Distribution of Externally Applied 15NH4 + or 15NO3 − in White Lupins at Different Developmental Stages

White lupins (Lupinus albus L., var. Kievsky mutant) were grown in Mitscherlich pots. ¹⁵NH₄NO₃ or NH₄ ¹⁵NO₃ (10 or 95 at% ¹⁵N exc.) was applied at the late vegetative stage (14–16 leaves) or at the generative stage (end of bloom), respectively. Dry weight increase (ΔDW) of the plants as well as the ¹⁵N distribution in different organs and N fractions (NO_{3 ?}-N, NH₂-N, protein N) were investigated after 5 days and 50 days (maturity). The following results were obtained:

1. In comparison with the ¹⁵N derived from NO₃ ^?, the ¹⁵N derived from NH₄ ⁺ was more strongly retained in the roots. When transported into the shoot, it was more strongly translocated into the tip.

2. More ¹⁵N derived from NH⁴ ₊ was incorporated into proteins and reduced soluble N compounds than ¹⁵N derived from NO₄ ^?. Even in the above-ground plant tissues, a considerable proportion of the latter was found as NO₃ ^?.

3. In the long-term experiment (harvest at maturity 50 days after application), no difference in translocation and incorporation could be found between the two offered ¹⁵N sources.     

Approximation with generalized hyperexponential distributions: Weak convergence results

Generalized hyperexponential (GH) distributions are linear combinations of exponential CDFs with mixing parameters (positive and negative) that sum to unity. The denseness of the class GH with respect to the class of all CDFs defined on [0, ) is established by showing that a GH distribution can be found that is as close to a given CDF as desired, with respect to a suitably defined metric. The metric induces the usual topology of weak convergence so that, equivalently, there exists a sequence of GH CDFs that converges weakly to a given CDF. This result is established by using a similar result for weak convergence of Erlang mixtures. Various set inclusion relations are also obtained relating the GH distributions to other commonly used classes of approximating distributions, including generalized Erlang (GE), mixed generalized Erlang (MGE), those with reciprocal polynomial Laplace transforms (K _n ), those with rational Laplace transforms (R _n ), and phase-type (PH) distributions. A brief survey of the history and use of approximating distributions in queueing theory is also included.This research was partially supported by the Office of Naval Research under Contract No. N00014-86-K0029. Much of this work is taken from the first-named author's doctoral dissertation, accepted by the faculty at the University of Virginia.     

Controlling stages of supramolecular crystallization in suspensions of monodispersed spherical silica particles

Supramolecular crystallization in alcohol suspensions of monodispersed spherical silica particles at pH ~ 7-7.8 is controlled by sedimentation of structural units. At pH ~ 7.8-10.5 or in the presence of indifferent (inert) electrolytes, the crystallization is controlled by the linear growth of supramolecular crystals. The peculiarities of crystallization and nucleation in both cases are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of H<Subscript>2</Subscript>SeO<Subscript>3</Subscript> on the early stages of nucleation and growth during Cu electrocrystallization in acidic medium on glassy carbon electrode

The early stages of Cu electrodeposition onto a GC electrode were investigated in 0.5 M H₂SO₄ + 0.01 M CuSO₄ solution without or with H₂SeO₃ when a molar concentration ratio [Cu(II)]/[Se(IV)] was 1.10⁴ to 2.10². The H₂SeO₃ solution in 0.5 M H₂SO₄ was also used. The electrochemical techniques such as cyclic voltammetry and chronoamperometry and structural investigation using ex situ AFM were applied to study the nucleation and growth of Cu onto a GC electrode. Chronoamperometric results were shown to follow an instantaneous 3D nucleation and diffusion-controlled growth model by Scharifker and Hills. The values of number of Cu nuclei N and average nuclei radius r _av were calculated. It was shown that, in the presence of H₂SeO₃ in amounts of 0.001 to 0.005 mM, N increases and r _av decreases. At higher concentrations of the additive, the changes of these parameters with the deposition potential E _dep were shown to be somewhat more complex. The dependences of N and r _av on the concentration of H₂SeO₃ in different regions of Cu overpotentials were also revealed.     

具有常恢复率的艾滋病梯度传染模型

本文建立了一个具有常恢复率的同质人群艾滋病梯度传染模型，其中的性接触数是人群变量的函数，讨论了平衡点的存在性和稳定性，推广和改进了一些相关的已有结果。     

Effects of tributyltin(IV) chloride exposure on early embryonic stages of Ciona intestinalis: In vivo and ultrastructural investigations

The effects of tributyltin(IV) chloride (TBT chloride) solutions on ascidian embryos of Ciona intestinalis have been tested at different stages of development. It has been observed, in vivo, that TBT chooride inhibited cleavage of fertilized eggs and of embryo blastomeres giving rise to cellular masses that are not delimited by plasma membrane. Electron-dense precipitates of TBT chloride, probably as inorganic tin, have been observed by transmission electron microscopy in the egg cytoplasm of cellular masses. The same type of precipitate was present also inside the mitochondria, whose structure appeared to be highly modified. These ultrastructure aspects are indicative of a degenerative process of the embroys after incubation in TBT chloride.     

