全文获取类型
收费全文 | 502篇 |
免费 | 19篇 |
国内免费 | 11篇 |
专业分类
林业 | 24篇 |
农学 | 18篇 |
47篇 | |
综合类 | 120篇 |
农作物 | 14篇 |
水产渔业 | 90篇 |
畜牧兽医 | 201篇 |
园艺 | 12篇 |
植物保护 | 6篇 |
出版年
2024年 | 2篇 |
2023年 | 2篇 |
2022年 | 2篇 |
2021年 | 6篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 2篇 |
2017年 | 16篇 |
2016年 | 8篇 |
2015年 | 14篇 |
2014年 | 28篇 |
2013年 | 26篇 |
2012年 | 27篇 |
2011年 | 33篇 |
2010年 | 19篇 |
2009年 | 34篇 |
2008年 | 37篇 |
2007年 | 42篇 |
2006年 | 37篇 |
2005年 | 25篇 |
2004年 | 24篇 |
2003年 | 14篇 |
2002年 | 11篇 |
2001年 | 6篇 |
2000年 | 15篇 |
1999年 | 16篇 |
1998年 | 8篇 |
1997年 | 10篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 1篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
排序方式: 共有532条查询结果,搜索用时 16 毫秒
41.
J. E. Parlevliet 《Euphytica》2007,153(3):353-362
Improved cultivars loose their identity and healthiness unless maintained properly. Contaminating and degrading forces, such
as outcrossing, volunteer plants, mixing, natural selection, mutation and seed-borne diseases, are at the root of this. Maintenance
selection can prevent this deterioration. How it is carried out depends on the reproduction system of the crop. Crops are
therefore classified into four categories; typical cross-pollinating crops, self-pollinating crops with a substantial amount
of outcrossing, typical self-pollinating crops with little outcrossing, and the vegetatively reproduced crops.
Generally some of the “breeder seed” is used to plant a small plot with spaced plants. A fair number of healthy plants of
the cultivar type is selected and the seed is harvested per plant. The progenies of the selected plants are grown in small
plots. Non-uniform or deviating plots and plots with a seed-borne disease are removed. The seed of the progenies that are
healthy, uniform and similar (and so of the cultivar type) are harvested per progeny to be tested next season on larger plots.
The same selection is applied and only the seed of the progenies that are healthy, uniform and similar are harvested together
to produce the “breeder seed”. The details of this maintenance selection vary with the reproduction system, the multiplication
rate of the crop and the possibilities available to the breeder. Seven crops, potato, common bean, barley, wheat, faba bean,
quinoa and maize are discussed here as they represent the different reproduction systems and multiplication rates, while being
important Andean food crops. 相似文献
42.
多球果型马尾松的形态和生殖生物学特征研究 总被引:2,自引:1,他引:2
本文对多球果型马尾松和常见型马尾松在生长状态、形态结构、生殖发育等方面的差异进行研究和分析.在多球果型马尾松雌性生殖枝上,能够着生40~80个球果(大孢子叶球),有的甚至可以达到193个以上.而在常见型马尾松的雌性生殖枝上, 通常只着生有1~8个球果,最常见的是着生2~4个球果.多球果型马尾松和常见型马尾松在雌球花的芽组织结构和雌球花的分化过程中有着明显的差异.通过显微分析我们发现:多球果型马尾松雌球花原基的发育在当年生过冬顶芽的下部,在它的上部,还发育有一系列的营养芽原基(10列以上).而常见型马尾松雌球花原基的发育在当年生过冬顶芽的顶端.雌球花发育机理的不同,决定了多球果型马尾松球果的着生部位发生了变异.多球果型马尾松球果着生在春梢的下(基)部,常见型马尾松球果着生在春梢的顶部.多球果型马尾松与常见型马尾松球果(大孢子叶球)的发育过程和发育起始时间不相同.通过对8~12月标本进行显微切片分析,多球果型马尾松球果的发育起始时间在当年的8月(过冬顶芽开始形成的同时),与小孢子叶球的发育起始时间同步.常见型马尾松球果的发育起始时间是当年的11月,两种类型马尾松的大孢子叶球都是在次年4月受粉,到第三年的11月种子成熟.多球果型马尾松球果比常见型马尾松球果多3个月的发育时间.多球果型马尾松个体大量的长串状多球果性状在历年都能重复出现,这说明该性状具有个体水平的稳定性.无性繁殖研究证明,多球果型马尾松的长串状多球果性状,可以通过嫩枝嫁接的方式,进行性状的传递.对多球果型马尾松进行有性繁殖研究时我们发现,多球果型马尾松的球果可以发育出种子,且成熟种子的发芽率为60%~76%.这说明多球果型马尾松与常见型马尾松一样,可以用种子进行后代繁殖.多球果型马尾松与常见型马尾松在形态和生长速率方面没有明显差异. 相似文献
43.
