全文获取类型
收费全文 | 2316篇 |
免费 | 181篇 |
国内免费 | 54篇 |
专业分类
2551篇 |
出版年
2024年 | 5篇 |
2023年 | 38篇 |
2022年 | 30篇 |
2021年 | 54篇 |
2020年 | 75篇 |
2019年 | 78篇 |
2018年 | 74篇 |
2017年 | 61篇 |
2016年 | 67篇 |
2015年 | 94篇 |
2014年 | 105篇 |
2013年 | 142篇 |
2012年 | 64篇 |
2011年 | 90篇 |
2010年 | 85篇 |
2009年 | 113篇 |
2008年 | 114篇 |
2007年 | 110篇 |
2006年 | 102篇 |
2005年 | 75篇 |
2004年 | 81篇 |
2003年 | 87篇 |
2002年 | 77篇 |
2001年 | 71篇 |
2000年 | 47篇 |
1999年 | 52篇 |
1998年 | 50篇 |
1997年 | 45篇 |
1996年 | 40篇 |
1995年 | 47篇 |
1994年 | 38篇 |
1993年 | 31篇 |
1992年 | 37篇 |
1991年 | 27篇 |
1990年 | 26篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 14篇 |
1984年 | 19篇 |
1983年 | 5篇 |
1982年 | 10篇 |
1981年 | 23篇 |
1980年 | 23篇 |
1979年 | 19篇 |
1978年 | 8篇 |
1977年 | 16篇 |
1976年 | 6篇 |
排序方式: 共有2551条查询结果,搜索用时 15 毫秒
41.
42.
利用光学显微镜和石蜡切片技术对生长在盐碱地的大车前和平车前的营养器官进行了比较解剖学研究。结果表明二者的解剖结构存在差异:平车前根的木栓层较大车前的狭窄,栓内层细胞层数较少;平车前叶表皮的细胞角质膜较薄,孔下室较小,构成维管束鞘的薄壁细胞较大车前小,束鞘细胞两层。二者都具有适应盐渍环境的结构特征:营养器官中通气组织发达;根结构中薄壁组织和木栓发达;叶表皮角质层发达。 相似文献
43.
Wood anatomy and carbon‐isotope discrimination support long‐term hydraulic deterioration as a major cause of drought‐induced dieback 下载免费PDF全文
Elena Pellizzari J. Julio Camarero Antonio Gazol Gabriel Sangüesa‐Barreda Marco Carrer 《Global Change Biology》2016,22(6):2125-2137
Hydraulic impairment due to xylem embolism and carbon starvation are the two proposed mechanisms explaining drought‐induced forest dieback and tree death. Here, we evaluate the relative role played by these two mechanisms in the long‐term by quantifying wood‐anatomical traits (tracheid size and area of parenchyma rays) and estimating the intrinsic water‐use efficiency (iWUE) from carbon isotopic discrimination. We selected silver fir and Scots pine stands in NE Spain with ongoing dieback processes and compared trees showing contrasting vigour (declining vs nondeclining trees). In both species earlywood tracheids in declining trees showed smaller lumen area with thicker cell wall, inducing a lower theoretical hydraulic conductivity. Parenchyma ray area was similar between the two vigour classes. Wet spring and summer conditions promoted the formation of larger lumen areas, particularly in the case of nondeclining trees. Declining silver firs presented a lower iWUE than conspecific nondeclining trees, but the reverse pattern was observed in Scots pine. The described patterns in wood anatomical traits and iWUE are coherent with a long‐lasting deterioration of the hydraulic system in declining trees prior to their dieback. Retrospective quantifications of lumen area permit to forecast dieback in declining trees 2–5 decades before growth decline started. Wood anatomical traits provide a robust tool to reconstruct the long‐term capacity of trees to withstand drought‐induced dieback. 相似文献
44.
