Temporal variation in vegetative development of Caulerpa scalpelliformis (Chlorophyta) from Baleia beach, Ilha Grande bay (Rio de janeiro, brazil)

Caulerpa scalpelliformis grows on rocky and sandy bottoms at different depths in Jacuacanga Cove, Ilha Grande Bay, where it can form dense patches. The invasive behavior of this and other species of Caulerpa is well documented in the literature. This study analyzed the variation of the vegetative development of C. scalpelliformis from Baleia Beach (23º01'63''S and 44º14'18''W) in Jacuacanga Cove, from June 2003 to September 2004, including plants from rocky and sandy substrates. Morphometric and dry weight data from the erect and prostrate portions were used. Plants were collected from 20 x 20 cm² plots (n=3), randomly positioned on the rocky boulders (≅1.5 m depth) and on contiguous sandy bottom (≅3.0 m depth). During the study period, C. scalpelliformis occurred on both substrates as a pseudo-perennial species, showing partial loss of the erect portions from winter to spring; the height and dry weight of the erect portions increased from summer to autumn on both substrates. The temporal variation of C. scalpelliformis vegetative development in Baleia Beach was similar to that described for the invasive C. racemosa and C. taxifolia from different geographical regions of the world.Caulerpa scalpelliformis cresce sobre substrato rochoso e arenoso a diferentes profundidades na Enseada de Jacuacanga, Baía da Ilha Grande, onde pode formar bancos densos. O comportamento invasor desta e de outras espécies de Caulerpa é bem documentado na literatura. Este estudo analisou a variação do desenvolvimento vegetativo de C. scalpelliformis da Praia da Baleia (23º01'63''S e 44º14'18''W), Enseada de Jacuacanga, de junho de 2003 a setembro de 2004, considerando plantas do substrato rochoso e do fundo de areia. Dados morfométricos e de peso seco das porções prostrada e ereta foram usados. Plantas foram coletadas em quadrados de 20 cm de lado (n=3), posicionados aleatoriamente sobre o substrato rochoso (profundidade ≅ 1,5 m) e sobre o substrato arenoso (profundidade ≅ 3,0 m). Durante o período de estudo, C. scalpelliformis ocorreu nos dois substratos como espécie pseudo-perene, mostrando perda parcial das porções eretas do talo do inverno para a primavera; altura e massa seca das porções eretas mostraram tendência de aumento do verão para o outono nos dois substratos. A variação temporal do desenvolvimento vegetativo de C. scalpelliformis na Praia da Baleia foi similar à descrita para C. racemosa e C. taxifolia, espécies invasoras em diferentes regiões do mundo

Ferroelastic and ferroelectric phase transition in a molecular crystal : tanane - 3. — From ab initio computation of the intermolecular forces to statistical mechanics of the transition

On the basis of structural data of the disordered tetragonal phase of tanane, the Van der Waals intermolecular forces are computed for the 16 nearest neighbours of a given molecule. Considering first the molecular ordering in a « rigid » tetragonal lattice, the pair energies between neighbouring molecules are analysed within a pseudo-spin formalism which shows that the main ordering interactions define linear chains of molecules along the ferroelectric Z-axis. As there is no exact procedure for a statistical treatment of the 3-d Ising Hamiltonian, we just calculate the transition temperature To in the mean-field approximation. The predicted result : T0 ~ 200 K is in good agreement with the ordering temperature in the « rigid » lattice deduced from experimental investigations : To = 235 K. In a second step, we consider the « piezo-electric-like » coupling between the molecular ordering and the uxy shear of the lattice. Using a « Bragg-Williams-like » treatment of the strain-dependent Ising Hamiltonian, we build a free energy expansion whose bilinear terms agree well with those determined from experiments (shear modulus C66 and « piezo-electric-like » coefficient D), and which yields for the predicted transition temperature : T c ~ 240 K (actual value : Tc = 287 K). This analysis also reveals an important biquadratic coupling term which is supposed to enhance the instability of the structure in the vicinity of T c.A partir de la structure de la phase quadratique désordonnée du tanane, nous avons calculé les énergies intermoléculaires de Van der Waals pour les 16 premières voisines d'une molécule donnée. Les énergies de paire, obtenues dans le cas du réseau quadratique « rigide », sont analysées à l'aide d'un formalisme de pseudo-spin qui montre que les principales interactions responsables de la mise en ordre définissent des chaînes linéaires de molécules le long de l'axe ferroélectrique Z. En l'absence de solution statistique exacte pour le modèle d'Ising à 3 dimensions, nous calculons simplement la température de transition To dans l'approximation du champ moléculaire. Le résultat obtenu : T0 ~ 200 K est en bon accord avec la température de transition, en réseau « rigide », déduite des études expérimentales : To = 235 K. Ensuite, nous considérons le couplage de type « piézo-électrique » entre la mise en ordre des molécules et le cisaillement uxy du réseau. A l'aide d'un traitement « à la Bragg-Williams » on construit, à partir de l'Hamiltonien d'Ising dépendant de la déformation, un développement de l'énergie libre qui prévoit des valeurs en bon accord avec l'expérience pour : le module de cisaillement C66, le coefficient de type « piézo-électrique » D et la température de transition Tc ~ 240 K (à comparer à la valeur réelle Tc = 287 K). Cette analyse révèle aussi un important couplage d'ordre supérieur, qui doit augmenter l'instabilité de la structure au voisinage de Tc