44.
采用胡萝卜片培养短体线虫技术,测试不同培养时间和接种密度对咖啡短体线虫(Pratylenchus coffeae)繁殖和形态的影响。结果表明,在25℃培养条件下,每片胡萝卜(厚10 mm,直径3 cm)接种25头线虫雌虫,线虫的数量随培养时间的延长而增加,前3周线虫的繁殖系数为6.24,线虫的群体数量增长缓慢;第3~6周线虫的繁殖系数略有增高,为8.99;第6~9周其繁殖系数高达17.88,线虫群体数量快速增长;之后,线虫群体数量增速下降,其繁殖系数降为1.42。在接种不同密度线虫的实验中,线虫的群体数量均显著增长,其繁殖系数在不同接种密度处理中差异不显著;随机挑取10头雄虫与雌虫,比较不同线虫密度对线虫形态特征的影响,结果表明,线虫密度除对雌虫的b值(体长/头顶至食道腺与肠连接处长)有显著影响外,对雄虫和雌虫的其他形态特征测量值均无显著影响。 相似文献
45.
A. Marm Kilpatrick 《Biological conservation》2006,128(4):475-485
Research has shown that avian malaria plays an important role in limiting the distribution and population sizes of many Hawaiian birds, and that projected climate change is likely to eliminate most disease-free habitat in Hawai’i in the next century. I used a modeling approach, parameterized with demographic data from the literature and the field, to examine alternate management scenarios for the conservation of native Hawaiian birds. I examined the feasibility of using management in the form of rodent control to facilitate the evolution of resistance to malaria by increasing the survival and reproduction of native birds. Analysis of demographic data from seven native species, Akepa (Loxops coccineus), ‘Akohekohe (Palmeria dolei), Elepaio (Chasiempis sandwichensis), Hawai’i’amakihi (Hemignathus virens), Hawai’i creeper (Oreomystis mana), Omao (Myadestes obscurus), and Palila (Loxioides bailleui), suggest that differences in life history cause some species to be more susceptible to local extinctions from the transmission of malaria. Modeling results demonstrated that rodent control at middle, but not high, elevations can facilitate the evolution of resistance to malaria in several species of Hawaiian birds. Advocating a management approach that encourages evolutionary change in endangered species contrasts with the traditional conservation paradigm but it may be the best strategy to reduce the impacts of one of the multiple stressors that have devastated the native bird community of Hawai’i. 相似文献
46.
Matt W. Hayward Paul J. de Tores Michael J. Dillon Barry J. Fox 《Biological conservation》2003,110(3):343-355
We investigated the population structure of the quokka (Setonix brachyurus) on the mainland of Western Australia using mark–recapture techniques. Seven previously known local populations and one unconfirmed site supporting the preferred, patchy and discrete, swampy habitat of the quokka were trapped. The quokka is now considered as locally extinct at three sites. The five remaining sites had extremely low numbers, ranging from 1 to 36 individuals. Population density at these sites ranged from 0.07 to 4.3 individuals per hectare. There has been no response to the on-going, 6 year fox control programme occurring in the region despite the quokkas’ high fecundity and this is due to low recruitment levels. The remaining quokka populations in the northern jarrah forest appear to be the terminal remnants of a collapsing metapopulation. 相似文献
47.
To understand the life cycle of an organism, it is important to understand the physiological processes that govern growth and reproduction. In this paper, we re-analyse a life-cycle data set for the earthworm Eisenia veneta, using a process-based model. The data set comprises measurements of body size and cocoon production over 200 days, at two temperatures (15-25 °C) and two densities (five and 10 worms per container, but with the same worm:soil weight ratio). The model consists of a set of simple equations, derived from Dynamic Energy Budget (DEB) theory. The dynamics of growth and reproduction are simultaneously described by the model, using very few parameters (five parameters for four curves). This supports the use of this model for efficient analysis of earthworm life-cycle data, and to interpret the effects of stressors. However, there was considerable inter-individual variation in the response, hampering the interpretation of the temperature and density effects. A temperature increase corresponded to an increase in the rate constants for growth and reproduction (with the same factor), without affecting the other parameters, as expected from DEB theory. Changing the earthworm density hardly affected the growth curves, but had an unexpected effect on reproduction: at higher densities, the worms start to produce cocoons at a larger body size and the maximum reproduction rate was lower. This study confirms the use of DEB as a reference model for earthworms, and using this model, we can recognise that temperature has a predictable effect on the life cycle of E. veneta. Furthermore, this analysis reveals that the effects of density are less clear and may involve a change in energy allocation that requires further study. 相似文献
48.
49.
50.