非参数计量经济联立模型的局部线性两阶段最小二乘估计

联立方程模型在经济政策制定，经济结构分析和经济预测方面起重要作用，本在随机设计（模型中所有变量为随机变量）下，提出了非参数计量经济联立模型的局部线性两阶段最小二乘估计并利用概率论中大数定理和中心极限定理在内点处研究了它的大样本性质，证明了它的一致性和渐近正态性，它在内点处的收敛速度达到了非参数函数估计的最优收敛速度。     

Photoionization of the hydrogen atom in strong magnetic fields of White Dwarfs

The photoionization cross-sections of the hydrogen atom in strong magnetic fields of magnetic White Dwarf stars were calculated with a direct numerical integration method using the Landau basis as a high field approach. The validity regime of these solutions overlap with those of the spherical symmetry and complement other recently developed methods. An important result is the relation between the density of states and the normalization for the more than one open channel regime leading to additional coupling terms not taken into account by multi channel quantum defect methods. Received: 16 March 1998 / Revised and Accepted: 14 July 1998     

濒危植物夏蜡梅花部综合特征与繁育系统

野外定点观察夏蜡梅（Sinocalycathus chinensis）的开花进程和花朵的功能形态特征,运用杂交指数、花粉-胚珠比、人工授粉和套袋试验等方法测定夏蜡梅的繁育系统．结果显示：夏蜡梅花各部形态和数量存在普遍的变异．单花花期为7～9d．开花进程中,雄蕊向心聚合,最后包围雌蕊,柱头与花药基本等高．夏蜡梅花雌雄异熟,雌蕊先熟,雌雄蕊无明显异位．单花花期依其形态和雌雄性功能表达可分为6个时期：蕾期、开花前期、雌性期、两性期、雄性期和谢花期．夏蜡梅杂交指数为3,花粉-胚珠比为29571±5839．授粉实验结果显示其不存在无融合生殖,为自交亲和,虫媒在传粉过程中起了较大的作用,表明夏蜡梅的繁育系统以异交为主,但自交亲和,为混合交配系统．     

可见光光谱的冬小麦苗期地上生物量估算

地上生物量是表征冬小麦苗期长势的重要参数之一,对于监测冬小麦苗期长势,预测产量具有重要的实际意义。目前,通过计算光谱指数进行生物量估算是冬小麦苗期地上生物量无损测量的主要方法,但该方法需要一定的平台支撑,在便捷性方面存在一定的不足。为此,利用可见光图像数据获取方便、准确率高的特点,基于冬小麦苗期冠层可见光图像数据开展冬小麦苗期地上生物量估算研究。采用数码相机,采集冬小麦苗期冠层可见光图像并利用Canopeo进行冬小麦冠层与背景的分割。在获取冠层分割图像后,提取了CC（canopy cover）、ExG（excessg reen）、ExR（excess red）、ExGR（ExG-ExR）、NGRDI（normalized green-red difference index）、GLI（green leaf index）、RGRI（red-green ratio index）和RGBVI（RGB vegetation index）共8个可见光图像特征。利用相关性分析进行特征优选,选择与冬小麦苗期地上生物量实测数据相关性较高的图像特征构建估算模型。利用优选的图像特征,分别构建偏最小二乘回归（PLSR）、BP神经网络（BPNN）、支持向量机回归（SVR）和随机森林（RF）模型,开展冬小麦苗期地上生物量估算研究,并定量分析特征数量和播种密度对估算模型准确率的影响。结果表明,ExR,GLI和RGBVI与生物量实测数据相关性较低,因此,将这3个特征剔除。CC,ExG,ExGR,NGRDI和RGRI与生物量实测数据的相关性较高,其中CC,ExG和ExGR与生物量实测数据呈正相关,而NGRDI和RGRI与生物量实测数据呈负相关。利用优选的图像特征构建估算模型,研究结果表明,基于优选的5个图像特征,PLSR的估算准确率最高,模型R2为0.801 5,RMSE为0.0788 kg·m-2,表明PLSR能够实现冬小麦苗期地上生物量的准确估算。特征数量是影响估算模型准确率的因素之一,随着特征数量的减少,模型估算的准确率逐步下降。利用不同播种密度数据集对估算模型进行测试,结果表明,PLSR在不同的播种密度数据集上均取得了最高的估算准确率,模型R2分别为0.897,0.827 9和0.788 6,RMSE分别为0.062,0.072和0.079 1 kg·m-2,表明PLSR估算的冬小麦苗期地上生物量数据与实测生物量数据之间具有良好的相关关系。随着播种密度的增加,所有估算模型的准确率均出现下降,而PLSR的准确率下降程度最小。由此可见,基于可见光图像数据,能够实现冬小麦苗期地上生物量的准确估算,为冬小麦苗期田间管理提供参考。