Ultrastructural Aspects of Unilateral Interspecific Incompatibility between Lycopersicum Peruvianum and L. Esculentum

SUMMARYObservations have been made, at the electron microscope, of the pollen tubes present in the styles of Lycopersicum esculentum and L. peruvianum after reciprocal crosses between the two species.The unilateral incompatibility barrier which isolates the two species when L. peruvianum is used as pistillate parent was then compared to the processes of pollen tube rejection which have been recently analysed (J. Cell Sci., 1972) after self-pollination in this self-incompatible species. Such a comparison, which was also carried out by means of fluorescence techniques, has permitted to find out that for both types of incompatibility the rejection process was characterised by a progressive disappearance of the callose-rich inner wall of the pollen tube and by an accumulation of bi-partite particles in the tube cytoplasm.In the case of unilateral incompatibility, however, the tube outer wall is gradually disaggregated while the callosic inner wall remains quite thick at the tube apex, becoming thinner and fin..

Critical behaviour of the Rouse model for gelling polymers

It is shown that the traditionally accepted "Rouse values" for the critical exponents at the gelation transition do not arise from the Rouse model for gelling polymers. The true critical behaviour of the Rouse model for gelling polymers is obtained from spectral properties of the connectivity matrix of the fractal clusters that are formed by the molecules. The required spectral properties are related to the return probability of a "blind ant"-random walk on the critical percolating cluster. The resulting scaling relations express the critical exponents of the shear-stress-relaxation function, and hence those of the shear viscosity and of the first normal stress coefficient, in terms of the spectral dimension $d_{s}$ of the critical percolating cluster and the exponents $\sigma$ and $\tau$ of the cluster-size distribution.Comment: 9 pages, slightly extended version, to appear in J. Phys.

Elasticity of Gaussian and nearly-Gaussian phantom networks

We study the elastic properties of phantom networks of Gaussian and nearly-Gaussian springs. We show that the stress tensor of a Gaussian network coincides with the conductivity tensor of an equivalent resistor network, while its elastic constants vanish. We use a perturbation theory to analyze the elastic behavior of networks of slightly non-Gaussian springs. We show that the elastic constants of phantom percolation networks of nearly-Gaussian springs have a power low dependence on the distance of the system from the percolation threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur

Electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ

We study the electronic structure of the quasi-one-dimensional organic conductor TTF-TCNQ by means of density-functional band theory, Hubbard model calculations, and angle-resolved photoelectron spectroscopy (ARPES). The experimental spectra reveal significant quantitative and qualitative discrepancies to band theory. We demonstrate that the dispersive behavior as well as the temperature-dependence of the spectra can be consistently explained by the finite-energy physics of the one-dimensional Hubbard model at metallic doping. The model description can even be made quantitative, if one accounts for an enhanced hopping integral at the surface, most likely caused by a relaxation of the topmost molecular layer. Within this interpretation the ARPES data provide spectroscopic evidence for the existence of spin-charge separation on an energy scale of the conduction band width. The failure of the one-dimensional Hubbard model for the {\it low-energy} spectral behavior is attributed to interchain coupling and the additional effect of electron-phonon interaction.Comment: 18 pages, 9 figure

Nuclear relaxation and spin dynamics in NMP TCNQ

Proton nuclear relaxation measurements have been performed as a function of the frequency in selectively deuterated NMP TCNQ compounds. The results show that the main source of nuclear relaxation is provided by localized electronic spins, associated with back charge transfer on the NMP stacks. Once their contribution to the relaxation has been subtracted, it appears that the spin dynamics in the TCNQ stacks has a one-dimensional diffusive behaviour, as in other TCNQ conducting compounds.Des mesures de relaxation nucléaire des protons ont été effectuées en fonction de la fréquence dans des echantillons de NMP TCNQ sélectivement deutériés. Les résultats montrent que la principale source de relaxation nucléaire provient de spins électroniques localisés sur les chaines de NMP et associés à un transfert de charge incomplet. Après soustraction de leur contribution à la relaxation, il apparait que la dynamique de spin dans les chaines de TCNQ a un caractère diffusif unidimensionnel, comme dans les autres sels conducteurs de TCNQ

NMR study of transport properties in pure and irradiated Qn TCNQ2

From proton nuclear relaxation measurements, we deduce the parallel and perpendicular spin diffusion coefficients in pure and neutron-irradiated 1D organic conductor Qn TCNQ2. Comparison with conductivity data shows that the transport properties are determined by the temperature-dependent mobility. From our NMR results, we estimate the conductivity anisotropy, discuss the conduction mechanisms and the effects of irradiation induced defects.A l'aide de mesures de relaxation nucléaire des protons nous déterminons les coefficients de diffusion de spin parallèle et transverse dans des échantillons purs et irradiés par neutrons du conducteur organique Qn TCNQ2- Une comparaison avec la conductivité montre que les propriétés de transport sont dominées par la dépendance thermique de la mobilité. A partir des mesures de RMN, nous donnons une estimation de l'anisotropie de la conductivité et nous discutons les mécanismes de conduction, ainsi que les effets des défauts créés par irradiation

Temps de relaxation nucléaire T1D et T1ρ en présence de mouvement de spins électroniques

The theoretical expressions of the dipolar relaxation time T 1D and of the relaxation time in the rotating frame T1ρ are derived from a thermodynamical formalism for the case of nuclear relaxation controlled by electronic spin dynamics. It is showed that T1D and T1ρ are influenced by very low frequency motions (104 s-1). Experimental results obtained in linear exchange paramagnets agree with theoretical calculations.Les expressions théoriques du temps de relaxation dipolaire T 1D et du temps de relaxation dans le référentiel tournant T1ρ sont établies à l'aide d'un formalisme thermodynamique, dans le cas où la relaxation nucléaire provient de la modulation du champ hyperfin par le mouvement des spins électroniques. Il apparaît que T1D et T1ρ sont sensibles à des mouvements à très basse fréquence (104 s-1). Une confrontation expérimentale est effectuée dans le cas de systèmes présentant des interactions d'échange linéaire