Wakana Ogawa Naoki Endo Yumi Takeda Miyuki Kodaira Masaki Fukuda Akiyoshi Yamada 《Mycoscience》2019,60(1):45-53
Species of fleshy yellow Cantharellus are known as chanterelles, which are among the most popular wild edible mycorrhizal mushrooms in the world. However, pure culture isolates of Cantharellus are rare. We report an efficient isolation technique of the Japanese golden chanterelle, Cantharellus anzutake, from its ectomycorrhizal root tips. Field-sampled fresh ectomycorrhizal root tips of C. anzutake on various hosts such as pines, spruce, and oaks were vortexed with 0.005% Tween 80 solution, surface sterilized with 1% calcium hypochlorite solution, rinsed with sterilized distilled water, and placed on modified Norkrans’ C (MNC) agar plate medium. Most ectomycorrhizal root tips of C. anzutake produced yellowish mycelial colonies within a few months. In contrast, tissue isolation from basidiomata provided limited cultures of C. anzutake but much contamination of bacteria and molds, even on media that contained antibiotics. The established C. anzutake cultures had clamp connections on the hyphae and contained intracellular oily droplets. These cultured isolates were identified as C. anzutake by sequence analysis of the rRNA internal transcribed spacer (ITS) region and translation elongation factor EF1-alpha (tef-1) genes. 相似文献
45.
Coding characters from different life stages for phylogenetic reconstruction: a case study on dragonfly adults and larvae,including a description of the larval head anatomy of Epiophlebia superstes (Odonata: Epiophlebiidae) 下载免费PDF全文
Alexander Blanke Sebastian Büsse Ryuichiro Machida 《Zoological Journal of the Linnean Society》2015,174(4):718-732
The exclusive use of characters coding for specific life stages may bias tree reconstruction. If characters from several life stages are coded, the type of coding becomes important. Here, we simulate the influence on tree reconstruction of morphological characters of Odonata larvae incorporated into a data matrix based on the adult body under different coding schemes. For testing purposes, our analysis is focused on a well‐supported hypothesis: the relationships of the suborders Zygoptera, ‘Anisozygoptera’, and Anisoptera. We studied the cephalic morphology of Epiophlebia, a key taxon among Odonata, and compared it with representatives of Zygoptera and Anisoptera in order to complement the data matrix. Odonate larvae are characterized by a peculiar morphology, such as the specific head form, mouthpart configuration, ridge configuration, cephalic musculature, and leg and gill morphology. Four coding strategies were used to incorporate the larval data: artificial coding (AC), treating larvae as independent terminal taxa; non‐multistate coding (NMC), preferring the adult life stage; multistate coding (MC); and coding larval and adult characters separately (SC) within the same taxon. As expected, larvae are ‘monophyletic’ in the AC strategy, but with anisopteran and zygopteran larvae as sister groups. Excluding larvae in the NMC approach leads to strong support for both monophyletic Odonata and Epiprocta, whereas MC erodes phylogenetic signal completely. This is an obvious result of the larval morphology leading to many multistate characters. SC results in the strongest support for Odonata, and Epiprocta receives the same support as with NMC. Our results show the deleterious effects of larval morphology on tree reconstruction when multistate coding is applied. Coding larval characters separately is still the best approach in a phylogenetic framework. © 2015 The Linnean Society of London 相似文献
46.
Four different ratios of river sand, ceramic pellets, vermiculite and perlite (1:1), and field soil were selected as the
substrates in this experiment, and four gradient levels of root waterlogging, half waterlogging, full waterlogging
and normal were set to investigate the effects of different gradients of waterlogging stress on the root morphology
of Taxus chinensis var. mairei seedlings under different substrates. In this study, the root anatomical structure of
Taxus chinensis var. mairei under different waterlogging stress was observed by the paraffin section method. The
roots of T. chinensis var. mairei were diarch, with no pith and resin canals. There was a large number of tannins in
the pericycle of the aerial adventitious roots of seedlings adapted to waterlogging. Also, the endodermis has
obvious casparian strip thickening, and there were 4-5 layers of large parenchymatous cells in the close to the
inner side of the pericycle in the vascular cylinder, which could increase the storage capacity, and transport capacity of the root. Under the treatment of root waterlogging stress, the development of plant roots in the mixed
substrate of vermiculite and, perlite was the earliest. Under half waterlogging stress, T. chinensis var. mairei seedlings treated with various substrates all could better adapt to the environment of waterlogging stress. Under the
stress of fully waterlogging, the roots of seedlings planted in river sand substrate developed secondary growth. 相似文献
47.
