaroyl)-glucoside]. This is the first report of the occurrence of 7-polyacylated anthocyanins in the family of Goodeniaceae, although others have been found in the families of the Ranunculaceae, Campanulaceae, and Compositae. Moreover, delphinidin 3-glycoside-7-di-(glucosylcaffeoyl)-glucoside has been reported only in the flowers of Platycodon grandiflorum (Campanulaceae). From a chemotaxonomical viewpoint, the Goodeniaceae may be closely related to the Campanulaceae. 相似文献
219.
220.