首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3921篇
  免费   184篇
  国内免费   177篇
  2024年   6篇
  2023年   46篇
  2022年   57篇
  2021年   88篇
  2020年   99篇
  2019年   113篇
  2018年   111篇
  2017年   107篇
  2016年   147篇
  2015年   98篇
  2014年   114篇
  2013年   744篇
  2012年   100篇
  2011年   121篇
  2010年   86篇
  2009年   122篇
  2008年   134篇
  2007年   168篇
  2006年   160篇
  2005年   124篇
  2004年   150篇
  2003年   122篇
  2002年   109篇
  2001年   92篇
  2000年   78篇
  1999年   80篇
  1998年   68篇
  1997年   57篇
  1996年   89篇
  1995年   80篇
  1994年   60篇
  1993年   49篇
  1992年   59篇
  1991年   52篇
  1990年   45篇
  1989年   43篇
  1988年   29篇
  1987年   37篇
  1986年   36篇
  1985年   39篇
  1984年   44篇
  1983年   10篇
  1982年   14篇
  1981年   22篇
  1980年   14篇
  1979年   17篇
  1978年   9篇
  1977年   8篇
  1976年   12篇
  1972年   3篇
排序方式: 共有4282条查询结果,搜索用时 14 毫秒
991.
马桑绣球(绣球科)的花器官发生和发育   总被引:3,自引:0,他引:3  
在扫描电镜下观察了马桑绣球Hydrangea aspera孕性花的发生及发育过程。马桑绣球的花器官向心轮状发生:花萼原基以2/5螺旋式相继发生,花瓣原基几乎同步发生。花瓣开始发育时,与花萼相对的雄蕊发生。与花瓣相对的雄蕊原基与心皮原基几乎同时出现。初始心皮向上扩展,分化出花柱和柱头,向下延伸,嵌入花托,发育为下位子房。花发育成熟时,隔膜于子房的下部连续,而中部和上部不连续,即子房为不完全2室。经过与绣球属已观察过的另外5种1亚种花器官发生和发育比较,发现马桑绣球与藤绣球H. ano mala subs  相似文献   
992.
白桦雌花发育、大孢子发生及胚胎发育的解剖学观察   总被引:6,自引:2,他引:4  
姜静  李同华  庄振东  杨传平 《植物研究》2003,23(1):T007-T009
白桦雌花从开花到雌性器官的成熟需经历1个月左右的时间,解剖学观察表明:四月下旬越冬的雌蕊原基开始了活跃的分裂和分化。子房和柱头开始生长。四月末开花,五月初授粉。此后胚珠开始长大。五月中旬即分化形成珠被,珠心,珠被为单层珠被,胚珠为厚珠心胚珠,胚珠倒生,五月中下旬,珠心内产生大孢子母细胞,一周左右发育为成熟胚囊-七细胞八核胚囊,五月末完成双受精,白桦胚胎发育经过合子,原胚,球形胚,心形胚和鱼雷形胚等时期最后发育成熟,胚乳发育与胚胎同步,即受精的极核进行几次分裂后形成核型胚乳,胚乳核不断增多,在形成心形胚后胚乳细胞形成细胞壁。  相似文献   
993.
舞钢玉兰芽种类与成枝成花规律的研究   总被引:17,自引:1,他引:16  
报道了舞钢玉兰芽的种类、分枝习性与成枝生长规律,拟花蕾、着生位置、解剖结构及其分化发育成花规律。从中发现:(1)当年生枝上有休眠芽、叶芽(侧叶芽和顶叶芽)、拟花蕾3种;(2)拟花蕾有缩台枝、芽鳞状托叶、雏枝、雏芽及雏蕾组成,因其外形似“花蕾“,称为“拟花蕾“;(3)缩台枝是枝与花着生的中间过滤枝变阶段,是由母枝顶端节间缩短、增粗的枝段和当年由雏枝生长的1次极短新枝所组成;(4)4-5月及7-8月前后两批形成的拟花蕾,均经过未分化发育期、花被分化发育期、雄蕊群分化发育期及离心皮雌蕊群分化发育期,各期均依次递后交错进行,但不逆转,也不能截然分开,直到翌春花分化发育全部结束,开花后才能结实;(5)芽鳞状托叶是托叶的变态,最外层薄革质,外面密被短柔毛,始落期6月中下旬,其余纸质--膜质,外面密被或疏被毛柔毛,翌春开花时脱落完毕;(6)雏蕾有雏梗、雏花及包被雏花的佛焰苞状托叶组成;(7)分枝习性与成枝生长规律与预生分枝及预生一同生分枝呈单阶无歧、单阶1歧生长规律,稀有单阶2歧生长规律。  相似文献   
994.
Shortly after the initial detection of western flower thrips (WFT), Frankiniella occidentalis (Pergande), in Australia during 1993 a resistance management strategy based on the alternation of chemical groups was implemented. This study aimed to verify this strategy by field testing α-cypermethrin against WFT with and without chemical alternation. Up to 114 times α-cypermethrin resistance (at LC50) was detected and resistance increased with and without chemical alternation; however, chemical alternation did significantly reduce numbers of thrips compared with a nonalternation strategy. Resistance has the potential to undermine the sustainable use of those chemicals to which there is no current detectable resistance. Consequently, chemicals with a high frequency and level of resistance against WFT need to be identified through monitoring and quickly eliminated from WFT chemical control recommendations.  相似文献   
995.
