216 research outputs found
Osmotically driven flows in microchannels separated by a semipermeable membrane
We perform experimental investigations of osmotically driven flows in
artificial microchannels by studying the dynamics and structure of the front of
a sugar solution traveling in 200 um wide and 50-200 um deep microchannels. We
find that the sugar front travels with constant speed, and that this speed is
proportional to the concentration of the sugar solution and inversely
proportional to the depth of the channel. We propose a theoretical model,
which, in the limit of low axial flow resistance, predicts that the sugar front
indeed should travel with a constant velocity. The model also predicts an
inverse relationship between the depth of the channel and the speed and a
linear relation between the sugar concentration and the speed. We thus find
good agreement between the experimental results and the predictions of the
model. Our motivation for studying osmotically driven flows is that they are
believed to be responsible for the translocation of sugar in plants through the
phloem sieve element cells. Also, we suggest that osmotic elements can act as
integrated pumps with no movable parts in lab-on-a-chip systems.Comment: 7 Pages, submitted to Lab-on-a-chip on October 24th, 200
Why paper mills clean up : determinants of pollution abatement in four Asian countries
The authors find strong evidence that despite weak or nonexistent formal regulation and enforcement of environmental standards, many plants in South and Southeast Asia are clean. At the same time, many plants are among the world's worst polluters. To account for the extreme variation among plants, the authors review evidence from a survey of pollution abatement by 26 pulp and paper plants in four countries: Bangladesh, India, Indonesia, and Thailand. They incorporate 3 sets of factors affecting pollution intensity: plant characteristics, economic considerations, and external pressure from the government and private stakeholders. They find that the level of pollution abatement is positively associated with scale and competitiveness, negatively associated with public ownership, and unaffected by foreign links (in ownership or financing). Informal regulation, or community pressure on plants works to abate pollution, with high income being a powerful predictor of effectiveness. Privatization, to the extent that it increases plant efficiency, can significantly improve environmental performance. To prevent environmental injustice in poor or marginalized communities, the authors conclude, governments may want to consider strategies for improving their participation, and may want to target regulation to address pollution problems among them.Environmental Economics&Policies,Water and Industry,Water Conservation,Pollution Management&Control,Sanitation and Sewerage,Environmental Economics&Policies,Water and Industry,Pollution Management&Control,Sanitation and Sewerage,TF030632-DANISH CTF - FY05 (DAC PART COUNTRIES GNP PER CAPITA BELOW USD 2,500/AL
A model-based approach to recovering the structure of a plant from images
We present a method for recovering the structure of a plant directly from a
small set of widely-spaced images. Structure recovery is more complex than
shape estimation, but the resulting structure estimate is more closely related
to phenotype than is a 3D geometric model. The method we propose is applicable
to a wide variety of plants, but is demonstrated on wheat. Wheat is made up of
thin elements with few identifiable features, making it difficult to analyse
using standard feature matching techniques. Our method instead analyses the
structure of plants using only their silhouettes. We employ a generate-and-test
method, using a database of manually modelled leaves and a model for their
composition to synthesise plausible plant structures which are evaluated
against the images. The method is capable of efficiently recovering accurate
estimates of plant structure in a wide variety of imaging scenarios, with no
manual intervention
Agnostic notes on regression adjustments to experimental data: Reexamining Freedman's critique
Freedman [Adv. in Appl. Math. 40 (2008) 180-193; Ann. Appl. Stat. 2 (2008)
176-196] critiqued ordinary least squares regression adjustment of estimated
treatment effects in randomized experiments, using Neyman's model for
randomization inference. Contrary to conventional wisdom, he argued that
adjustment can lead to worsened asymptotic precision, invalid measures of
precision, and small-sample bias. This paper shows that in sufficiently large
samples, those problems are either minor or easily fixed. OLS adjustment cannot
hurt asymptotic precision when a full set of treatment-covariate interactions
is included. Asymptotically valid confidence intervals can be constructed with
the Huber-White sandwich standard error estimator. Checks on the asymptotic
approximations are illustrated with data from Angrist, Lang, and Oreopoulos's
[Am. Econ. J.: Appl. Econ. 1:1 (2009) 136--163] evaluation of strategies to
improve college students' achievement. The strongest reasons to support
Freedman's preference for unadjusted estimates are transparency and the dangers
of specification search.Comment: Published in at http://dx.doi.org/10.1214/12-AOAS583 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Structured Light-Based 3D Reconstruction System for Plants.
Camera-based 3D reconstruction of physical objects is one of the most popular computer vision trends in recent years. Many systems have been built to model different real-world subjects, but there is lack of a completely robust system for plants. This paper presents a full 3D reconstruction system that incorporates both hardware structures (including the proposed structured light system to enhance textures on object surfaces) and software algorithms (including the proposed 3D point cloud registration and plant feature measurement). This paper demonstrates the ability to produce 3D models of whole plants created from multiple pairs of stereo images taken at different viewing angles, without the need to destructively cut away any parts of a plant. The ability to accurately predict phenotyping features, such as the number of leaves, plant height, leaf size and internode distances, is also demonstrated. Experimental results show that, for plants having a range of leaf sizes and a distance between leaves appropriate for the hardware design, the algorithms successfully predict phenotyping features in the target crops, with a recall of 0.97 and a precision of 0.89 for leaf detection and less than a 13-mm error for plant size, leaf size and internode distance
Leaf Eh and pH: A Novel Indicator of Plant Stress. Spatial, Temporal and Genotypic Variability in Rice (Oryza sativa L.)
A wealth of knowledge has been published in the last decade on redox regulations in plants. However, these works remained largely at cellular and organelle levels. Simple indicators of oxidative stress at the plant level are still missing. We developed a method for direct measurement of leaf Eh and pH, which revealed spatial, temporal, and genotypic variations in rice. Eh (redox potential) and Eh@pH7 (redox potential corrected to pH 7) of the last fully expanded leaf decreased after sunrise. Leaf Eh was high in the youngest leaf and in the oldest leaves, and minimum for the last fully expanded leaf. Leaf pH decreased from youngest to oldest leaves. The same gradients in Eh-pH were measured for various varieties, hydric conditions, and cropping seasons. Rice varieties differed in Eh, pH, and/or Eh@pH7. Leaf Eh increases and leaf pH decreases with plant age. These patterns and dynamics in leaf Eh-pH are in accordance with the pattern and dynamics of disease infections. Leaf Eh-pH can bring new insight on redox processes at plant level and is proposed as a novel indicator of plant stress/health. It could be used by agronomists, breeders, and pathologists to accelerate the development of crop cultivation methods leading to agroecological crop protection
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