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   4446篇
  免费   150篇
  国内免费   240篇
林业   406篇
农学   727篇
基础科学   60篇
  267篇
综合类   1866篇
农作物   647篇
水产渔业   24篇
畜牧兽医   440篇
园艺   338篇
植物保护   61篇
  2024年   9篇
  2023年   53篇
  2022年   94篇
  2021年   79篇
  2020年   73篇
  2019年   82篇
  2018年   41篇
  2017年   128篇
  2016年   149篇
  2015年   150篇
  2014年   191篇
  2013年   219篇
  2012年   286篇
  2011年   312篇
  2010年   262篇
  2009年   319篇
  2008年   291篇
  2007年   302篇
  2006年   295篇
  2005年   217篇
  2004年   173篇
  2003年   144篇
  2002年   114篇
  2001年   85篇
  2000年   112篇
  1999年   82篇
  1998年   87篇
  1997年   65篇
  1996年   64篇
  1995年   71篇
  1994年   57篇
  1993年   59篇
  1992年   33篇
  1991年   49篇
  1990年   36篇
  1989年   25篇
  1988年   12篇
  1987年   11篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1977年   1篇
排序方式: 共有4836条查询结果,搜索用时 15 毫秒
41.
华山松针叶中矿质元素含量的地理变异   总被引:1,自引:1,他引:1  
张云跃  李佩瑜 《林业科学》1996,32(3):206-212
以12年生的种源林为取材对象,在1991年4月30日、7月30日、10月30日及1992年1月30日同时采当年和1年生针叶,测定N、P、K、Ca和Mg浓度。结果表明,除K外,其它矿质元素浓度种源间存在极显著差异。地理变异呈纬向渐变型。极端温度与元素地理差异有密切关系,并构成主导自然选择压。元素浓度的季节动态与针叶生长节律循环相伴。南、北华山松已分化成两个营养能力显著不同的地理种群,但动态模式基本相似。  相似文献   
42.
青海云杉的种实害虫有十几种,危害严重的有三种,即云杉球果小卷蛾、云杉蛀果斑螟和云杉梢斑螟,主要发生在甘肃祁连山林区。本文经过调查,根据青海云杉和种实害虫的生物学特性,提出加强青海云杉林的经营管理,尽量减少广谱性化学药剂的大面积使用;尽可能多的应用营林技术为主的、不杀伤天敌的防治措施,以达到青海云杉种实害虫综合治理的良性循环;并提出青海云杉种实害虫综合治理的策略、原则以及评价青海云杉种实害虫综合治理的步骤和方法。  相似文献   
43.
采用常规的水浸催芽方法,对引自美国的5种共7个种源的灌木树种种实的萌发特性进行了初步研究。对水浸催芽不易萌发的种实再采取其它催芽方法进行实验。结果表明:黑油脂木(Sarcoba tusverm icula tus)和沙生胶兔木(Chrysotham nus nauseosus ssp.turbina tus)的果实特别容易萌发,采用常规水浸处理即可得到较高的发芽率;不作任何处理即进行直播,其出苗率也很高;而四翅滨藜(Atriplex canescens)、巨四翅滨藜(A.canescens ssp.gigan tea)、三齿滨藜(Atriplex.triden ta ta)和内华达麻黄(Ephedranevadensis)的种实,常规水浸处理发芽率很低,通过混沙催芽、层积催芽等方法可以提高种实的发芽率和出苗率。  相似文献   
44.
Fruits of Pyracantha crenulata were collected from two locations varying by approximately 550 m in elevation from south aspect for assessing seed maturation indicators. The mean seed size (length×diameter) across the collection dates varied between 1.61 ± 0.7 and 4.93 ± 0.06 mm2 across both the locations. The seed moisture content negatively correlated with germination. The change in fruit colour from dark green to light orange, the range of fruit moisture content (30.43% ± 0.06 to 36.10% ± 0.25) and the seed moisture content between 68.8% ± 0.68 and 71.6 ± 0.62 coincided with maximum germination and appear to be major indicators of seed maturation in Pyracantha crenulata.  相似文献   
45.
Effects of stratification, desiccation, radicle pruning, and season of sowing on Quercus vulcanica germination and growth were studied to identify optimum nursery procedures for artificial regeneration of this species. Following stratification (0, 2, 4 and 8 weeks), acorns were germinated, and acorn moisture content and germination performance were also determined at various times during desiccation. In early December unsprouted acorns were planted in containers, and in early April sprouted acorns, which had been stored in polyethylene bags at 4 °C, with radicles left intact or with radicles pruned back to 1.0 cm were planted in containers.Stratification for 2 and 4 weeks did not increase germination percentage but significantly increased germination rate. Germination percentage of the seeds dropped when the moisture content of the seeds was reduced by desiccation, and the critical minimum moisture content of the recalcitrant Q. vulcanica acorns was found to be 11–16%. Spring sowing of sprouted acorns altered the morphology the containerized seedlings and caused the formation of significantly more main roots, but resulted in significantly less shoot height and shoot dry weight. Spring-sown sprouted acorns had also a significant advantage over the fall-sown unsprouted acorns in seedling survival, and thus nursery personnel should not be concerned if acorns sprout before sowing.  相似文献   
46.