Gynoecium diversity and systematics in basal monocots   总被引:5,自引:0,他引:5  
Gynoecium and ovule structure was comparatively studied in representatives of the basal monocots, including Acorales (Acoraceae), Alismatales (Araceae, Alismataceae, Aponogetonaceae, Butomaceae, Hydrocharitaceae, Junc‐aginaceae, Limnocharitaceae, Potamogetonaceae, Scheuchzeriaceae, Tofieldiaceae), Dioscoreales (Dioscoreaceae, Taccaceae), and Triuridaceae as a family of uncertain position in monocots. In all taxa studied the carpels or gynoecia are closed at anthesis. This closure is attained in different ways: (1) by secretion without postgenital fusion (Araceae, Hydrocharitaceae); (2) by partly postgenitally fused periphery but with a completely unfused canal (Alismataceae, Aponogetonaceae, Butomaceae, Limnocharitaceae, Scheuchzeriaceae, Dioscoreaceae, Taccaceae); (3) by completely postgenitally fused periphery but with an unfused canal in the centre (Acoraceae, Tofieldiaceae); (4) by complete postgenital fusion and without an (unfused) canal (Juncaginaceae, Potamogetonaceae). In many Alismatales (but without Araceae) carpels have two lateral lobes. The stigmatic surface is restricted to the uppermost part of the ventral slit (if the carpel is plicate); it is never distinctly double‐crested (Butomaceae?). Stigmas are commonly unicellular‐papillate and secretory in most taxa. The locules are filled with a (often) mucilaginous secretion in a number of taxa. Superficial (probably intrusive) ethereal oil cells were found in the carpel wall of Acorus gramineus (as in Piperales!). Idioblasts in carpels are otherwise rare. A number of basal monocots has orthotropous ovules, which is perhaps the plesiomorphic condition in the group. The presence of almost tenuinucellar (pseudocrassinucellar) ovules is relatively common (Acoraceae, many Araceae, some Alismatales s.s.), whereas completely tenuinucellar ovules are rare and do not characterize larger groups. However, crassinucellar ovules occur in the largest number of families among the study group (basal Araceae, many Alismatales s.s.) The outer integument is always annular in orthotropous ovules. The inner integument is often lobed and it mostly forms the micropyle, whereas the outer integument is always unlobed. Gynoecium structure supports the isolated position of Acoraceae as sister to all other monocots. However, in an overall view, if compared with all other families, Acoraceae clearly shows the greatest similarities with Araceae.  相似文献   
996.
In the cyanic flowers ofDahlia variabilis (Asteraceae), an enzyme was demonstrated which catalyzes a glucosyl group transfer from UDP-glucose to the 5 position of anthocyanidin 3-O-glucoside and 3-O-malonylglucoside. The anthocyanin 5-O-glucosyltransferase (5GT) was purified 88-fold at 8 percnt; yield by (NH4)2SO4 precipitation followed by successive chromatography on DEAE-cellulose, Sephacryl S-200 and Mono P. 5GT exhibited a pH optimum at 8.0 and a pI of 4. 2. Its apparent molecular weight calculated from Sephacryl S-200 was 53 kDa. Its activity was stimulated by 2-ME and DTE but strongly inhibited by PCMB and NEM. It was slightly activated by Mg2+ and Ca2+ but strongly inhibited by Hg2+, Zn2+, Cu2+, Mn2+, Fe3+ and Al3+. No effect of EDTA was observed. The apparent Km values for cyanidin 3-O-glucoside, cyanidin 3-O-(6′′-O-malonyl)glucoside and UDP-glucose were 120 μmol/L, 75 μmol/L and 250 μmol/L, respectively. Pelargonidin 3-O-glucoside and malonylglucoside were also considerable substrates, but low relative activity was observed for delphinidin 3-O-glucoside which has yet not been found inDahlia flowers.Dahlia 5GT showed substrate specificities different from those reported forSilene, Petunia, Matthiola andPerilla. Neither ADP-glucose nor UDP-galactose could serve as glycosyl donor.  相似文献   
997.
998.
999.
1000.
Certain sleep-wake schedule disorders (SWSDs) cannot be successfully managed clinically using conventional methods of sleep therapy. We describe two cases of SWSD, the first following head trauma and the second originating during childhood, that had been misdiagnosed by physicians for many years. After conventional treatment for SWSD with light therapy and melatonin failed to bring about substantial improvement, it was determined that they were suffering from an incurable disability. Hence, we propose new medical terminology for such cases—SWSD disability. SWSD disability is an untreatable pathology of the circadian time structure. Patients suffering from SWSD disability should be encouraged to accept the fact that they suffer from a permanent disability, and that their quality of life can only be improved if they are willing to undergo rehabilitation. It is imperative that physicians recognize the medical condition of SWSD disability in their patients and bring it to the notice of the public institutions responsible for vocational and social rehabilitation. (Chronobiology International, 18(6), 1019-1027, 2001)  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号