甘草根和根状茎的发育解剖学研究 总被引:1,自引:1,他引:1
甘草(GlycyrrhizauralensisFisch)实生苗具直根系。其根端原分生组织具三层原始细胞,外层产生表皮和根冠,中层和内层分别产生皮层和中柱。根的初生木质部四原型,由中柱鞘细胞产生第一次周皮。成长根中央的初生木质部由于导管周围薄壁细胞的增殖而被分割成几部分,使导管分子星散分布于薄壁细胞间。实生苗生长到第三年春由于叶节处的更新芽萌发形成最初的根状茎,根状茎顶端的原分生组织由两层原套细胞和一团原体细胞组成。初生结构的分化开始于皮层和髓两部分基本分生组织,而原形成层早期在根状茎横切面上呈连续的分生组织环。以后在环内逐渐分化出原形成层束,由它分化出初生锥管束,根状茎第一次周皮产生于初生韧皮部外方的第五或第六层皮层细胞。 相似文献
48.
【目的】高加索三叶草是优良的蜜源植物,了解其花蜜腺形态特征、组织结构及其发育规律对认识其小花糖分变化规律和合理利用该植物资源具有重要意义。【方法】选择生长良好的高加索三叶草标准株,分别在小花的不同发育时期,即现蕾期、露冠期、初花期、盛花期和谢花期采样。利用石蜡切片技术,观察小花细胞组织结构。采用毛细管法吸取小花花蜜,用生理试剂盒-分光光度法测定初花期、盛花期和谢花期小花的可溶性糖(包含蔗糖、果糖、葡萄糖)和淀粉等糖分营养指标。【结果】结果表明,(1)单个小花的花蜜量为0.394±0.095μL,每平方米花蜜量为4536.93±1319.34μL。(2)蜜腺位于9枚合生雄蕊的花丝内侧基部,属于雄蕊蜜腺。经PAS 染色,蜜腺组织在整个泌蜜过程中淀粉粒有明显的动态变化,属于淀粉型蜜腺。(3)小花的可溶性糖含量在盛花期最高,为7.978mg/g;可溶性糖各组分中果糖占比最大,初花期占比最高,为66%,后逐渐下降;淀粉含量占全糖含量的81%~82%,其中盛花期最高,为35.173mg/g,初花期最低,为24.168 mg/g。【结论】高加索三叶草花蜜腺属于雄蕊蜜腺,蜜腺由分泌表皮细胞和泌蜜细胞构成。原蜜由蜜腺相邻的花托维管束提供,经蜜腺细胞加工成蜜汁,最后由表皮的气孔泌出。高加索三叶草小花的可溶性糖含量在盛花期时最高,其中果糖的占比最大。 相似文献
49.
采用石蜡切片和半薄切片法研究组织培养中防风体细胞胚发生和发育的形态和结构特征,以了解体细胞再生形成胚胎结构的细胞分化和形态发生特征。结果表明:(1)胚性细胞内含有丰富的淀粉体,随细胞分裂和分化逐渐减少;(2)体细胞胚的发生发育经历了类似合子胚的各阶段;(3)多细胞原胚形成的分裂方式以平周分裂为主,并由此表现出有序生长;(4)在体细胞胚迅速发育过程中,不同阶段的体细胞胚始终与周围细胞存在明显界限;(5)胚柄发育不明显;(6)畸形胚常见,其中连体胚是胚性细胞发生密度过高造成的,而次生胚则在原来胚的基础上又产生新的胚的发生中心。 相似文献
50.