多效抗旱驱鼠剂室内促长效果研究   总被引:1,自引:0,他引:1  
室内用多效抗旱驱鼠剂对小麦、黄豆、油松、侧柏种子进行浸种、拌种处理。结果表明,300x的林用型和200x的农用型RPA对侧柏种子浸种效果最好,30x的林用型和80x的农用型RPA拌种效果最好,经检验分析,差异显著;500x的林用型和600x的农用型RPA对油松促长效更为明显,发芽率比对照提高25.2%-27.0%,平均高生长增加2.1-2.8cm,拌种型的两种药剂6个浓度对油松的发芽及生长影响比较明显,30x的林用型和40x的农用型可以使油松发芽率提高27.7%-29.8%,苗高增长0.6-1.3cm;对黄豆的发芽率提高12.0%-17.0%、13.8%-23.2%;侧根数量增加0.2-2.6、0.3-1.6条,平均苗高及生物量也有不同程度的提高;浸种型的RPA使小麦发芽率提高13.3%-23.3%,苗高增长5.1-7.5cm,根长增加0.6-1.8cm;拌种型RPA发芽率提高9.3%-22.6%,苗高增长4.7-6.6cm,根长增加0.4-1.6cm。  相似文献   
47.
杜仲种子发芽试验分析   总被引:3,自引:0,他引:3  
杜仲为我国特有的经济树种和名贵中药,其皮为重要的中药材。采用传统的种子育苗法其发芽率一般不超过50%。将杜仲种子剥去果皮或剥去果皮并弃去胚根端部分胚乳,接种于Ms+0.8%琼脂的培养基上进行发芽试验,种子发芽率分别为50%和94%。将种子分别置于10℃、15℃、20℃、25℃、30℃的温度下培养发现,杜仲的最适发芽温度为20℃。  相似文献   
48.
Uma Shankar 《New Forests》2006,31(2):305-320
In species with seeds losing viability shortly after dispersal and exhibiting inherently low germination, quick decisions are required with respect to seeds that should be selected to maximize germination success and vigorous growth of seedlings. In ‘hollong’ (Dipterocarpus macrocarpus Vesque), I address the following questions: (a) are seeds that germinate randomly distributed within a seed population, (b) are subpopulations of viable and non-viable seeds separable, (c) does seed size predict which seeds germinate and (d) does seed size predict time required for germination and seedling vigour? Two estimators of seed size, diameter and weight, demonstrated a significant positive linear relationship endorsing assumption that accumulation of mass increases with increase in diameter of seeds. A threshold for selection of potential seeds for germination could not be derived from diameter measurements since seeds in an entire range of diameter did or did not germinate. All seeds <11 g did not germinate, but this threshold lies at the far lower end of the weight range and allows rejection of only a few seeds and acceptance of many seeds that will not germinate. A risk of potential seeds being rejected or non-viable seeds being picked exists if selection was derived from either diameter or weight. However, viable seeds could be better predicted from a scatter-plot of diameter on x-axis and weight on y-axis. Seeds showed a fan-shaped scatter and those developing the lower blade of the fan did not germinate while those following the handle were successful. Hence, two subpopulations segregated, but with a fuzzy edge. Mean diameter and mean weight of germinated seeds were significantly greater than those of ungerminated seeds. Most ungerminated seeds were those that had relatively smaller weight compared to their diameter. Although some large diameter seeds with small weight did germinate, but failed to develop into seedlings. Germination time and seedling vigour parameters (height, leaf number and collar diameter) were correlated both with diameter and weight. However, weight appears to have mattered more than diameter in germination success and early seedling growth.  相似文献   
49.
 Two kinds of bamboo vinegar from madake bamboo (Phyllostachys bambusoides) and moso bamboo (Phyllostachys pubescens) were prepared to analyze their components by gas chromatography (GC). The original vinegar, distilled vinegar, ether-extracted vinegar, and three fractions including acidic, neutral, and phenolic fractions separated from ether-extracted vinegar were diluted with distilled water 102 to 107 times. These diluted vinegar solutions were used to investigate the effect of bamboo vinegar on the germination and radicle growth of seed plants. High concentrations of all kinds of treated bamboo vinegars (e.g., 102 of original vinegar and 103 of ether-extracted vinegar) showed strong inhibition against germination of the seeds. However, an appropriate dilution of bamboo vinegar showed an obvious promotional effect on germination and radicle growth for the four kinds of tested seeds (lettuce, watercress, honewort, chrysanthemum). Received: December, 12, 2001 / Accepted: July 29, 2002 RID="*" ID="*" Part of this paper was presented at the 51st Annual Meeting of the Japan Wood Research Society, Tokyo, April 2001  相似文献   
50.
Pandit  Atul  Pant  Kavita  Ram  Jeet 《New Forests》2002,23(2):121-130
Seed maturity indices of Populus ciliata were investigated in collections from Sukhatal and Bhowali. Across both seed sources, mean capsule size varied from 68.0±1.5 mm2 to 72.0±0.9 mm2, while mean number of capsules/100 grams varied from 268±8 to 295±12. Mean weight of 100 capsules varied from 25.0±1.4 g to 27.0±1.4 g. Weight of 100 capsules among sources and individual trees differed significantly (P < 0.05), while number of capsules/100 grams and weight of 100 capsules was negatively correlated (r2 = 0.69). As capsules matured moisture content decreased from 83.2±1.1% to 54.3±0.3% at Sukhatal and from 77.7±0.4% to 46.4±0.3% at Bhowali. The optimum germination was at 58–60% moisture content. Maturation was related to capsule colour changes as well as moisture content. These parameters were judged to be good indicators of when to collect P. ciliata seeds.  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